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 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 (see language support).Programming style, like writing style, is somewhat of an art and cannot be codified by inflexible rules, although discussions about style often seem to center exclusively around such rules. In the case of the goto statement, it has long been observed that unfettered use of goto's quickly leads to unmaintainable spaghetti code. However, a simple, unthinking ban on the goto statement does not necessarily lead immediately to beautiful programming: an unstructured programmer is just as capable of constructing a Byzantine tangle without using any goto's (perhaps substituting oddly-nested loops and Boolean control variables, instead). Many programmers adopt a moderate stance: goto's are usually to be avoided, but are acceptable in a few well-constrained situations, if necessary: as multi-level break statements, to coalesce common actions inside a switch statement, or to centralize cleanup tasks in a function with several error returns. (...) Blindly avoiding certain constructs or following rules without understanding them can lead to just as many problems as the rules were supposed to avert. Furthermore, many opinions on programming style are just that: opinions. They may be strongly argued and strongly felt, they may be backed up by solid-seeming evidence and arguments, but the opposing opinions may be just as strongly felt, supported, and argued. It's usually futile to get dragged into 'style wars', because on certain issues, opponents can never seem to agree, or agree to disagree, or stop arguing. 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 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 (see language support). 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, with the caveat that code duplication and additional variables may need to be introduced. In the past there was considerable debate in academia and industry on the merits of the use of goto statements. Use of goto was formerly common, but since the advent of structured programming in the 1960s and 1970s its use has declined significantly. The primary criticism is that code that uses goto statements is harder to understand than alternative constructions. Goto remains in use in certain common usage patterns, but alternatives are generally used if available. Debates over its (more limited) uses continue in academia and software industry circles. goto label 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 goto statement. For example, the C programming language does not permit a jump to a label contained within another function, however jumps within a single call chain are possible using the setjmp/longjmp functions. At the pre-ALGOL meeting held in 1959 Heinz Zemanek explicitly threw doubt on the necessity for GOTO statements; at the time no one paid attention to his remark, including Edsger W. Dijkstra, who later became the iconic opponent of GOTO. The 1970s and 1980s saw a decline in the use of GOTO statements in favor of the 'structured programming' paradigm, with goto criticized as leading to 'unmaintainable spaghetti code' (see below). Some programming style coding standards, for example the GNU Pascal Coding Standards, recommend against the use of GOTO statements. The Böhm-Jacopini proof (1966) 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 because its application requires the introduction of additional local variables. It did, however, spark 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). The letter itself sparked a debate, including a ''GOTO Considered Harmful' Considered Harmful' letter sent to Communications of the ACM (CACM) in March 1987, as well as further replies by other people, including Dijkstra's On a Somewhat Disappointing Correspondence. 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. In their quasi-standard book on the C programming language, Dennis Ritchie and Brian Kernighan warn that goto is 'infinitely abusable', but also suggest that it could be used for end-of-function error handlers and for multi-level breaks from loops. These two patterns can be found in numerous subsequent books on C by other authors; a 2007 introductory textbook notes that the error handling pattern is a way to work around the 'lack of built-in exception handling within the C language'. Other programmers, including 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 clarity, but only when used in a sensible way by a comparably sensible programmer. According to computer science professor John Regehr, in 2013, there were about 100,000 instances of goto in the Linux kernel code.