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In computer programming languages, a switch statement is a type of selection control mechanism used to allow the value of a variable or expression to change the control flow of program execution via a multiway branch. Switch statements exist in most high-level imperative programming languages such as Pascal, Ada, C/C++, C# and Java, and in many other types of language, using such keywords as switch, case, select or inspect.Switch statements come in two main variants: a structured switch, as in Pascal, which takes exactly one branch, and an unstructured switch, as in C, which functions as a type of goto. The main reasons for using a switch include improving clarity, by reducing otherwise repetitive coding, and (if the heuristics permit) also offering the potential for faster execution through easier compiler optimization in many cases. == History == In his 1952 text ''Introduction to Metamathematics'', Stephen Kleene formally proved that the CASE function (the IF-THEN-ELSE function being its simplest form) is a primitive recursive function, where he defines the notion definition by cases in the following manner::"#F. The function φ defined thus :: φ(x1 , ... , xn ) = ::: *φ1(x1 , ... , xn ) if Q1(x1 , ... , xn ), ::: * . . . . . . . . . . . . ::: *φm(x1 , ... , xn ) if Qm(x1 , ... , xn ), ::: *φm+1(x1 , ... , xn ) otherwise, :where Q1 , ... , Qm are mutually exclusive predicates (or φ(x1 , ... , xn) shall have the value given by the first clause which applies) is primitive recursive in φ1, ..., φm+1, Q1, ..., Qm+1.〔"Definition by cases", Kleene 1952:229〕 Kleene provides a proof of this in terms of the Boolean-like recursive functions "sign-of" sg( ) and "not sign of" ~sg( ) (Kleene 1952:222-223); the first returns 1 if its input is positive and −1 if its input is negative. Boolos-Burgess-Jeffrey make the additional observation that "definition by cases" must be both mutually exclusive and collectively exhaustive. They too offer a proof of the primitive recursiveness of this function (Boolos-Burgess-Jeffrey 2002:74-75). The IF-THEN-ELSE is the basis of the McCarthy formalism: its usage replaces both primitive recursion and the mu-operator. 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「switch statement」の詳細全文を読む スポンサード リンク
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