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Concept
Conditional (computer programming)
Summary
In computer science, conditionals (that is, conditional statements, conditional expressions and conditional constructs) are programming language commands for handling decisions. Specifically, conditionals perform different computations or actions depending on whether a programmer-defined Boolean condition evaluates to true or false. In terms of control flow, the decision is always achieved by selectively altering the control flow based on some condition (apart from the case of branch predication).
Although dynamic dispatch is not usually classified as a conditional construct, it is another way to select between alternatives at runtime. Conditional statements are the checkpoints in the programe that determines behaviour according to situation.
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.
The if–then construct (sometimes called if–then–else) is common across many programming languages. Although the syntax varies from language to language, the basic structure (in pseudocode form) looks like this:
If (boolean condition) Then
(consequent)
Else
(alternative)
End If
For example:
If stock=0 Then
message= order new stock
Else
message= there is stock
End If
In the example code above, the part represented by (boolean condition) constitutes a conditional expression, having intrinsic value (e.g., it may be substituted by either of the values True or False) but having no intrinsic meaning. In contrast, the combination of this expression, the If and Then surrounding it, and the consequent that follows afterward constitute a conditional statement, having intrinsic meaning (e.g., expressing a coherent logical rule) but no intrinsic value.
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.
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