If statement

An if statement is a selection statement that allows more than one possible flow of control.

You can optionally specify an else clause on the if statement. If the test expression evaluates to false (or in C, a zero value) and an else clause exists, the statement associated with the else clause runs. If the test expression evaluates to true, the statement following the expression runs and the else clause is ignored.

When if statements are nested and else clauses are present, a given else is associated with the closest preceding if statement within the same block.

A single statement following any selection statements (if, switch) is treated as a compound statement containing the original statement. As a result any variables declared on that statement will be out of scope after the if statement.

Pseudocode

Typical syntax is

 if criterion1
    action1
 else 
    if criterion2
        action2 
    else
        action3

When the purpose of the if statement is to return/assign one of two values depending on the condition, the ternary operator may be useful.

When several if statements are used in a cascade to choose between several options, a Switch Statement might be worth considering as a more readable and maintainable replacement.

Examples

Curly bracket programming languages

 if (expr) {
     // statement 1
 } else {
     // statement 2
 }

expr must be a condition (see above).

 if (false) {
     // This will never happen!
 }

for if statements where there is only one option the brackets can be left out.

if(expr)
   do_this();
else 
   do_that();

where the statement controls how the function exits you can leave out the else too.

if(expr)
   return do_this();

return do_that();

Ruby

var = false
if var {
     print "foobar is y\n" 
}
else { 
   print "foobar is n\n" 
}

Visual Basic

If booleanExpression Then
   statement1
Else
   statement2
End If

Python

if expr:
    # statement 1
else:
    # statement 2

or

if var==1:
    #Something
elif var>4:
    #Something
else:
    #Something

See Also

• Switch Statement
• ternary operator
• Boolean
• null

References

Control structures If statement - Switch statement - While loop - For loop - Foreach loop - Do while loop