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In computer programming languages, a switch statement is a type of selection control mechanism used to allow the value of a
variable Variable may refer to: * Variable (computer science), a symbolic name associated with a value and whose associated value may be changed * Variable (mathematics), a symbol that represents a quantity in a mathematical expression, as used in many ...
or expression to change the
control flow In computer science, control flow (or flow of control) is the order in which individual statements, instructions or function calls of an imperative program are executed or evaluated. The emphasis on explicit control flow distinguishes an ''imper ...
of program execution via search and map. Switch statements function somewhat similarly to the if statement used in programming languages like C/ C++, C#, Visual Basic .NET,
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 List ...
and exists in most high-level
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 c ...
languages such as
Pascal Pascal, Pascal's or PASCAL may refer to: People and fictional characters * Pascal (given name), including a list of people with the name * Pascal (surname), including a list of people and fictional characters with the name ** Blaise Pascal, Fren ...
,
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, Tur ...
, C/ C++, C#, Visual Basic .NET,
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 List ...
, and in many other types of language, using such
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 syste ...
s as switch, case, select or inspect. Switch statements come in two main variants: a structured switch, as in Pascal, which takes exactly one branch, and an unstructured switch, as in C, which functions as a type of
goto 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 ca ...
. The main reasons for using a switch include improving clarity, by reducing otherwise repetitive coding, and (if the
heuristic A heuristic (; ), or heuristic technique, is any approach to problem solving or self-discovery that employs a practical method that is not guaranteed to be optimal, perfect, or rational, but is nevertheless sufficient for reaching an immediate, ...
s permit) also offering the potential for faster execution through easier
compiler optimization In computing, an optimizing compiler is a compiler that tries to minimize or maximize some attributes of an executable computer program. Common requirements are to minimize a program's execution time, memory footprint, storage size, and power con ...
in many cases.


History

In his 1952 text ''Introduction to Metamathematics'', Stephen Kleene formally proved that the CASE function (the IF-THEN-ELSE function being its simplest form) is a primitive recursive function, where he defines the notion definition by cases in the following manner: :"#F. The function φ defined thus :: φ(x1 , ... , xn ) = :::*φ1(x1 , ... , xn ) if Q1(x1 , ... , xn ), :::* . . . . . . . . . . . . :::*φm(x1 , ... , xn ) if Qm(x1 , ... , xn ), :::*φm+1(x1 , ... , xn ) otherwise, :where Q1 , ... , Qm are mutually exclusive predicates (or φ(x1 , ... , xn) shall have the value given by the first clause which applies) is primitive recursive in φ1, ..., φm+1, Q1, ..., Qm+1. Kleene provides a proof of this in terms of the Boolean-like recursive functions "sign-of" sg( ) and "not sign of" ~sg( ) (Kleene 1952:222-223); the first returns 1 if its input is positive and −1 if its input is negative. Boolos-Burgess-Jeffrey make the additional observation that "definition by cases" must be both mutually exclusive and collectively exhaustive. They too offer a proof of the primitive recursiveness of this function (Boolos-Burgess-Jeffrey 2002:74-75). The IF-THEN-ELSE is the basis of the McCarthy formalism: its usage replaces both primitive recursion and the mu-operator.


Typical syntax

In most languages, programmers write a switch statement across many individual lines using one or two keywords. A typical syntax involves: * the first select, followed by an expression which is often referred to as the ''control expression'' or ''control variable'' of the switch statement * subsequent lines defining the actual cases (the values), with corresponding sequences of statements for execution when a match occurs * In languages with fallthrough behaviour, a break statement typically follows a case statement to end said statement.
ells Ells may refer to: * Ell, a measure of length * Ell (architecture) * Ells (surname), a surname * Ells Field, an airport in Mendocino County, California, United States * Ells River, in Alberta, Canada * Euroleague for Life Sciences See also * E ...
* In some languages, e.g., PL/I, the control expression is optional; if there is no control expression then each alternative begins with a WHEN clause containing a boolean expression and a match occurs for the first case for which that expression evaluates to true. This usage is similar to the if/then/elseif/else structures in some other languages, e.g.,
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 offici ...
. * In some languages, e.g.,
Rexx Rexx (Restructured Extended Executor) is a programming language that can be interpreted or compiled. It was developed at IBM by Mike Cowlishaw. It is a structured, high-level programming language designed for ease of learning and reading. ...
, no control expression is allowed and each alternative begins with a WHEN clause containing a boolean expression and a match occurs for the first case for which that expression evaluates to true. Each alternative begins with the particular value, or list of values (see below), that the control variable may match and which will cause the control to
goto 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 ca ...
the corresponding sequence of statements. The value (or list/range of values) is usually separated from the corresponding statement sequence by a colon or by an implication arrow. In many languages, every case must also be preceded by a keyword such as case or when. An optional default case is typically also allowed, specified by a default, otherwise, or else keyword. This executes when none of the other cases match the control expression. In some languages, such as C, if no case matches and the default is omitted the switch statement simply exits. In others, like PL/I, an error is raised.


Semantics

Semantically, there are two main forms of switch statements. The first form are structured switches, as in Pascal, where exactly one branch is taken, and the cases are treated as separate, exclusive blocks. This functions as a generalized if–then–else conditional, here with any number of branches, not just two. The second form are unstructured switches, as in C, where the cases are treated as labels within a single block, and the switch functions as a generalized goto. This distinction is referred to as the treatment of fallthrough, which is elaborated below.


Fallthrough

In many languages, only the matching block is executed, and then execution continues at the end of the switch statement. These include the
Pascal Pascal, Pascal's or PASCAL may refer to: People and fictional characters * Pascal (given name), including a list of people with the name * Pascal (surname), including a list of people and fictional characters with the name ** Blaise Pascal, Fren ...
family (Object Pascal, Modula, Oberon, Ada, etc.) as well as PL/I, modern forms of Fortran and
BASIC BASIC (Beginners' All-purpose Symbolic Instruction Code) is a family of general-purpose, high-level programming languages designed for ease of use. The original version was created by John G. Kemeny and Thomas E. Kurtz at Dartmouth College ...
dialects influenced by Pascal, most functional languages, and many others. To allow multiple values to execute the same code (and avoid needing to duplicate code), Pascal-type languages permit any number of values per case, given as a comma-separated list, as a range, or as a combination. Languages derived from C language, and more generally those influenced by Fortran's computed GOTO, instead feature fallthrough, where control moves to the matching case, and then execution continues ("falls through") to the statements associated with the ''next'' case in the source text. This also allows multiple values to match the same point without any special syntax: they are just listed with empty bodies. Values can be special conditioned with code in the case body. In practice, fallthrough is usually prevented with a break keyword at the end of the matching body, which exits execution of the switch block, but this can cause bugs due to unintentional fallthrough if the programmer forgets to insert the break statement. This is thus seen by many as a language wart, and warned against in some lint tools. Syntactically, the cases are interpreted as labels, not blocks, and the switch and break statements explicitly change control flow. Some languages influenced by C, such as
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 Website, websites use JavaScript on the Client (computing), client side ...
, retain default fallthrough, while others remove fallthrough, or only allow it in special circumstances. Notable variations on this in the C-family include C#, in which all blocks must be terminated with a break or return unless the block is empty (i.e. fallthrough is used as a way to specify multiple values). In some cases languages provide optional fallthrough. For example,
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 offici ...
does not fall through by default, but a case may explicitly do so using a continue keyword. This prevents unintentional fallthrough but allows it when desired. Similarly,
Bash Bash or BASH may refer to: Arts and entertainment * ''Bash!'' (Rockapella album), 1992 * ''Bash!'' (Dave Bailey album), 1961 * '' Bash: Latter-Day Plays'', a dramatic triptych * ''BASH!'' (role-playing game), a 2005 superhero game * "Bash" ('' ...
defaults to not falling through when terminated with ;;, but allow fallthrough with ;& or ;;& instead. An example of a switch statement that relies on fallthrough is Duff's device.


Compilation

Optimizing compilers In computing, an optimizing compiler is a compiler that tries to minimize or maximize some attributes of an executable computer program. Common requirements are to minimize a program's execution time, memory footprint, storage size, and power cons ...
such as GCC or Clang may compile a switch statement into either a
branch table In computer programming, a branch table or jump table is a method of transferring program control (Branch (computer science), branching) to another part of a program (or a different program that may have been dynamically loaded) using a table of b ...
or a binary search through the values in the cases. A branch table allows the switch statement to determine with a small, constant number of instructions which branch to execute without having to go through a list of comparisons, while a binary search takes only a logarithmic number of comparisons, measured in the number of cases in the switch statement. Normally, the only method of finding out if this optimization has occurred is by actually looking at the resultant
assembly Assembly may refer to: Organisations and meetings * Deliberative assembly, a gathering of members who use parliamentary procedure for making decisions * General assembly, an official meeting of the members of an organization or of their representa ...
or machine code output that has been generated by the compiler.


Advantages and disadvantages

In some languages and programming environments, the use of a case or switch statement is considered superior to an equivalent series of ''if else if'' statements because it is: * Easier to debug (e.g. setting breakpoints on code vs. a call table, if the debugger has no conditional breakpoint capability) * Easier for a person to read * Easier to understand, and consequently easier to maintain * Fixed depth: a sequence of "if else if" statements may yield deep nesting, making compilation more difficult (especially in automatically generated code) * Easier to verify that all values are handled. Compilers can issue a warning if some enum values are not handled. Additionally, an optimized implementation may execute much faster than the alternative, because it is often implemented by using an indexed
branch table In computer programming, a branch table or jump table is a method of transferring program control (Branch (computer science), branching) to another part of a program (or a different program that may have been dynamically loaded) using a table of b ...
. For example, deciding program flow based on a single character's value, if correctly implemented, is vastly more efficient than the alternative, reducing instruction path lengths considerably. When implemented as such, a switch statement essentially becomes a
perfect hash In computer science, a perfect hash function for a set is a hash function that maps distinct elements in to a set of integers, with no collisions. In mathematical terms, it is an injective function. Perfect hash functions may be used to impl ...
. In terms of the control-flow graph, a switch statement consists of two nodes (entrance and exit), plus one edge between them for each option. By contrast, a sequence of "if...else if...else if" statements has an additional node for every case other than the first and last, together with a corresponding edge. The resulting control-flow graph for the sequences of "if"s thus has many more nodes and almost twice as many edges, with these not adding any useful information. However, the simple branches in the if statements are individually conceptually easier than the complex branch of a switch statement. In terms of cyclomatic complexity, both of these options increase it by ''k''−1 if given ''k'' cases.


Switch expressions

''Switch expressions'' are introduced in Java SE 12, 19 March 2019, as a preview feature. Here a whole switch expression can be used to return a value. There is also a new form of case label, where the right-hand-side is a single expression. This also prevents fall though and requires that cases are exhaustive. In Java SE 13 the yield statement is introduced, and in Java SE 14 switch expressions becomes a standard language feature. For example: int ndays = switch(month) ;


Alternative uses

Many languages evaluate expressions inside switch blocks at runtime, allowing a number of less obvious uses for the construction. This prohibits certain compiler optimizations, so is more common in dynamic and scripting languages where the enhanced flexibility is more important than the performance overhead.


PHP

For example, in
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 ...
, a constant can be used as the "variable" to check against, and the first case statement which evaluates to that constant will be executed: switch (true) switch (5) This feature is also useful for checking multiple variables against one value rather than one variable against many values. COBOL also supports this form (and others forms) in the EVALUATE statement. PL/I has an alternative form of the SELECT statement where the control expression is omitted altogether and the first WHEN that evaluates to ''true'' is executed.


Ruby

In
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 sa ...
, due to its handling of

equality, the statement can be used to test for variable’s class: case input when Array then puts 'input is an Array!' when Hash then puts 'input is a Hash!' end Ruby also returns a value that can be assigned to a variable, and doesn’t actually require the case to have any parameters (acting a bit like an else if statement): catfood = case when cat.age <= 1 junior when cat.age > 10 senior else normal end


Assembler

A switch statement in
assembly language In computer programming, assembly language (or assembler language, or symbolic machine code), often referred to simply as Assembly and commonly abbreviated as ASM or asm, is any low-level programming language with a very strong correspondence be ...
: switch: cmp ah, 00h je a cmp ah, 01h je b jmp swtend ; No cases match or "default" code here a: push ah mov al, 'a' mov ah, 0Eh mov bh, 00h int 10h pop ah jmp swtend ; Equivalent to "break" b: push ah mov al, 'b' mov ah, 0Eh mov bh, 00h int 10h pop ah jmp swtend ; Equivalent to "break" ... swtend:


Python

For Python 3.10.6,
PEPs The ("Laval University Sports and Physical Education Complex"), usually called PEPS for short, is a sports complex located in Quebec City, Quebec, on the Université Laval campus. PEPS opened in 1970 and includes an outdoor stadium, an indoor st ...
634-636 were accepted, which added and keywords. Unlike other languages, Python notably doesn't exhibit fallthrough behavior. letter = input("Put in a single letter: ").strip() casefold() # first non-whitespace character of the input, lowercase match letter: case 'a' , 'e' , 'i' , 'o' , 'u': # Unlike conditions in if statements, the `or` keyword cannot be used here to differentiate between cases print(f"Letter is a vowel!") case 'y': print(f"Letter may be a vowel, just not in English.") case _: # `case _` is equivalent to `default` from C and others print(f"Letter is not a vowel!")


Exception handling

A number of languages implement a form of switch statement in exception handling, where if an exception is raised in a block, a separate branch is chosen, depending on the exception. In some cases a default branch, if no exception is raised, is also present. An early example is Modula-3, which use the TRY...EXCEPT syntax, where each EXCEPT defines a case. This is also found in
Delphi Delphi (; ), in legend previously called Pytho (Πυθώ), in ancient times was a sacred precinct that served as the seat of Pythia, the major oracle who was consulted about important decisions throughout the ancient classical world. The oracle ...
, Scala, and Visual Basic .NET.


Alternatives

Some alternatives to switch statements can be: * A series of ''if-else''
conditionals Conditional (if then) may refer to: *Causal conditional, if X then Y, where X is a cause of Y *Conditional probability, the probability of an event A given that another event B has occurred *Conditional proof, in logic: a proof that asserts a co ...
that examine the target one value at a time. Fallthrough behavior can be achieved with a sequence of ''if'' conditionals each without the ''else'' clause. * A
lookup table In computer science, a lookup table (LUT) is an array that replaces runtime computation with a simpler array indexing operation. The process is termed as "direct addressing" and LUTs differ from hash tables in a way that, to retrieve a value v wi ...
, which contains, as keys, the case values and, as values, the part under the case statement. ::(In some languages, only actual data types are allowed as values in the lookup table. In other languages, it is also possible to assign functions as lookup table values, gaining the same flexibility as a real switch statement. See
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 wa ...
article for more detail on this). :: Lua does not support case/switch statements. This lookup technique is one way to implement switch statements in the Lua language, which has no built-in switch.Switch statement in Lua
/ref> ::In some cases, lookup tables are more efficient than non- optimized switch statements since many languages can optimize table lookups, whereas switch statements are not optimized unless the range of values is small with few gaps. A non-optimized, non- binary search lookup, however, will almost certainly be slower than either a non-optimized switch or the equivalent multiple ''if-else'' statements. * A
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 wa ...
(that may be implemented as a simple lookup table) can also be customized to accommodate multiple conditions on multiple inputs if required and usually exhibits greater 'visual compactness' than an equivalent switch (that can occupy many statements). * Pattern matching, which is used to implement switch-like functionality in many
functional Functional may refer to: * Movements in architecture: ** Functionalism (architecture) ** Form follows function * Functional group, combination of atoms within molecules * Medical conditions without currently visible organic basis: ** Functional sy ...
languages.


See also

*
Algorithmic efficiency In computer science, algorithmic efficiency is a property of an algorithm which relates to the amount of computational resources used by the algorithm. An algorithm must be analyzed to determine its resource usage, and the efficiency of an algor ...
*
Branch table In computer programming, a branch table or jump table is a method of transferring program control (Branch (computer science), branching) to another part of a program (or a different program that may have been dynamically loaded) using a table of b ...
*
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 wa ...
* Duff's device *
Index mapping Index mapping (or direct addressing, or a trivial hash function) in computer science describes using an array, in which each position corresponds to a key in the universe of possible values. The technique is most effective when the universe of keys ...


References


Further reading

* Stephen Kleene, 1952 (10th reprint 1991), ''Introduction to Metamathematics'', North-Holland Publishing Company, Amsterdam NL, *
George Boolos George Stephen Boolos (; 4 September 1940 – 27 May 1996) was an American philosopher and a mathematical logician who taught at the Massachusetts Institute of Technology. Life Boolos is of Greek-Jewish descent. He graduated with an A.B. in ...
, John Burgess, and Richard Jeffrey, 2002, ''Computability and Logic: Fourth Edition'', Cambridge University Press, Cambridge UK, {{ISBN, 0-521-00758-5 paperback. cf page 74-75. Conditional constructs ru:Оператор ветвления#Переключатель (оператор множественного выбора)