In
computer science
Computer science is the study of computation, automation, and information. Computer science spans theoretical disciplines (such as algorithms, theory of computation, information theory, and automation) to practical disciplines (includi ...
, control flow (or flow of control) is the order in which individual
statements
Statement or statements may refer to: Common uses
*Statement (computer science), the smallest standalone element of an imperative programming language
*Statement (logic), declarative sentence that is either true or false
*Statement, a declarative ...
,
instructions or
function call
In computer programming, a function or subroutine is a sequence of program instructions that performs a specific task, packaged as a unit. This unit can then be used in programs wherever that particular task should be performed.
Functions may ...
s of an
imperative program are
executed
Capital punishment, also known as the death penalty, is the state-sanctioned practice of deliberately killing a person as a punishment for an actual or supposed crime, usually following an authorized, rule-governed process to conclude that t ...
or evaluated. The emphasis on explicit control flow distinguishes an ''
imperative programming
In computer science, imperative programming is a programming paradigm of software that uses statements that change a program's state. In much the same way that the imperative mood in natural languages expresses commands, an imperative program ...
'' language from a ''
declarative programming'' language.
Within an imperative
programming language
A programming language is a system of notation for writing computer programs. Most programming languages are text-based formal languages, but they may also be graphical. They are a kind of computer language.
The description of a programming ...
, a ''control flow statement'' is a statement that results in a choice being made as to which of two or more paths to follow. For
non-strict functional languages, functions and
language construct
In computer programming, a language construct is a syntactically allowable part of a program that may be formed from one or more lexical tokens in accordance with the rules of the programming language.
The term "language construct" is often used ...
s exist to achieve the same result, but they are usually not termed control flow statements.
A set of statements is in turn generally structured as a
block
Block or blocked may refer to:
Arts, entertainment and media Broadcasting
* Block programming, the result of a programming strategy in broadcasting
* W242BX, a radio station licensed to Greenville, South Carolina, United States known as ''96.3 ...
, which in addition to grouping, also defines a
lexical scope
In computer programming, the scope of a name binding (an association of a name to an entity, such as a variable) is the part of a program where the name binding is valid; that is, where the name can be used to refer to the entity. In other parts ...
.
Interrupt
In digital computers, an interrupt (sometimes referred to as a trap) is a request for the processor to ''interrupt'' currently executing code (when permitted), so that the event can be processed in a timely manner. If the request is accepted, ...
s and
signals
In signal processing, a signal is a function that conveys information about a phenomenon. Any quantity that can vary over space or time can be used as a signal to share messages between observers. The ''IEEE Transactions on Signal Processing'' ...
are low-level mechanisms that can alter the flow of control in a way similar to a
subroutine, but usually occur as a response to some external stimulus or event (that can occur
asynchronously), rather than execution of an ''in-line'' control flow statement.
At the level of
machine language
In computer programming, machine code is any low-level programming language, consisting of machine language instructions, which are used to control a computer's central processing unit (CPU). Each instruction causes the CPU to perform a very ...
or
assembly language, control flow instructions usually work by altering the
program counter. For some
central processing unit
A central processing unit (CPU), also called a central processor, main processor or just Processor (computing), processor, is the electronic circuitry that executes Instruction (computing), instructions comprising a computer program. The CPU per ...
s (CPUs), the only control flow instructions available are conditional or unconditional
branch
A branch, sometimes called a ramus in botany, is a woody structural member connected to the central trunk of a tree (or sometimes a shrub). Large branches are known as boughs and small branches are known as twigs. The term ''twig'' usually ...
instructions, also termed jumps.
Categories
![For-loop-diagram](https://upload.wikimedia.org/wikipedia/commons/0/06/For-loop-diagram.png)
The kinds of control flow statements supported by different languages vary, but can be categorized by their effect:
* Continuation at a different statement (unconditional
branch
A branch, sometimes called a ramus in botany, is a woody structural member connected to the central trunk of a tree (or sometimes a shrub). Large branches are known as boughs and small branches are known as twigs. The term ''twig'' usually ...
or jump)
* Executing a set of statements only if some condition is met (choice - i.e.,
conditional branch)
* Executing a set of statements zero or more times, until some condition is met (i.e., loop - the same as
conditional branch)
* Executing a set of distant statements, after which the flow of control usually returns (
subroutines,
coroutine
Coroutines are computer program components that generalize subroutines for non-preemptive multitasking, by allowing execution to be suspended and resumed. Coroutines are well-suited for implementing familiar program components such as cooperative ...
s, and
continuation
In computer science, a continuation is an abstract representation of the control state of a computer program. A continuation implements ( reifies) the program control state, i.e. the continuation is a data structure that represents the computati ...
s)
* Stopping the program, preventing any further execution (unconditional halt)
Primitives
Labels
A
label
A label (as distinct from signage) is a piece of paper, plastic film, cloth, metal, or other material affixed to a container or product, on which is written or printed information or symbols about the product or item. Information printed ...
is an explicit name or number assigned to a fixed position within the
source code
In computing, source code, or simply code, is any collection of code, with or without comments, written using a human-readable programming language, usually as plain text. The source code of a program is specially designed to facilitate the w ...
, and which may be referenced by control flow statements appearing elsewhere in the source code. A label marks a position within source code and has no other effect.
Line number
In computing, a line number is a method used to specify a particular sequence of characters in a text file. The most common method of assigning numbers to lines is to assign every line a unique number, starting at 1 for the first line, and increme ...
s are an alternative to a named label used in some languages (such as
BASIC). They are
whole numbers placed at the start of each line of text in the source code. Languages which use these often impose the constraint that the line numbers must increase in value in each following line, but may not require that they be consecutive. For example, in BASIC:
10 LET X = 3
20 PRINT X
In other languages such as
C and
Ada
Ada may refer to:
Places
Africa
* Ada Foah, a town in Ghana
* Ada (Ghana parliament constituency)
* Ada, Osun, a town in Nigeria
Asia
* Ada, Urmia, a village in West Azerbaijan Province, Iran
* Ada, Karaman, a village in Karaman Province, ...
, a label is an
identifier, usually appearing at the start of a line and immediately followed by a colon. For example, in C:
Success: printf("The operation was successful.\n");
The language
ALGOL 60 allowed both whole numbers and identifiers as labels (both linked by colons to the following statement), but few if any other
ALGOL
ALGOL (; short for "Algorithmic Language") is a family of imperative computer programming languages originally developed in 1958. ALGOL heavily influenced many other languages and was the standard method for algorithm description used by the ...
variants allowed whole numbers. Early
Fortran compilers only allowed whole numbers as labels. Beginning with Fortran-90, alphanumeric labels have also been allowed.
Goto
The ''goto'' statement (a combination of the English words ''
go'' and ''
to'', and pronounced accordingly) is the most basic form of unconditional transfer of control.
Although the
keyword
Keyword may refer to:
Computing
* Keyword (Internet search), a word or phrase typically used by bloggers or online content creator to rank a web page on a particular topic
* Index term, a term used as a keyword to documents in an information syst ...
may either be in upper or lower case depending on the language, it is usually written as:
goto ''label''
The effect of a goto statement is to cause the next statement to be executed to be the statement appearing at (or immediately after) the indicated label.
Goto statements have been
considered harmful
GoTo (goto, GOTO, GO TO or other case combinations, depending on the programming language) is a statement found in many computer programming languages. It performs a one-way transfer of control to another line of code; in contrast a function c ...
by many computer scientists, notably
Dijkstra.
Subroutines
The terminology for
subroutines varies; they may alternatively be known as routines, procedures, functions (especially if they return results) or methods (especially if they belong to
classes or
type classes).
In the 1950s, computer memories were very small by current standards so subroutines were used mainly to reduce program size. A piece of code was written once and then used many times from various other places in a program.
Today, subroutines are more often used to help make a program more structured, e.g., by isolating some algorithm or hiding some data access method. If many programmers are working on one program, subroutines are one kind of
modularity that can help divide the work.
Sequence
In structured programming, the ordered sequencing of successive commands is considered one of the basic control structures, which is used as a building block for programs alongside iteration, recursion and choice.
Minimal structured control flow
In May 1966, Böhm and Jacopini published an article in ''Communications of the ACM'' which showed that any program with gotos could be transformed into a goto-free form involving only choice (IF THEN ELSE) and loops (WHILE condition DO xxx), possibly with duplicated code and/or the addition of Boolean variables (true/false flags). Later authors showed that choice can be replaced by loops (and yet more Boolean variables).
That such minimalism is possible does not mean that it is necessarily desirable; after all, computers theoretically need only
one machine instruction (subtract one number from another and branch if the result is negative), but practical computers have dozens or even hundreds of machine instructions.
What Böhm and Jacopini's article showed was that all programs could be goto-free. Other research showed that control structures with one entry and one exit were much easier to understand than any other form, mainly because they could be used anywhere as a statement without disrupting the control flow. In other words, they were ''composable''. (Later developments, such as
non-strict programming language
A strict programming language is a programming language which employs a strict programming paradigm, allowing only strict functions (functions whose parameters must be evaluated completely before they may be called) to be defined by the user. A no ...
s – and more recently, composable
software transactions – have continued this strategy, making components of programs even more freely composable.)
Some academics took a purist approach to the Böhm–Jacopini result and argued that even instructions like
break
and
return
from the middle of loops are bad practice as they are not needed in the Böhm–Jacopini proof, and thus they advocated that all loops should have a single exit point. This purist approach is embodied in the language
Pascal (designed in 1968–1969), which up to the mid-1990s was the preferred tool for teaching introductory programming in academia.
[Roberts, E. ]995
Year 995 ( CMXCV) was a common year starting on Tuesday (link will display the full calendar) of the Julian calendar.
Events
By place
Japan
* 17 May - Fujiwara no Michitaka (imperial regent) dies.
* 3 June: Fujiwara no Michikane gain ...
Loop Exits and Structured Programming: Reopening the Debate
” ACM SIGCSE Bulletin, (27)1: 268–272. The direct application of the Böhm–Jacopini theorem may result in additional local variables being introduced in the structured chart, and may also result in some
code duplication In computer programming, duplicate code is a sequence of source code that occurs more than once, either within a program or across different programs owned or maintained by the same entity. Duplicate code is generally considered undesirable for a ...
.
Pascal is affected by both of these problems and according to empirical studies cited by
Eric S. Roberts, student programmers had difficulty formulating correct solutions in Pascal for several simple problems, including writing a function for searching an element in an array. A 1980 study by Henry Shapiro cited by Roberts found that using only the Pascal-provided control structures, the correct solution was given by only 20% of the subjects, while no subject wrote incorrect code for this problem if allowed to write a return from the middle of a loop.
Control structures in practice
Most programming languages with control structures have an initial keyword which indicates the type of control structure involved. Languages then divide as to whether or not control structures have a final keyword.
* No final keyword:
ALGOL 60,
C,
C++
C++ (pronounced "C plus plus") is a high-level general-purpose programming language created by Danish computer scientist Bjarne Stroustrup as an extension of the C programming language, or "C with Classes". The language has expanded significan ...
,
Haskell
Haskell () is a general-purpose, statically-typed, purely functional programming language with type inference and lazy evaluation. Designed for teaching, research and industrial applications, Haskell has pioneered a number of programming lan ...
,
Java
Java (; id, Jawa, ; jv, ꦗꦮ; su, ) is one of the Greater Sunda Islands in Indonesia. It is bordered by the Indian Ocean to the south and the Java Sea to the north. With a population of 151.6 million people, Java is the world's mos ...
,
Pascal,
Perl
Perl is a family of two high-level, general-purpose, interpreted, dynamic programming languages. "Perl" refers to Perl 5, but from 2000 to 2019 it also referred to its redesigned "sister language", Perl 6, before the latter's name was offic ...
,
PHP
PHP is a general-purpose scripting language geared toward web development. It was originally created by Danish-Canadian programmer Rasmus Lerdorf in 1993 and released in 1995. The PHP reference implementation is now produced by The PHP Group. ...
,
PL/I
PL/I (Programming Language One, pronounced and sometimes written PL/1) is a procedural, imperative computer programming language developed and published by IBM. It is designed for scientific, engineering, business and system programming. I ...
,
Python
Python may refer to:
Snakes
* Pythonidae, a family of nonvenomous snakes found in Africa, Asia, and Australia
** ''Python'' (genus), a genus of Pythonidae found in Africa and Asia
* Python (mythology), a mythical serpent
Computing
* Python (pro ...
,
PowerShell
PowerShell is a task automation and configuration management program from Microsoft, consisting of a command-line shell and the associated scripting language. Initially a Windows component only, known as Windows PowerShell, it was made open-sou ...
. Such languages need some way of grouping statements together:
** ALGOL 60 and Pascal:
begin
...
end
** C, C++, Java, Perl, PHP, and PowerShell:
curly brackets
** PL/I:
DO
...
END
** Python: uses
indent level (see
Off-side rule
A computer programming language is said to adhere to the off-side rule of syntax if blocks in that language are expressed by their indentation. The term was coined by Peter Landin, possibly as a pun on the offside rule in association football ...
)
** Haskell: either
indent level or curly brackets can be used, and they can be freely mixed
** Lua: uses
do
...
end
* Final keyword:
Ada
Ada may refer to:
Places
Africa
* Ada Foah, a town in Ghana
* Ada (Ghana parliament constituency)
* Ada, Osun, a town in Nigeria
Asia
* Ada, Urmia, a village in West Azerbaijan Province, Iran
* Ada, Karaman, a village in Karaman Province, ...
,
ALGOL 68
ALGOL 68 (short for ''Algorithmic Language 1968'') is an imperative programming language that was conceived as a successor to the ALGOL 60 programming language, designed with the goal of a much wider scope of application and more rigorously d ...
,
Modula-2,
Fortran 77,
Mythryl,
Visual Basic Visual Basic is a name for a family of programming languages from Microsoft. It may refer to:
* Visual Basic .NET (now simply referred to as "Visual Basic"), the current version of Visual Basic launched in 2002 which runs on .NET
* Visual Basic ( ...
. The forms of the final keyword vary:
** Ada: final keyword is
end
+ ''space'' + initial keyword e.g.,
if
...
end if
,
loop
...
end loop
** ALGOL 68, Mythryl: initial keyword spelled backwards e.g.,
if
...
fi
,
case
...
esac
** Fortran 77: final keyword is
END
+ initial keyword e.g.,
IF
...
ENDIF
,
DO
...
ENDDO
** Modula-2: same final keyword
END
for everything
** Visual Basic: every control structure has its own keyword.
If
...
End If
;
For
...
Next
;
Do
...
Loop
;
While
...
Wend
Choice
If-then-(else) statements
Conditional expressions and conditional constructs are features of a
programming language
A programming language is a system of notation for writing computer programs. Most programming languages are text-based formal languages, but they may also be graphical. They are a kind of computer language.
The description of a programming ...
which perform different computations or actions depending on whether a programmer-specified
boolean ''condition'' evaluates to true or false.
*
IF..GOTO
. A form found in unstructured languages, mimicking a typical machine code instruction, would jump to (GOTO) a label or line number when the condition was met.
*
IF..THEN..(ENDIF)
. Rather than being restricted to a jump, any simple statement, or nested block, could follow the THEN key keyword. This a structured form.
*
IF..THEN..ELSE..(ENDIF)
. As above, but with a second action to be performed if the condition is false. This is one of the most common forms, with many variations. Some require a terminal
ENDIF
, others do not.
C and related languages do not require a terminal keyword, or a 'then', but do require parentheses around the condition.
* Conditional statements can be and often are nested inside other conditional statements. Some languages allow
ELSE
and
IF
to be combined into
ELSEIF
, avoiding the need to have a series of
ENDIF
or other final statements at the end of a compound statement.
Less common variations include:
* Some languages, such as
Fortran, have a ''three-way'' or ''
arithmetic if
The arithmetic IF statement is a three-way arithmetic conditional statement, first seen in the first release of Fortran in 1957, and found in all later versions, and some other programming languages, such as FOCAL. Unlike the logical IF stateme ...
'', testing whether a numeric value is positive, negative or zero.
*Some languages have a
functional form of an
if
statement, for instance
Lisp's cond
.
*Some languages have an
operator form of an
if
statement, such as C's
ternary operator
In mathematics, a ternary operation is an ''n''-ary operation with ''n'' = 3. A ternary operation on a set ''A'' takes any given three elements of ''A'' and combines them to form a single element of ''A''.
In computer science, a ternary operator i ...
.
*
Perl
Perl is a family of two high-level, general-purpose, interpreted, dynamic programming languages. "Perl" refers to Perl 5, but from 2000 to 2019 it also referred to its redesigned "sister language", Perl 6, before the latter's name was offic ...
supplements a C-style
if
with
when
and
unless
.
*
Smalltalk uses
ifTrue
and
ifFalse
messages to implement conditionals, rather than any fundamental language construct.
Case and switch statements
Switch statements (or ''case statements'', or ''multiway branches'') compare a given value with specified constants and take action according to the first constant to match. There is usually a provision for a default action ("else", "otherwise") to be taken if no match succeeds. Switch statements can allow compiler optimizations, such as
lookup tables. In
dynamic language
In computer science, a dynamic programming language is a class of high-level programming languages, which at runtime execute many common programming behaviours that static programming languages perform during compilation. These behaviors cou ...
s, the cases may not be limited to constant expressions, and might extend to
pattern matching
In computer science, pattern matching is the act of checking a given sequence of tokens for the presence of the constituents of some pattern. In contrast to pattern recognition, the match usually has to be exact: "either it will or will not be ...
, as in the
shell script example on the right, where the
*)
implements the default case as a
glob matching any string. Case logic
can also be implemented in functional form, as in
SQL's
decode
statement.
Loops
A loop is a sequence of statements which is specified once but which may be carried out several times in succession.
The code "inside" the loop (the ''body'' of the loop, shown below as ''xxx'') is obeyed a specified number of times, or once for each of a collection of items, or until some condition is met, or
indefinitely.
In
functional programming
In computer science, functional programming is a programming paradigm where programs are constructed by applying and composing functions. It is a declarative programming paradigm in which function definitions are trees of expressions that ...
languages, such as
Haskell
Haskell () is a general-purpose, statically-typed, purely functional programming language with type inference and lazy evaluation. Designed for teaching, research and industrial applications, Haskell has pioneered a number of programming lan ...
and
Scheme, both
recursive
Recursion (adjective: ''recursive'') occurs when a thing is defined in terms of itself or of its type. Recursion is used in a variety of disciplines ranging from linguistics to logic. The most common application of recursion is in mathematics ...
and
iterative
Iteration is the repetition of a process in order to generate a (possibly unbounded) sequence of outcomes. Each repetition of the process is a single iteration, and the outcome of each iteration is then the starting point of the next iteration. ...
processes are expressed with
tail recursive procedures instead of looping constructs that are syntactic.
Count-controlled loops
Most programming languages have constructions for repeating a loop a certain number of times.
In most cases counting can go downwards instead of upwards and step sizes other than 1 can be used.
FOR I = 1 TO N , for I := 1 to N do begin
xxx , xxx
NEXT I , end;
------------------------------------------------------------
DO I = 1,N , for ( I=1; I<=N; ++I )
In these examples, if N < 1 then the body of loop may execute once (with I having value 1) or not at all, depending on the programming language.
In many programming languages, only integers can be reliably used in a count-controlled loop. Floating-point numbers are represented imprecisely due to hardware constraints, so a loop such as
for X := 0.1 step 0.1 to 1.0 do
might be repeated 9 or 10 times, depending on rounding errors and/or the hardware and/or the compiler version. Furthermore, if the increment of X occurs by repeated addition, accumulated rounding errors may mean that the value of X in each iteration can differ quite significantly from the expected sequence 0.1, 0.2, 0.3, ..., 1.0.
Condition-controlled loops
Most programming languages have constructions for repeating a loop until some condition changes.
Some variations test the condition at the start of the loop; others test it at the end.
If the test is at the start, the body may be skipped completely; if it is at the end, the body is always executed at least once.
DO WHILE (test) , repeat
xxx , xxx
LOOP , until test;
----------------------------------------------
while (test) , while (test);
A
control break
In computer programming a control break is a change in the value of one of the keys on which a file is sorted which requires some extra processing. For example, with an input file sorted by post code, the number of items found in each postal dist ...
is a value change detection method used within ordinary loops to trigger processing for groups of values. Values are monitored within the loop and a change diverts program flow to the handling of the group event associated with them.
DO UNTIL (End-of-File)
IF new-zipcode <> current-zipcode
display_tally(current-zipcode, zipcount)
current-zipcode = new-zipcode
zipcount = 0
ENDIF
zipcount++
LOOP
Collection-controlled loops
Several programming languages (e.g.,
Ada
Ada may refer to:
Places
Africa
* Ada Foah, a town in Ghana
* Ada (Ghana parliament constituency)
* Ada, Osun, a town in Nigeria
Asia
* Ada, Urmia, a village in West Azerbaijan Province, Iran
* Ada, Karaman, a village in Karaman Province, ...
,
D,
C++11
C++11 is a version of the ISO/ IEC 14882 standard for the C++ programming language. C++11 replaced the prior version of the C++ standard, called C++03, and was later replaced by C++14. The name follows the tradition of naming language versions b ...
,
Smalltalk,
PHP
PHP is a general-purpose scripting language geared toward web development. It was originally created by Danish-Canadian programmer Rasmus Lerdorf in 1993 and released in 1995. The PHP reference implementation is now produced by The PHP Group. ...
,
Perl
Perl is a family of two high-level, general-purpose, interpreted, dynamic programming languages. "Perl" refers to Perl 5, but from 2000 to 2019 it also referred to its redesigned "sister language", Perl 6, before the latter's name was offic ...
,
Object Pascal
Object Pascal is an extension to the programming language Pascal that provides object-oriented programming (OOP) features such as classes and methods.
The language was originally developed by Apple Computer as ''Clascal'' for the Lisa Worksh ...
,
Java
Java (; id, Jawa, ; jv, ꦗꦮ; su, ) is one of the Greater Sunda Islands in Indonesia. It is bordered by the Indian Ocean to the south and the Java Sea to the north. With a population of 151.6 million people, Java is the world's mos ...
,
C#,
MATLAB
MATLAB (an abbreviation of "MATrix LABoratory") is a proprietary multi-paradigm programming language and numeric computing environment developed by MathWorks. MATLAB allows matrix manipulations, plotting of functions and data, implementa ...
,
Visual Basic Visual Basic is a name for a family of programming languages from Microsoft. It may refer to:
* Visual Basic .NET (now simply referred to as "Visual Basic"), the current version of Visual Basic launched in 2002 which runs on .NET
* Visual Basic ( ...
,
Ruby
A ruby is a pinkish red to blood-red colored gemstone, a variety of the mineral corundum ( aluminium oxide). Ruby is one of the most popular traditional jewelry gems and is very durable. Other varieties of gem-quality corundum are called ...
,
Python
Python may refer to:
Snakes
* Pythonidae, a family of nonvenomous snakes found in Africa, Asia, and Australia
** ''Python'' (genus), a genus of Pythonidae found in Africa and Asia
* Python (mythology), a mythical serpent
Computing
* Python (pro ...
,
JavaScript
JavaScript (), often abbreviated as JS, is a programming language that is one of the core technologies of the World Wide Web, alongside HTML and CSS. As of 2022, 98% of websites use JavaScript on the client side for webpage behavior, of ...
,
Fortran 95 and later) have special constructs which allow implicit looping through all elements of an array, or all members of a set or collection.
someCollection do:
xxx
for Item in Collection do begin xxx end;
foreach (item; myCollection)
foreach someArray
foreach ($someArray as $k => $v)
Collection
coll; for (String s : coll)
foreach (string s in myStringCollection)
someCollection , ForEach-Object
forall ( index = first:last:step... )
Scala has for-expressions, which generalise collection-controlled loops, and also support other uses, such as asynchronous programming
Asynchrony, in computer programming, refers to the occurrence of events independent of the main control flow, program flow and ways to deal with such events. These may be "outside" events such as the arrival of Unix signal, signals, or actions ins ...
. Haskell
Haskell () is a general-purpose, statically-typed, purely functional programming language with type inference and lazy evaluation. Designed for teaching, research and industrial applications, Haskell has pioneered a number of programming lan ...
has do-expressions and comprehensions, which together provide similar function to for-expressions in Scala.
General iteration
General iteration constructs such as C's for
statement and Common Lisp's do
form can be used to express any of the above sorts of loops, and others, such as looping over some number of collections in parallel. Where a more specific looping construct can be used, it is usually preferred over the general iteration construct, since it often makes the purpose of the expression clearer.
Infinite loops
Infinite loop
In computer programming, an infinite loop (or endless loop) is a sequence of instructions that, as written, will continue endlessly, unless an external intervention occurs ("pull the plug"). It may be intentional.
Overview
This differs from:
* ...
s are used to assure a program segment loops forever or until an exceptional condition arises, such as an error. For instance, an event-driven program (such as a server
Server may refer to:
Computing
*Server (computing), a computer program or a device that provides functionality for other programs or devices, called clients
Role
* Waiting staff, those who work at a restaurant or a bar attending customers and su ...
) should loop forever, handling events as they occur, only stopping when the process is terminated by an operator.
Infinite loops can be implemented using other control flow constructs. Most commonly, in unstructured programming this is jump back up (goto), while in structured programming this is an indefinite loop (while loop) set to never end, either by omitting the condition or explicitly setting it to true, as while (true) ...
. Some languages have special constructs for infinite loops, typically by omitting the condition from an indefinite loop. Examples include Ada (loop ... end loop
), Fortran (DO ... END DO
), Go (for
), and Ruby (loop do ... end
).
Often, an infinite loop is unintentionally created by a programming error in a condition-controlled loop, wherein the loop condition uses variables that never change within the loop.
Continuation with next iteration
Sometimes within the body of a loop there is a desire to skip the remainder of the loop body and continue with the next iteration of the loop. Some languages provide a statement such as continue
(most languages), skip
, or next
(Perl and Ruby), which will do this. The effect is to prematurely terminate the innermost loop body and then resume as normal with the next iteration. If the iteration is the last one in the loop, the effect is to terminate the entire loop early.
Redo current iteration
Some languages, like Perl and Ruby, have a redo
statement that restarts the current iteration from the start.
Restart loop
Ruby has a retry
statement that restarts the entire loop from the initial iteration.
Early exit from loops
When using a count-controlled loop to search through a table, it might be desirable to stop searching as soon as the required item is found. Some programming languages provide a statement such as break
(most languages), Exit
(Visual Basic), or last
(Perl), which effect is to terminate the current loop immediately, and transfer control to the statement immediately after that loop. Another term for early-exit loops is loop-and-a-half.
The following example is done in Ada
Ada may refer to:
Places
Africa
* Ada Foah, a town in Ghana
* Ada (Ghana parliament constituency)
* Ada, Osun, a town in Nigeria
Asia
* Ada, Urmia, a village in West Azerbaijan Province, Iran
* Ada, Karaman, a village in Karaman Province, ...
which supports both ''early exit from loops'' and '' loops with test in the middle''. Both features are very similar and comparing both code snippets will show the difference: ''early exit'' must be combined with an if statement while a ''condition in the middle'' is a self-contained construct.
with Ada.Text IO;
with Ada.Integer Text IO;
procedure Print_Squares is
X : Integer;
begin
Read_Data : loop
Ada.Integer Text IO.Get(X);
exit Read_Data when X = 0;
Ada.Text IO.Put (X * X);
Ada.Text IO.New_Line;
end loop Read_Data;
end Print_Squares;
Python
Python may refer to:
Snakes
* Pythonidae, a family of nonvenomous snakes found in Africa, Asia, and Australia
** ''Python'' (genus), a genus of Pythonidae found in Africa and Asia
* Python (mythology), a mythical serpent
Computing
* Python (pro ...
supports conditional execution of code depending on whether a loop was exited early (with a break
statement) or not by using an else-clause with the loop. For example,
for n in set_of_numbers:
if isprime(n):
print("Set contains a prime number")
break
else:
print("Set did not contain any prime numbers")
The else
clause in the above example is linked to the for
statement, and not the inner if
statement. Both Python's for
and while
loops support such an else clause, which is executed only if early exit of the loop has not occurred.
Some languages support breaking out of nested loops; in theory circles, these are called multi-level breaks. One common use example is searching a multi-dimensional table. This can be done either via multilevel breaks (break out of ''N'' levels), as in bash and PHP, or via labeled breaks (break out and continue at given label), as in Java and Perl. Alternatives to multilevel breaks include single breaks, together with a state variable which is tested to break out another level; exceptions, which are caught at the level being broken out to; placing the nested loops in a function and using return to effect termination of the entire nested loop; or using a label and a goto statement. C does not include a multilevel break, and the usual alternative is to use a goto to implement a labeled break. Python does not have a multilevel break or continue – this was proposed i
PEP 3136
and rejected on the basis that the added complexity was not worth the rare legitimate use.
The notion of multi-level breaks is of some interest in theoretical computer science
computer science (TCS) is a subset of general computer science and mathematics that focuses on mathematical aspects of computer science such as the theory of computation, lambda calculus, and type theory.
It is difficult to circumscribe the ...
, because it gives rise to what is today called the ''Kosaraju hierarchy''. In 1973 S. Rao Kosaraju
Sambasiva Rao Kosaraju is a professor of computer science at Johns Hopkins University, and division director for Computing & Communication Foundations at the National Science Foundation.structured program theorem
The structured program theorem, also called the Böhm–Jacopini theorem, is a result in programming language theory. It states that a class of control-flow graphs (historically called flowcharts in this context) can compute any computable function ...
by proving that it is possible to avoid adding additional variables in structured programming, as long as arbitrary-depth, multi-level breaks from loops are allowed. Furthermore, Kosaraju proved that a strict hierarchy of programs exists: for every integer ''n'', there exists a program containing a multi-level break of depth ''n'' that cannot be rewritten as a program with multi-level breaks of depth less than ''n'' without introducing added variables.
One can also return
out of a subroutine executing the looped statements, breaking out of both the nested loop and the subroutine. There are other proposed control structures for multiple breaks, but these are generally implemented as exceptions instead.
In his 2004 textbook, David Watt uses Tennent's notion of sequencer to explain the similarity between multi-level breaks and return statements. Watt notes that a class of sequencers known as ''escape sequencers'', defined as "sequencer that terminates execution of a textually enclosing command or procedure", encompasses both breaks from loops (including multi-level breaks) and return statements. As commonly implemented, however, return sequencers may also carry a (return) value, whereas the break sequencer as implemented in contemporary languages usually cannot.
Loop variants and invariants
Loop variants and loop invariant
In computer science, a loop invariant is a property of a program loop that is true before (and after) each iteration. It is a logical assertion, sometimes checked within the code by an assertion call. Knowing its invariant(s) is essential in und ...
s are used to express correctness of loops.
In practical terms, a loop variant is an integer expression which has an initial non-negative value. The variant's value must decrease during each loop iteration but must never become negative during the correct execution of the loop. Loop variants are used to guarantee that loops will terminate.
A loop invariant is an assertion which must be true before the first loop iteration and remain true after each iteration. This implies that when a loop terminates correctly, both the exit condition and the loop invariant are satisfied. Loop invariants are used to monitor specific properties of a loop during successive iterations.
Some programming languages, such as Eiffel contain native support for loop variants and invariants. In other cases, support is an add-on, such as the Java Modeling Language's specification fo
loop statements
in Java
Java (; id, Jawa, ; jv, ꦗꦮ; su, ) is one of the Greater Sunda Islands in Indonesia. It is bordered by the Indian Ocean to the south and the Java Sea to the north. With a population of 151.6 million people, Java is the world's mos ...
.
Loop sublanguage
Some Lisp dialects provide an extensive sublanguage for describing Loops. An early example can be found in Conversional Lisp of Interlisp
Interlisp (also seen with a variety of capitalizations) is a programming environment built around a version of the programming language Lisp. Interlisp development began in 1966 at Bolt, Beranek and Newman (renamed BBN Technologies) in Cambridge, ...
. Common Lisp provides a Loop macro which implements such a sublanguage.
Loop system cross-reference table
# while (true)
does not count as an infinite loop for this purpose, because it is not a dedicated language structure.
# C's for (''init''; ''test''; ''increment'')
loop is a general loop construct, not specifically a counting one, although it is often used for that.
# Deep breaks may be accomplished in APL, C, C++ and C# through the use of labels and gotos.
# Iteration over objects wa
added
in PHP 5.
# A counting loop can be simulated by iterating over an incrementing list or generator, for instance, Python's range()
.
# Deep breaks may be accomplished through the use of exception handling.
# There is no special construct, since the while
function can be used for this.
# There is no special construct, but users can define general loop functions.
# The C++11
C++11 is a version of the ISO/ IEC 14882 standard for the C++ programming language. C++11 replaced the prior version of the C++ standard, called C++03, and was later replaced by C++14. The name follows the tradition of naming language versions b ...
standard introduced the range-based for. In the STL, there is a std::for_each
template
Template may refer to:
Tools
* Die (manufacturing), used to cut or shape material
* Mold, in a molding process
* Stencil, a pattern or overlay used in graphic arts (drawing, painting, etc.) and sewing to replicate letters, shapes or designs
...
function which can iterate on STL containers
A container is any receptacle or enclosure for holding a product used in storage, packaging, and transportation, including shipping.
Things kept inside of a container are protected on several sides by being inside of its structure. The term ...
and call a unary function
A unary function is a function that takes one argument. A unary operator belongs to a subset of unary functions, in that its range coincides with its domain. In contrast, a unary function's domain may or may not coincide with its range.
Exampl ...
for each element. The functionality also can be constructed as macro on these containers.
# Count-controlled looping is effected by iteration across an integer interval; early exit by including an additional condition for exit.
# Eiffel supports a reserved word retry
, however it is used in exception handling
In computing and computer programming, exception handling is the process of responding to the occurrence of ''exceptions'' – anomalous or exceptional conditions requiring special processing – during the execution of a program. In general, an ...
, not loop control.
# Requires Java Modeling Language (JML) behavioral interface specification language.
# Requires loop variants to be integers; transfinite variants are not supported.
# D supports infinite collections, and the ability to iterate over those collections. This does not require any special construct.
# Deep breaks can be achieved using GO TO
and procedures.
# Common Lisp predates the concept of generic collection type.
Structured non-local control flow
Many programming languages, especially those favoring more dynamic styles of programming, offer constructs for ''non-local control flow''. These cause the flow of execution to jump out of a given context and resume at some predeclared point. '' Exception handling#Condition systems, Conditions'', '' exceptions'' and ''continuation
In computer science, a continuation is an abstract representation of the control state of a computer program. A continuation implements ( reifies) the program control state, i.e. the continuation is a data structure that represents the computati ...
s'' are three common sorts of non-local control constructs; more exotic ones also exist, such as generators, coroutine
Coroutines are computer program components that generalize subroutines for non-preemptive multitasking, by allowing execution to be suspended and resumed. Coroutines are well-suited for implementing familiar program components such as cooperative ...
s and the async keyword.
Conditions
PL/I
PL/I (Programming Language One, pronounced and sometimes written PL/1) is a procedural, imperative computer programming language developed and published by IBM. It is designed for scientific, engineering, business and system programming. I ...
has some 22 standard conditions (e.g., ZERODIVIDE SUBSCRIPTRANGE ENDFILE) which can be raised and which can be intercepted by: ON ''condition'' action; Programmers can also define and use their own named conditions.
Like the ''unstructured if'', only one statement can be specified so in many cases a GOTO is needed to decide where flow of control should resume.
Unfortunately, some implementations had a substantial overhead in both space and time (especially SUBSCRIPTRANGE), so many programmers tried to avoid using conditions.
Common Syntax examples:
ON ''condition'' GOTO ''label''
Exceptions
Modern languages have a specialized structured construct for exception handling which does not rely on the use of GOTO
or (multi-level) breaks or returns. For example, in C++ one can write:
try catch (someClass& someId) catch (someType& anotherId) catch (...)
Any number and variety of catch
clauses can be used above. If there is no catch
matching a particular throw
, control percolates back through subroutine calls and/or nested blocks until a matching catch
is found or until the end of the main program is reached, at which point the program is forcibly stopped with a suitable error message.
Via C++'s influence, catch
is the keyword reserved for declaring a pattern-matching exception handler in other languages popular today, like Java or C#. Some other languages like Ada use the keyword exception
to introduce an exception handler and then may even employ a different keyword (when
in Ada) for the pattern matching. A few languages like AppleScript
AppleScript is a scripting language created by Apple Inc. that facilitates automated control over scriptable Mac applications. First introduced in System 7, it is currently included in all versions of macOS as part of a package of system aut ...
incorporate placeholders in the exception handler syntax to automatically extract several pieces of information when the exception occurs. This approach is exemplified below by the on error
construct from AppleScript:
try
set myNumber to myNumber / 0
on error e number n from f to t partial result pr
if ( e = "Can't divide by zero" ) then display dialog "You must not do that"
end try
David Watt's 2004 textbook also analyzes exception handling in the framework of sequencers (introduced in this article in the section on early exits from loops). Watt notes that an abnormal situation, generally exemplified with arithmetic overflows or input/output
In computing, input/output (I/O, or informally io or IO) is the communication between an information processing system, such as a computer, and the outside world, possibly a human or another information processing system. Inputs are the signals ...
failures like file not found, is a kind of error that "is detected in some low-level program unit, but or which
Or or OR may refer to:
Arts and entertainment Film and television
* "O.R.", a 1974 episode of M*A*S*H
* Or (My Treasure), a 2004 movie from Israel (''Or'' means "light" in Hebrew)
Music
* ''Or'' (album), a 2002 album by Golden Boy with Mis ...
a handler is more naturally located in a high-level program unit". For example, a program might contain several calls to read files, but the action to perform when a file is not found depends on the meaning (purpose) of the file in question to the program and thus a handling routine for this abnormal situation cannot be located in low-level system code. Watts further notes that introducing status flags testing in the caller, as single-exit structured programming or even (multi-exit) return sequencers would entail, results in a situation where "the application code tends to get cluttered by tests of status flags" and that "the programmer might forgetfully or lazily omit to test a status flag. In fact, abnormal situations represented by status flags are by default ignored!" Watt notes that in contrast to status flags testing, exceptions have the opposite default behavior, causing the program to terminate unless the programmer explicitly deals with the exception in some way, possibly by adding explicit code to ignore it. Based on these arguments, Watt concludes that jump sequencers or escape sequencers aren't as suitable as a dedicated exception sequencer with the semantics discussed above.
In Object Pascal, D, Java, C#, and Python a finally
clause can be added to the try
construct. No matter how control leaves the try
the code inside the finally
clause is guaranteed to execute. This is useful when writing code that must relinquish an expensive resource (such as an opened file or a database connection) when finished processing:
FileStream stm = null; // C# example
try
finally
Since this pattern is fairly common, C# has a special syntax:
using (var stm = new FileStream("logfile.txt", FileMode.Create))
Upon leaving the using
-block, the compiler guarantees that the stm
object is released, effectively binding the variable to the file stream while abstracting from the side effects of initializing and releasing the file. Python's with
statement and Ruby's block argument to File.open
are used to similar effect.
All the languages mentioned above define standard exceptions and the circumstances under which they are thrown.
Users can throw exceptions of their own; in fact C++ allows users to throw and catch almost any type, including basic types like int
, whereas other languages like Java aren't as permissive.
Continuations
Async
C# 5.0 introduced the async keyword for supporting asynchronous I/O
In computer science, asynchronous I/O (also non-sequential I/O) is a form of input/output processing that permits other processing to continue before the transmission has finished. A name used for asynchronous I/O in the Windows API is overlappe ...
in a "direct style".
Generators
Generators, also known as semicoroutines, allow control to be yielded to a consumer method temporarily, typically using a keyword
yield description
. Like the async keyword, this supports programming in a "direct style".
Coroutines
Coroutines are functions that can yield control to each other - a form of co-operative multitasking
Cooperative multitasking, also known as non-preemptive multitasking, is a style of computer multitasking in which the operating system never initiates a context switch from a running process to another process. Instead, in order to run multiple ...
without threads.
Coroutines can be implemented as a library if the programming language provides either continuations or generators - so the distinction between coroutines and generators in practice is a technical detail.
Non-local control flow cross reference
Proposed control structures
In a spoof Datamation article in 1973, R. Lawrence Clark suggested that the GOTO statement could be replaced by the COMEFROM
In computer programming, COMEFROM (or COME FROM) is an obscure control flow structure used in some programming languages, originally as a joke. COMEFROM is the inverse of GOTO in that it can take the execution state from any arbitrary point in code ...
statement, and provides some entertaining examples. COMEFROM was implemented in one esoteric programming language
An esoteric programming language (sometimes shortened to esolang) is a programming language designed to test the boundaries of computer programming language design, as a proof of concept, as software art, as a hacking interface to another language ...
named INTERCAL
The Compiler Language With No Pronounceable Acronym (INTERCAL) is an esoteric programming language that was created as a parody by Don Woods and , two Princeton University students, in 1972. It satirizes aspects of the various programming langu ...
.
Donald Knuth
Donald Ervin Knuth ( ; born January 10, 1938) is an American computer scientist, mathematician, and professor emeritus at Stanford University. He is the 1974 recipient of the ACM Turing Award, informally considered the Nobel Prize of computer sc ...
's 1974 article "Structured Programming with go to Statements", identifies two situations which were not covered by the control structures listed above, and gave examples of control structures which could handle these situations. Despite their utility, these constructs have not yet found their way into mainstream programming languages.
Loop with test in the middle
The following was proposed by Dahl in 1972:
loop loop
xxx1 read(char);
while test; while not atEndOfFile;
xxx2 write(char);
repeat; repeat;
If ''xxx1'' is omitted, we get a loop with the test at the top (a traditional while loop). If ''xxx2'' is omitted, we get a loop with the test at the bottom, equivalent to a do while loop in many languages. If while is omitted, we get an infinite loop. The construction here can be thought of as a do loop with the while check in the middle. Hence this single construction can replace several constructions in most programming languages.
Languages lacking this construct generally emulate it using an equivalent infinite-loop-with-break idiom:
while (true)
A possible variant is to allow more than one while test; within the loop, but the use of exitwhen (see next section) appears to cover this case better.
In Ada
Ada may refer to:
Places
Africa
* Ada Foah, a town in Ghana
* Ada (Ghana parliament constituency)
* Ada, Osun, a town in Nigeria
Asia
* Ada, Urmia, a village in West Azerbaijan Province, Iran
* Ada, Karaman, a village in Karaman Province, ...
, the above loop construct (loop-while-repeat) can be represented using a standard infinite loop (loop - end loop) that has an exit when clause in the middle (not to be confused with the exitwhen statement in the following section).
with Ada.Text_IO;
with Ada.Integer_Text_IO;
procedure Print_Squares is
X : Integer;
begin
Read_Data : loop
Ada.Integer_Text_IO.Get(X);
exit Read_Data when X = 0;
Ada.Text IO.Put (X * X);
Ada.Text IO.New_Line;
end loop Read_Data;
end Print_Squares;
Naming a loop (like ''Read_Data'' in this example) is optional but permits leaving the outer loop of several nested loops.
Multiple early exit/exit from nested loops
This was proposed by Zahn in 1974. A modified version is presented here.
exitwhen EventA or EventB or EventC;
xxx
exits
EventA: actionA
EventB: actionB
EventC: actionC
endexit;
exitwhen is used to specify the events which may occur within ''xxx'',
their occurrence is indicated by using the name of the event as a statement. When some event does occur, the relevant action is carried out, and then control passes just after endexit. This construction provides a very clear separation between determining that some situation applies, and the action to be taken for that situation.
exitwhen is conceptually similar to exception handling
In computing and computer programming, exception handling is the process of responding to the occurrence of ''exceptions'' – anomalous or exceptional conditions requiring special processing – during the execution of a program. In general, an ...
, and exceptions or similar constructs are used for this purpose in many languages.
The following simple example involves searching a two-dimensional table for a particular item.
exitwhen found or missing;
for I := 1 to N do
for J := 1 to M do
if table ,J= target then found;
missing;
exits
found: print ("item is in table");
missing: print ("item is not in table");
endexit;
Security
One way to attack a piece of software is to redirect the flow of execution of a program. A variety of control-flow integrity techniques, including stack canaries, buffer overflow protection, shadow stacks, and vtable
In computer programming, a virtual method table (VMT), virtual function table, virtual call table, dispatch table, vtable, or vftable is a mechanism used in a programming language to support dynamic dispatch (or run-time method binding).
Wh ...
pointer verification, are used to defend against these attacks.
See also
* Branch (computer science)
A branch is an instruction in a computer program that can cause a computer to begin executing a different instruction sequence and thus deviate from its default behavior of executing instructions in order. ''Branch'' (or ''branching'', ''branc ...
* Control-flow analysis
* Control-flow diagram
* Control-flow graph
In computer science, a control-flow graph (CFG) is a representation, using graph notation, of all paths that might be traversed through a program during its execution. The control-flow graph was discovered by Frances E. Allen, who noted that ...
* Control table
Control tables are tables that control the control flow or play a major part in program control. There are no rigid rules about the structure or content of a control table—its qualifying attribute is its ability to direct control flow in some w ...
* Coroutine
Coroutines are computer program components that generalize subroutines for non-preemptive multitasking, by allowing execution to be suspended and resumed. Coroutines are well-suited for implementing familiar program components such as cooperative ...
* Cyclomatic complexity
Cyclomatic complexity is a software metric used to indicate the complexity of a program. It is a quantitative measure of the number of linearly independent paths through a program's source code. It was developed by Thomas J. McCabe, Sr. in 1976. ...
* Drakon-chart
* Flowchart
* GOTO
* Jeroo Jeroo is a cross-platform educational tool for learning object oriented programming concepts. In particular, the program helps learning concepts such as objects, methods and basic control structures. Jeroo supports three syntactic styles: Java/C#/ ...
, helps learn control structures
* Main loop In computer science, the event loop is a programming construct or design pattern that waits for and dispatches events or messages in a program. The event loop works by making a request to some internal or external "event provider" (that generally ...
* Recursion
Recursion (adjective: ''recursive'') occurs when a thing is defined in terms of itself or of its type. Recursion is used in a variety of disciplines ranging from linguistics to logic. The most common application of recursion is in mathemati ...
* Scheduling (computing)
In computing, scheduling is the action of assigning ''resources'' to perform ''tasks''. The ''resources'' may be processors, network links or expansion cards. The ''tasks'' may be threads, processes or data flows.
The scheduling activity is c ...
* Spaghetti code
Spaghetti code is a pejorative phrase for unstructured and difficult-to- maintain source code. Spaghetti code can be caused by several factors, such as volatile project requirements, lack of programming style rules, and software engineers with ins ...
* Structured programming
* Subroutine
* Switch statement, alters control flow conditionally
References
Further reading
* Hoare, C. A. R. "Partition: Algorithm 63," "Quicksort: Algorithm 64," and "Find: Algorithm 65." Comm. ACM 4, 321-322, 1961.
External links
*
Go To Statement Considered Harmful
*
* {{small, (31.4 MB)
Articles with example Ada code