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Zahn's construct, in computer science, was a proposed structure for structured control flow in computer programming languages first described by Charles T. Zahn in 1974.〔Zahn, C. T. "A control statement for natural top-down structured programming" presented at Symposium on Programming Languages, Paris, 1974.〕 The construct is primarily described in terms of an extension to looping constructs to recognize multiple means by which a loop could terminate. For example, a search loop might terminate early, when the target is found; or it might terminate after the search has been completed unsuccessfully. Zahn's construct can be used to avoid GO TO statements when determining which case was encountered. Zahn does this by introducing a new kind of variable called a ''situation indicator'' in a CASE-like construct following the loop.Donald Knuth, in his paper "Structured Programming with Go To Statements",〔Knuth, D. E. ("Structured Programming with Go To Statements" ), ''Computing Surveys'', Volume 6, December 1974, page 275〕 describes two forms of Zahn's construct as follows: loop until repeat; then ... fi and: begin until end; then ... fi There must also be a statement to set a specific situation indicator and exit the body of the construct. The following simple example involves searching a two-dimensional table for a particular item. exitwhen found or missing; for I := 1 to N do for J := 1 to M do if table() = target then found; missing; exits found: print ("item is in table"); missing: print ("item is not in table"); endexit; Try-catch blocks, used in modern programming languages for exception handling, are variations of Zahn's construct. The major difference is that the scope of Zahn's proposals were limited to individual loops within a program, whereas exception-handling capabilities often allow exceptions to be "thrown" from deep within a call stack and "caught" at a point higher up in the stack. ==References== 〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Zahn's construct」の詳細全文を読む スポンサード リンク
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