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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.


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: (The example is actually perfectly valid Visual Basic or QuickBASIC syntax.)

 IF (boolean condition)
    (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.