GOTO

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This page is about the programming language command. See Goto for other uses.

In computer science GOTO is a statement found in many computer programming languages. When executed it causes an unconditional transfer of control to another statement. The jumped-to statement is specified using some kind of label, which may be an identifier or a line number depending on the language. At the machine code level a goto is a form of branch or jump statement.

In some languages, goto functionality may be present without explicit use of the keyword goto, such as where a break or continue keyword may be followed by with an identifier denoting a label. The SNOBOL programming language supports a form of statement suffix which causes an unconditional transfer of control after the statement has finished executing.

GOTO statements are found in nearly every high-level language. The few high-level languages that do not support a goto statement include, Java, where goto is a reserved word but does not presently serve any function. Hypertalk, TranScript (or Revolution), AppleScript, and Ruby do not have a goto.

1 Criticism of goto usage
2 Variations
3 See also
4 References
5 External links

[edit] Criticism of goto usage

The GOTO statement has been the target of much criticism, the claim being that use of GOTO produces unreadable and generally unmaintainable "spaghetti code" (whether the alternatives would be just as unreadable and generally unmaintainable is not usually discussed). As structured programming became more popular in the 1960s and 1970s, many computer scientists came to the conclusion that programs should always use so-called 'structured' flow-control commands such as loops and if-then-else statements in place of GOTO. However, others believed that use of GOTO did not unconditionally lead to poor quality code and that there are some tasks that cannot be straightforwardly accomplished in many programming languages without the use of one or more GOTO statements, such as breaking out of nested loops and exception handling.

Probably the most famous criticism of GOTO is a 1968 letter by Edsger Dijkstra called Go To Statement Considered Harmful ^[1]. 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 ^[2] which analyzes many common programming tasks and finds that in some of them GOTO is the most optimal language construct to use.

The Ada language design took note of the above criticisms, but the statement was included in the language despite the criticisms, mainly to support automatically generated code where the goto might prove indispensable. However, the labels used as the destination of a goto statement take the unusual form of an identifier enclosed in double angle brackets (e.g. <<Start_Again>>) and this syntax is not used anywhere else in the language. This makes it easy to check a program for the existence of goto destinations. The goto statement itself takes the simple form goto Start_Again;.

[edit] Variations

There are a number of different language constructs whose semantics could be said to have a goto component.

The switch statement (known as a case statement in some languages) performs a multi-way goto where the destination is selected by the value of an expression.

A computed GOTO (originally Fortran terminology) either jumps to one of several labels based on the value of an expression, or jumps to a label that has been stored in a variable. The ON GOTO statement in BASIC supports the first kind of computed GOTO and is useful for case-by-case branching, as in C's switch statement. Some C compilers (e.g., gcc) 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.[1]

A continuation is similar to a computed GOTO in that it transfers control from an arbitrary point in the program to a previously marked point. A continuation can be more flexible than GOTO in some languages because it can leave the current function, something that a GOTO cannot do in most languages. Executing a continuation usually involves some adjustment of 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 the esoteric programming language INTERCAL, which is a parody of languages like BASIC, COME FROM is used instead of GOTO.

In Perl, 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 call): the new function will not return to the GOTO, but instead to the place from which the original function was called. Early versions of COBOL had the ALTER verb to accomplish this.