Label (computer Science)
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In
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 ...
s, a label is a sequence of characters that identifies a location within
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 wo ...
. In most languages, labels take the form of an
identifier An identifier is a name that identifies (that is, labels the identity of) either a unique object or a unique ''class'' of objects, where the "object" or class may be an idea, physical countable object (or class thereof), or physical noncountable ...
, often followed by a punctuation character (e.g., a colon). In many high-level languages, the purpose of a label is to act as the destination of a
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 ...
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 ...
, labels can be used anywhere an
address An address is a collection of information, presented in a mostly fixed format, used to give the location of a building, apartment, or other structure or a plot of land, generally using political boundaries and street names as references, along ...
can (for example, as the operand of a JMP or MOV instruction). Also in Pascal and its derived variations. Some languages, such as 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 ...
, support numeric labels. Labels are also used to identify an entry point into a
compiled In computing, a compiler is a computer program that translates computer code written in one programming language (the ''source'' language) into another language (the ''target'' language). The name "compiler" is primarily used for programs that ...
sequence of statements (e.g., during
debugging In computer programming and software development, debugging is the process of finding and resolving '' bugs'' (defects or problems that prevent correct operation) within computer programs, software, or systems. Debugging tactics can involve in ...
).


C

In C a label identifies a statement in the code. A single statement can have multiple labels. Labels just indicate locations in the code and reaching a label has no effect on the actual execution.


Function labels

Function labels consist of an identifier, followed by a colon. Each such label points to a statement in a function and its identifier must be unique within that function. Other functions may use the same name for a label. Label identifiers occupy their own namespace – one can have variables and functions with the same name as a label. void foo(int number) Here ''error'' is the label. The statement
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 ...
can be used to jump to a labeled statement in the code. After a goto, program execution continues with the statement after the label.


Switch labels

Two types of labels can be put in a switch statement. A case label consists of the keyword case, followed by an expression that evaluates to integer constant. A default label consists of the keyword default. Case labels are used to associate an integer value with a statement in the code. When a switch statement is reached, program execution continues with the statement after the case label with value that matches the value in the parentheses of the switch. If there is no such case label, but there is a default label, program execution continues with the statement after the default label. If there is no default label, program execution continues after the switch. switch (die) Within a single
switch statement In computer programming languages, a switch statement is a type of selection control mechanism used to allow the value of a variable or expression to change the control flow of program execution via search and map. Switch statements function some ...
, the integer constant associated with each case label must be unique. There may or may not be a default statement. There is no restriction on the order of the labels within a switch. The requirement that case labels values evaluate to integer constants gives the compiler more room for optimizations.


Examples


Javascript

In JavaScript language
syntax In linguistics, syntax () is the study of how words and morphemes combine to form larger units such as phrases and sentences. Central concerns of syntax include word order, grammatical relations, hierarchical sentence structure ( constituency) ...
statements may be preceded by the label: top: //Label the outermost for-loop. for (var i = 0; i < 4; i++) alert("i=" + i + ", j=" + j); //i=2, j=3 It also possible to use break statement to break out of the code blocks: top: // Which would output: // > foo // > bar


Common Lisp

In
Common Lisp Common Lisp (CL) is a dialect of the Lisp programming language, published in ANSI standard document ''ANSI INCITS 226-1994 (S20018)'' (formerly ''X3.226-1994 (R1999)''). The Common Lisp HyperSpec, a hyperlinked HTML version, has been derived fro ...
two ways of defining labels exist. The first one involves the tagbody special operator. Distinguishing its usage from many other programming languages that permit global navigation, such as C, the labels are only accessible in the context of this operator. Inside of a tagbody labels are defined as forms starting with a symbol; the go special form permits a transfer of control between these labels. (let ((iteration NIL)) (tagbody start (print 'started) (setf iteration 0) increase (print iteration) (incf iteration 1) (go check) check (if (>= iteration 10) (go end) (go increase)) end (print 'done))) A second method utilizes the reader macros #''n''= and #''n''#, the former of which labels the object immediately following it, the latter refers to its evaluated value. Labels in this sense constitute rather an alternative to variables, with #''n''= declaring and initializing a “variable” and #''n''# accessing it. The placeholder ''n'' designates a chosen unsigned decimal integer identifying the label. (progn #1="hello" (print #1#)) Apart from that, some forms permit or mandate the declaration of a label for later referral, including the special form block which prescribes a naming, and the loop macro that can be identified by a named clause. Immediate departure from a named form is possible by using the return-from special operator. (block myblock (loop for iteration from 0 do (if (>= iteration 10) (return-from myblock 'done) (print iteration)))) (loop named myloop for iteration from 0 do (if (>= iteration 10) (return-from myloop 'done) (print iteration))) In a fashion similar to C, the macros case, ccase, ecase, typecase, ctypecase and etypecase define switch statements. (let ((my-value 5)) (case my-value (1 (print "one")) (2 (print "two")) ((3 4 5) (print "three four or five")) (otherwise (print "any other value")))) (let ((my-value 5)) (typecase my-value (list (print "a list")) (string (print "a string")) (number (print "a number")) (otherwise (print "any other type"))))


See also

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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 ...
*
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 ...
*
Switch statement In computer programming languages, a switch statement is a type of selection control mechanism used to allow the value of a variable or expression to change the control flow of program execution via search and map. Switch statements function some ...


References

{{reflist Source code Control flow Programming language concepts