Editing Control flow (section)

== Minimal structured control flow ==
{{See also|Structured program theorem}}
In May 1966, Böhm and Jacopini published an article<ref>Böhm, Jacopini. "Flow diagrams, turing machines and languages with only two formation rules" [[Comm. ACM]], 9(5):366-371, May 1966.</ref> in ''Communications of the ACM'' which showed that any program with '''goto'''s could be transformed into a goto-free form involving only choice (IF THEN ELSE) and loops (WHILE condition DO xxx), possibly with duplicated code and/or the addition of Boolean variables (true/false flags). Later authors showed that choice can be replaced by loops (and yet more Boolean variables).

That such minimalism is possible does not mean that it is necessarily desirable; computers theoretically need only [[One instruction set computer|one machine instruction]] (subtract one number from another and branch if the result is negative), but practical computers have dozens or even hundreds of machine instructions.

Other research showed that control structures with one entry and one exit were much easier to understand than any other form,{{citation needed|date=July 2014}} mainly because they could be used anywhere as a statement without disrupting the control flow. In other words, they were ''composable''. (Later developments, such as [[non-strict programming language]]s – and more recently, composable [[software transactional memory|software transactions]] – have continued this strategy, making components of programs even more freely composable.)

Some academics took a purist approach to the Böhm–Jacopini result and argued that even instructions like <code>break</code> and <code>return</code> from the middle of loops are bad practice as they are not needed in the Böhm–Jacopini proof, and thus they advocated that all loops should have a single exit point. This purist approach is embodied in the language [[Pascal (programming language)|Pascal]] (designed in 1968–1969), which up to the mid-1990s was the preferred tool for teaching introductory programming in academia.<ref name="roberts">Roberts, E. [1995] "[http://cs.stanford.edu/people/eroberts/papers/SIGCSE-1995/LoopExits.pdf Loop Exits and Structured Programming: Reopening the Debate] {{Webarchive|url=https://web.archive.org/web/20140725130816/http://cs.stanford.edu/people/eroberts/papers/SIGCSE-1995/LoopExits.pdf |date=2014-07-25 }}," ACM SIGCSE Bulletin, (27)1: 268–272.</ref> The direct application of the Böhm–Jacopini theorem may result in additional local variables being introduced in the structured chart, and may also result in some [[code duplication]].<ref name="WattFindlay2004">{{cite book|author1=David Anthony Watt|author2=William Findlay|title=Programming language design concepts|year=2004|publisher=John Wiley & Sons|isbn=978-0-470-85320-7|page=228}}</ref> Pascal is affected by both of these problems and according to empirical studies cited by [[Eric S. Roberts]], student programmers had difficulty formulating correct solutions in Pascal for several simple problems, including writing a function for searching an element in an array. A 1980 study by Henry Shapiro cited by Roberts found that using only the Pascal-provided control structures, the correct solution was given by only 20% of the subjects, while no subject wrote incorrect code for this problem if allowed to write a return from the middle of a loop.<ref name="roberts"/>