Goto
Encyclopedia
goto is a statement
found in many computer programming language
s. It is a combination of the English words go and to. It performs a one-way transfer of control to another line of code; in contrast a function call
normally returns control. The jumped-to locations are usually identified using labels
, though some languages use line numbers
. At the machine code
level, a goto is a form of branch or jump statement
.
Many languages support the goto statement, and many do not. In Java
, goto is a reserved word
, but is unusable.. In PHP
there was no native support for goto until version 5.3 (libraries were available to emulate its functionality).
The structured program theorem
proved that the goto statement is not necessary to write programs; some combination of the three programming constructs of sequence, selection/choice, and repetition/iteration are sufficient for any computation that can be performed by a Turing machine
.
In the past there was considerable debate in academia and industry on the merits of the use of goto statements.
The goto statement is often combined with the if statement to cause a conditional transfer of control.
IF condition THEN goto label
Programming languages impose different restrictions with respect to the destination of a
" paradigm
. Some programming style
coding standards prohibit use of GOTO statements, particularly in view of the aforementioned structured program theorem. The Böhm-Jacopini proof
did not settle the question of whether to adopt structured programming for software development, partly because the construction was more likely to obscure a program than to improve it. It has, however, sparked a prominent debate among computer scientists, educators, language designers and application programmers that saw a slow but steady shift away from the formerly ubiquitous use of the GOTO. Probably the most famous criticism of GOTO is a 1968 letter by Edsger Dijkstra
called Go To Statement Considered Harmful. In that letter Dijkstra argued that unrestricted GOTO statements should be abolished from higher-level languages because they complicated the task of analyzing and verifying the correctness of programs (particularly those involving loops). An alternative viewpoint is presented in Donald Knuth
's Structured Programming with go to Statements which analyzes many common programming tasks and finds that in some of them GOTO is the optimal language construct
to use. Some programmers, such as Linux
Kernel designer and coder Linus Torvalds
or software engineer and book author Steve McConnell
also object to Dijkstra's point of view, stating that GOTOs can be a useful language feature, improving program speed, size and code clearness, but only when used in a sensible way by a comparably sensible programmer.
The viewpoint that use of GOTO is sometimes undesirable is evident in the design of some programming languages, for instance Ada
visually emphasizes label definitions using angle brackets.
Fortran
introduced structured programming constructs in 1978 and in successive revisions the relatively loose semantic rules governing the allowable use of goto were tightened; the "extended range" in which a programmer could use a GOTO to enter and leave a still-executing DO loop was removed from the language in 1978, and by 1995 several of forms of Fortran GOTO, including the Computed GOTO and the Assigned GOTO, had been deleted from the language. Some widely used modern programming languages, such as Java
and Python
lack the GOTO statement, though most provide some means of breaking out of a selection, or either breaking out of or moving on to the next step of an iteration.
, three basic structures are used: sequence, repetition, and selection. Sequence is the execution of one program unit following another; repetition is the repeated execution of the same program unit until a desired program state is reached; and selection is the executing of one and only one of a fixed set of possible alternative program units depending on the program state. Here typically the "program unit" is a compound statement constructed in turn using the three basic structures.
Although these three control structures can all be hand-coded using
,
, and
statements, and
statements (multiway branch
ing - a form of multiple goto that is apparently more accepted than a single variety).
In practice the need arose to refine this basic three-structure template, because the attempt to eliminate exceptional changes of control altogether generated a need for quite complex program state data, and language design had to adapt in order to reduce the combinatorial explosion
. Typically the language designers provided a way to exit a structured unit prematurely (in C,
constructs were added, such as
, tail call optimization is mandatory.
Although Steele's paper did not introduce much that was new to computer science, at least as it was practised at MIT, it brought to light the scope for procedure call optimization, which made the modularity-promoting qualities of procedures into a more credible alternative to the then-common coding habits of large monolithic procedures with complex internal control structures and extensive state data. In particular, the tail call optimizations discussed by Steele turned the procedure into a credible way of implementing iteration through tail recursion.
and Smalltalk
were among the first to introduce the concepts of messages and objects. By encapsulating
state data, object-oriented programming
reduced software complexity to interactions (messages) between objects.
terminology) jumps to one of several labels based on the value of an expression. The
supports the computed GOTO and is useful for case-by-case branching, as in C's switch statement
.
Some variants of BASIC support a computed GOTO that can be any line number, not just one from a list. For example, one could write
Some C compilers (e.g., gcc
) support
standard had the ALTER verb which could be used to change the destination of an existing GO TO, which had to be in a paragraph by itself. The feature, which allowed polymorphism, was frequently condemned and seldom used.
is similar to a GOTO in that it transfers control from an arbitrary point in the program to a previously marked point. A continuation is more flexible than GOTO in those languages that support it, because it can transfer control out of the current function, something that a GOTO cannot do in most structured programming languages. In those language implementations that maintain stack frames for storage of local variables and function arguments, executing a continuation involves adjusting the program's call stack
in addition to a jump. The longjmp function of the C programming language
is an example of an escape continuation that may be used to escape the current context to a surrounding one. The Common Lisp
GO operator also has this stack unwinding property, despite the construct being lexically scoped, as the label to be jumped to can be referenced from a closure
.
In Scheme, continuations can even move control from an outer context to an inner one if desired. This almost limitless control over what code is executed next makes complex control structures such as coroutines and cooperative multitasking relatively easy to write.
, there is a variant of the
): the new function will not return to the GOTO, but instead to the place from which the original function was called.
Statement (programming)
In computer programming a statement can be thought of as the smallest standalone element of an imperative programming language. A program written in such a language is formed by a sequence of one or more statements. A statement will have internal components .Many languages In computer programming...
found in many computer programming language
Programming language
A programming language is an artificial language designed to communicate instructions to a machine, particularly a computer. Programming languages can be used to create programs that control the behavior of a machine and/or to express algorithms precisely....
s. It is a combination of the English words go and to. It performs a one-way transfer of control to another line of code; in contrast a function call
Subroutine
In computer science, a subroutine is a portion of code within a larger program that performs a specific task and is relatively independent of the remaining code....
normally returns control. The jumped-to locations are usually identified using labels
Label (programming language)
A label in a programming language is a sequence of characters that identifies a location within source code. In most languages labels take the form of an identifier, often followed by a punctuation character . In many high level programming languages the purpose of a label is to act as the...
, though some languages use line numbers
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 incrementing by 1 for each successive line.In the C...
. At the machine code
Machine code
Machine code or machine language is a system of impartible instructions executed directly by a computer's central processing unit. Each instruction performs a very specific task, typically either an operation on a unit of data Machine code or machine language is a system of impartible instructions...
level, a goto is a form of branch or jump statement
Branch (computer science)
A branch is sequence of code in a computer program which is conditionally executed depending on whether the flow of control is altered or not . The term can be used when referring to programs in high level languages as well as program written in machine code or assembly language...
.
Many languages support the goto statement, and many do not. In Java
Java (programming language)
Java is a programming language originally developed by James Gosling at Sun Microsystems and released in 1995 as a core component of Sun Microsystems' Java platform. The language derives much of its syntax from C and C++ but has a simpler object model and fewer low-level facilities...
, goto is a reserved word
Reserved word
Reserved words are one type of grammatical construct in programming languages. These words have special meaning within the language and are predefined in the language’s formal specifications...
, but is unusable.. In PHP
PHP
PHP is a general-purpose server-side scripting language originally designed for web development to produce dynamic web pages. For this purpose, PHP code is embedded into the HTML source document and interpreted by a web server with a PHP processor module, which generates the web page document...
there was no native support for goto until version 5.3 (libraries were available to emulate its functionality).
The structured program theorem
Structured program theorem
The structured program theorem is a result in programming language theory. It states that every computable function can be implemented in a programming language that combines subprograms in only three specific ways...
proved that the goto statement is not necessary to write programs; some combination of the three programming constructs of sequence, selection/choice, and repetition/iteration are sufficient for any computation that can be performed by a Turing machine
Turing machine
A Turing machine is a theoretical device that manipulates symbols on a strip of tape according to a table of rules. Despite its simplicity, a Turing machine can be adapted to simulate the logic of any computer algorithm, and is particularly useful in explaining the functions of a CPU inside a...
.
In the past there was considerable debate in academia and industry on the merits of the use of goto statements.
Usage
goto labelThe goto statement is often combined with the if statement to cause a conditional transfer of control.
IF condition THEN goto label
Programming languages impose different restrictions with respect to the destination of a
goto statement. For example, the C programming language does not permit a jump to a label contained within another function. The setjmp/longjmp functions provide support for non-local gotos.Criticism and decline
The 1960s and 1970s saw computer scientists move away from GOTO statements in favor of the "structured programmingStructured programming
Structured programming is a programming paradigm aimed on improving the clarity, quality, and development time of a computer program by making extensive use of subroutines, block structures and for and while loops - in contrast to using simple tests and jumps such as the goto statement which could...
" paradigm
Programming paradigm
A programming paradigm is a fundamental style of computer programming. Paradigms differ in the concepts and abstractions used to represent the elements of a program and the steps that compose a computation A programming paradigm is a fundamental style of computer programming. (Compare with a...
. Some programming style
Programming style
Programming style is a set of rules or guidelines used when writing the source code for a computer program. It is often claimed that following a particular programming style will help programmers to read and understand source code conforming to the style, and help to avoid introducing errors.A...
coding standards prohibit use of GOTO statements, particularly in view of the aforementioned structured program theorem. The Böhm-Jacopini proof
Structured program theorem
The structured program theorem is a result in programming language theory. It states that every computable function can be implemented in a programming language that combines subprograms in only three specific ways...
did not settle the question of whether to adopt structured programming for software development, partly because the construction was more likely to obscure a program than to improve it. It has, however, sparked a prominent debate among computer scientists, educators, language designers and application programmers that saw a slow but steady shift away from the formerly ubiquitous use of the GOTO. Probably the most famous criticism of GOTO is a 1968 letter by Edsger Dijkstra
Edsger Dijkstra
Edsger Wybe Dijkstra ; ) was a Dutch computer scientist. He received the 1972 Turing Award for fundamental contributions to developing programming languages, and was the Schlumberger Centennial Chair of Computer Sciences at The University of Texas at Austin from 1984 until 2000.Shortly before his...
called Go To Statement Considered Harmful. In that letter Dijkstra argued that unrestricted GOTO statements should be abolished from higher-level languages because they complicated the task of analyzing and verifying the correctness of programs (particularly those involving loops). An alternative viewpoint is presented in Donald Knuth
Donald Knuth
Donald Ervin Knuth is a computer scientist and Professor Emeritus at Stanford University.He is the author of the seminal multi-volume work The Art of Computer Programming. Knuth has been called the "father" of the analysis of algorithms...
's Structured Programming with go to Statements which analyzes many common programming tasks and finds that in some of them GOTO is the optimal language construct
Language construct
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 a programming language....
to use. Some programmers, such as Linux
Linux
Linux is a Unix-like computer operating system assembled under the model of free and open source software development and distribution. The defining component of any Linux system is the Linux kernel, an operating system kernel first released October 5, 1991 by Linus Torvalds...
Kernel designer and coder Linus Torvalds
Linus Torvalds
Linus Benedict Torvalds is a Finnish software engineer and hacker, best known for having initiated the development of the open source Linux kernel. He later became the chief architect of the Linux kernel, and now acts as the project's coordinator...
or software engineer and book author Steve McConnell
Steve McConnell
Steven C. McConnell is an author of many software engineering textbooks including Code Complete, Rapid Development, and Software Estimation...
also object to Dijkstra's point of view, stating that GOTOs can be a useful language feature, improving program speed, size and code clearness, but only when used in a sensible way by a comparably sensible programmer.
The viewpoint that use of GOTO is sometimes undesirable is evident in the design of some programming languages, for instance Ada
Ada (programming language)
Ada is a structured, statically typed, imperative, wide-spectrum, and object-oriented high-level computer programming language, extended from Pascal and other languages...
visually emphasizes label definitions using angle brackets.
Fortran
Fortran
Fortran is a general-purpose, procedural, imperative programming language that is especially suited to numeric computation and scientific computing...
introduced structured programming constructs in 1978 and in successive revisions the relatively loose semantic rules governing the allowable use of goto were tightened; the "extended range" in which a programmer could use a GOTO to enter and leave a still-executing DO loop was removed from the language in 1978, and by 1995 several of forms of Fortran GOTO, including the Computed GOTO and the Assigned GOTO, had been deleted from the language. Some widely used modern programming languages, such as Java
Java (programming language)
Java is a programming language originally developed by James Gosling at Sun Microsystems and released in 1995 as a core component of Sun Microsystems' Java platform. The language derives much of its syntax from C and C++ but has a simpler object model and fewer low-level facilities...
and Python
Python (programming language)
Python is a general-purpose, high-level programming language whose design philosophy emphasizes code readability. Python claims to "[combine] remarkable power with very clear syntax", and its standard library is large and comprehensive...
lack the GOTO statement, though most provide some means of breaking out of a selection, or either breaking out of or moving on to the next step of an iteration.
Structured programming
In classical structured programmingStructured programming
Structured programming is a programming paradigm aimed on improving the clarity, quality, and development time of a computer program by making extensive use of subroutines, block structures and for and while loops - in contrast to using simple tests and jumps such as the goto statement which could...
, three basic structures are used: sequence, repetition, and selection. Sequence is the execution of one program unit following another; repetition is the repeated execution of the same program unit until a desired program state is reached; and selection is the executing of one and only one of a fixed set of possible alternative program units depending on the program state. Here typically the "program unit" is a compound statement constructed in turn using the three basic structures.
Although these three control structures can all be hand-coded using
gotos and ifs, the desire for clarity and efficient optimization led to the introduction and refinement of supporting structures such as (in Pascal) procedure and function blocks, whileWhile loop
In most computer programming languages, a while loop is a control flow statement that allows code to be executed repeatedly based on a given boolean condition. The while loop can be thought of as a repeating if statement....
,
repeat untilDo while loop
In most computer programming languages, a do while loop, sometimes just called a do loop, is a control flow statement that allows code to be executed repeatedly based on a given Boolean condition. Note though that unlike most languages, Fortran's do loop is actually analogous to the for loop.The...
, and
forFor loop
In computer science a for loop is a programming language statement which allows code to be repeatedly executed. A for loop is classified as an iteration statement....
statements, and
caseSwitch statement
In computer programming, a switch, case, select or inspect statement is a type of selection control mechanism that exists in most imperative programming languages such as Pascal, Ada, C/C++, C#, Java, and so on. It is also included in several other types of languages...
statements (multiway branch
Multiway branch
Multiway branch is a computer science term used to describe the change to a program's control flow based upon a value matching a selected criteria. It is a form of conditional statement...
ing - a form of multiple goto that is apparently more accepted than a single variety).
In practice the need arose to refine this basic three-structure template, because the attempt to eliminate exceptional changes of control altogether generated a need for quite complex program state data, and language design had to adapt in order to reduce the combinatorial explosion
Combinatorial explosion
In administration and computing, a combinatorial explosion is the rapidly accelerating increase in lines of communication as organizations are added in a process...
. Typically the language designers provided a way to exit a structured unit prematurely (in C,
break and continue). For handling exceptional situations, specialized exception handlingException handling
Exception handling is a programming language construct or computer hardware mechanism designed to handle the occurrence of exceptions, special conditions that change the normal flow of program execution....
constructs were added, such as
try/catch/finally in Java.Tail calls
In a paper delivered to the ACM conference in Seattle in 1977, Guy L. Steele summarized the debate over the GOTO and structured programming, and observed that procedure calls in the tail position of a procedure can be most optimally treated as a direct transfer of control to the called procedure, typically eliminating unnecessary stack manipulation operations. Since such "tail calls" are very common in Lisp, a language where procedure calls are ubiquitous, this form of optimization considerably reduces the cost of a procedure call compared to the GOTO used in other languages. Steele argued that poorly implemented procedure calls had led to an artificial perception that the GOTO was cheap compared to the procedure call. Steele further argued that "in general procedure calls may be usefully thought of as GOTO statements which also pass parameters, and can be uniformly coded as [machine code] JUMP instructions", with the machine code stack manipulation instructions "considered an optimization (rather than vice versa!)". Steele cited evidence that well optimized numerical algorithms in Lisp could execute faster than code produced by then-available commercial Fortran compilers because the cost of a procedure call in Lisp was much lower. In Scheme, a Lisp dialect developed by Steele with Gerald Jay SussmanGerald Jay Sussman
Gerald Jay Sussman is the Panasonic Professor of Electrical Engineering at the Massachusetts Institute of Technology . He received his S.B. and Ph.D. degrees in mathematics from MIT in 1968 and 1973 respectively. He has been involved in artificial intelligence research at MIT since 1964...
, tail call optimization is mandatory.
Although Steele's paper did not introduce much that was new to computer science, at least as it was practised at MIT, it brought to light the scope for procedure call optimization, which made the modularity-promoting qualities of procedures into a more credible alternative to the then-common coding habits of large monolithic procedures with complex internal control structures and extensive state data. In particular, the tail call optimizations discussed by Steele turned the procedure into a credible way of implementing iteration through tail recursion.
Object oriented programming
The influential languages SimulaSimula
Simula is a name for two programming languages, Simula I and Simula 67, developed in the 1960s at the Norwegian Computing Center in Oslo, by Ole-Johan Dahl and Kristen Nygaard...
and Smalltalk
Smalltalk
Smalltalk is an object-oriented, dynamically typed, reflective programming language. Smalltalk was created as the language to underpin the "new world" of computing exemplified by "human–computer symbiosis." It was designed and created in part for educational use, more so for constructionist...
were among the first to introduce the concepts of messages and objects. By encapsulating
Encapsulation (object-oriented programming)
In a programming language encapsulation is used to refer to one of two related but distinct notions, and sometimes to the combination thereof:* A language mechanism for restricting access to some of the object's components....
state data, object-oriented programming
Object-oriented programming
Object-oriented programming is a programming paradigm using "objects" – data structures consisting of data fields and methods together with their interactions – to design applications and computer programs. Programming techniques may include features such as data abstraction,...
reduced software complexity to interactions (messages) between objects.
Variations
There are a number of different language constructs under the class of goto statements.Computed GOTO
A computedGOTO (originally FortranFortran
Fortran is a general-purpose, procedural, imperative programming language that is especially suited to numeric computation and scientific computing...
terminology) jumps to one of several labels based on the value of an expression. The
ON ... GOTO statement in BASICBASIC
BASIC is a family of general-purpose, high-level programming languages whose design philosophy emphasizes ease of use - the name is an acronym from Beginner's All-purpose Symbolic Instruction Code....
supports the computed GOTO and is useful for case-by-case branching, as in C's switch statement
Switch statement
In computer programming, a switch, case, select or inspect statement is a type of selection control mechanism that exists in most imperative programming languages such as Pascal, Ada, C/C++, C#, Java, and so on. It is also included in several other types of languages...
.
Some variants of BASIC support a computed GOTO that can be any line number, not just one from a list. For example, one could write
GOTO i*1000 to jump to the line numbered 1000 times the value of a variable i (which might represent a selected menu option, for example).Assigned GOTO
Originally found in Fortran, this variant transfers control to a statement label which is stored in (assigned to) a variable.Some C compilers (e.g., gcc
GNU Compiler Collection
The GNU Compiler Collection is a compiler system produced by the GNU Project supporting various programming languages. GCC is a key component of the GNU toolchain...
) support
goto with a label variable using the label value operator. The label value operator && returns the address of its operand, which must be a label defined in the current function or a containing function. The value is a constant of type void * and should be used only in a computed goto statement. The feature is an extension to C and C++, implemented to facilitate porting programs developed with GNU C. This C extension is sometimes referred to as a Computed goto, but it has the semantics of an assigned goto.ALTER
The 1985 ANSI COBOLCOBOL
COBOL is one of the oldest programming languages. Its name is an acronym for COmmon Business-Oriented Language, defining its primary domain in business, finance, and administrative systems for companies and governments....
standard had the ALTER verb which could be used to change the destination of an existing GO TO, which had to be in a paragraph by itself. The feature, which allowed polymorphism, was frequently condemned and seldom used.
Continuations
A continuationContinuation
In computer science and programming, a continuation is an abstract representation of the control state of a computer program. A continuation reifies the program control state, i.e...
is similar to a GOTO in that it transfers control from an arbitrary point in the program to a previously marked point. A continuation is more flexible than GOTO in those languages that support it, because it can transfer control out of the current function, something that a GOTO cannot do in most structured programming languages. In those language implementations that maintain stack frames for storage of local variables and function arguments, executing a continuation involves adjusting the program's call stack
Call stack
In computer science, a call stack is a stack data structure that stores information about the active subroutines of a computer program. This kind of stack is also known as an execution stack, control stack, run-time stack, or machine stack, and is often shortened to just "the stack"...
in addition to a jump. The longjmp function of the C programming language
C (programming language)
C is a general-purpose computer programming language developed between 1969 and 1973 by Dennis Ritchie at the Bell Telephone Laboratories for use with the Unix operating system....
is an example of an escape continuation that may be used to escape the current context to a surrounding one. The Common Lisp
Common Lisp
Common Lisp, commonly abbreviated CL, is a dialect of the Lisp programming language, published in ANSI standard document ANSI INCITS 226-1994 , . From the ANSI Common Lisp standard the Common Lisp HyperSpec has been derived for use with web browsers...
GO operator also has this stack unwinding property, despite the construct being lexically scoped, as the label to be jumped to can be referenced from a closure
Closure (computer science)
In computer science, a closure is a function together with a referencing environment for the non-local variables of that function. A closure allows a function to access variables outside its typical scope. Such a function is said to be "closed over" its free variables...
.
In Scheme, continuations can even move control from an outer context to an inner one if desired. This almost limitless control over what code is executed next makes complex control structures such as coroutines and cooperative multitasking relatively easy to write.
Perl GOTO
In PerlPerl
Perl is a high-level, general-purpose, interpreted, dynamic programming language. Perl was originally developed by Larry Wall in 1987 as a general-purpose Unix scripting language to make report processing easier. Since then, it has undergone many changes and revisions and become widely popular...
, there is a variant of the
goto statement that is not a traditional GOTO statement at all. It takes a function name and transfers control by effectively substituting one function call for another (a tail callTail call
In computer science, a tail call is a subroutine call that happens inside another procedure and that produces a return value, which is then immediately returned by the calling procedure. The call site is then said to be in tail position, i.e. at the end of the calling procedure. If a subroutine...
): the new function will not return to the GOTO, but instead to the place from which the original function was called.
See also
- COMEFROMCOMEFROMIn computer programming, COMEFROM is an obscure control flow structure used in some programming languages, originally as a joke....
- Control flow
- GOSUBGOSUBGOSUB is a command in many versions of the BASIC computer programming language. A GOSUB statement jumps to a line elsewhere in the program. That line and the following lines up to a RETURN are used as a simple kind of a subroutine without parameters or local variables.The GOSUB command may be used...
- Switch statementSwitch statementIn computer programming, a switch, case, select or inspect statement is a type of selection control mechanism that exists in most imperative programming languages such as Pascal, Ada, C/C++, C#, Java, and so on. It is also included in several other types of languages...
- a multiway branchMultiway branchMultiway branch is a computer science term used to describe the change to a program's control flow based upon a value matching a selected criteria. It is a form of conditional statement...
(or conditional goto) - Unstructured programming
- Considered harmfulConsidered harmfulIn computer science and related disciplines, considered harmful is a phrase popularly used in the titles of diatribes and other critical essays ....