Conditional statement

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In computer science, conditional statements are a vital part of a programming language. These statements are requests to the computer to make an execution choice based on a given condition.

One form is the If-Then clause, sometimes seen as :

If (condition) Then
(statements)
Else
(statements)
End If

It works this way - when the computer finds an If (a reserved word), it expects a data comparison, for example, x = 0 and evaluates the condition. If the condition is true, the statement block following the Then (another reserved word) shall be executed, otherwise the execution shall continue in the following statement block.

Depending on the language, the If may use Then, brackets, and another clause called Else or ElseIf.

There are many other conditional statements in programming languages, however, all of them follow these principles. Other conditional statements include switch statements, case statements and statements with guards.

[edit] Alternative Syntax

Main article: Ternary operation

Many languages support alternative syntax for if statements: (condition)?(evaluate if condition was true):(evaluate if condition was false).
This is mainly used for in-line if statements that wouldn't be suitable in the place used, for example:

//Invalid
$var = if(true) { 'foo' } else { 'bar' };
//Valid
$var = (true)?'foo':'bar'

To accomplish the same as the second (correct) line above, using standard if/else construct:

if (true) {
  $var = 'foo';
}
else {
  $var = 'bar';
}

The alternative syntax can also be used inside if statements themselves: that is to say more generally, "The alternative syntax, accomplishing the equivalent of an analogous if/else construct, may be used anywhere an if/else construct may be appropriately used".

[edit] Alternative Implementation

Contrary to other languages, in Smalltalk the conditional statement is not a language construct but defined as abstract method in the class Boolean as a method which takes two parameters, both clauses as closure. And then Boolean has two subclasses, True and False, which both define the method, True executing the first closure only, False executing the second closure only.

var := condition 
    ifTrue: [ 'foo' ]
    ifFalse: [ 'bar' ]

[edit] See also

• Control flow#Choice
• Conditional branch for a theoretical view
• Pattern matching for another way to make execution choices
• Logical conditional for a more general view of conditionals
• Ternary operation for shorthand syntax provided by most programming languages