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In computer science, conditional statements, conditional expressions and conditional constructs are features of a programming language which perform different computations or actions depending on whether a programmer-specified boolean ''condition'' evaluates to true or false. Apart from the case of branch predication, this is always achieved by selectively altering the control flow based on some condition. In imperative programming languages, the term "conditional statement" is usually used, whereas in functional programming, the terms "conditional expression" or "conditional construct" are preferred, because these terms all have distinct meanings. Although dynamic dispatch is not usually classified as a conditional construct, it is another way to select between alternatives at runtime. == If–then(–else) == The if–then construct (sometimes called if–then–else) is common across many programming languages. Although the syntax varies quite a bit from language to language, the basic structure (in pseudocode form) looks like this:If (boolean condition) Then (consequent) Else (alternative) End If When an interpreter finds an If, it expects a boolean condition – for example, x > 0, which means "the variable x contains a number that is greater than zero" – and evaluates that condition. If the condition is true, the statements following the then are executed. Otherwise, the execution continues in the following branch – either in the else block (which is usually optional), or if there is no else branch, then after the end If.After either branch has been executed, control returns to the point after the end If.In early programming languages, especially some dialects of BASIC in the 1980s home computers, an if–then statement could only contain GOTO statements. This led to a hard-to-read style of programming known as spaghetti programming, with programs in this style called ''spaghetti code''. As a result, structured programming, which allows (virtually) arbitrary statements to be put in statement blocks inside an if statement, gained in popularity, until it became the norm even in most BASIC programming circles. Such mechanisms and principles were based on the older but more advanced ALGOL family of languages, and ALGOL-like languages such as Pascal and Modula-2 influenced modern BASIC variants for many years. While it is possible while using only GOTO statements in if-then statements to write programs that are not spaghetti code and are just as well structured and readable as programs written in a structured programming language, structured programming makes this easier and enforces it. Structured if-then-else statements like the example above are one of the key elements of structured programming, and they are present in most popular high-level programming languages such as C, Java, JavaScript and Visual Basic .A subtlety is that the optional else clause found in many languages means that the context-free grammar is ambiguous, since nested conditionals can be parsed in multiple ways. Specifically, if a then if b then s else s2 can be parsed as if a then (if b then s) else s2 or if a then (if b then s else s2) depending on whether the else is associated with the first if or second if. This is known as the dangling else problem, and is resolved in various ways, depending on the language.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「Conditional (computer programming)」の詳細全文を読む スポンサード リンク
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