If Statement

The if statement evaluates an expression that results in a boolean value. If the result is true, any statements that follow the THEN keyword are executed. All if statements begin with the IF keyword and end with the END_IF keyword followed by a ;.

The if statement is considered a 'selection' statement along with the case statement as they both handle selective execution (or avoidance) of a set of statements.

Only one set of block statements will be executed

Simple If

Below is an example of a 'simple' if statement. The expression statement b := 1; is conditionally executed because a is true. While this example has only a single statement contained within the block, any number of statements are permitted. The statement begins with the IF keyword and is followed by a condition. The condition is any expression that results in a boolean value (true or false). This can include boolean variables, operators such as comparison or equality, or other functions. The THEN keyword indicates the beginning of the block statements to be executed when the condition is true. The END_IF keyword and ; identify the end of the block statements.

a : BOOL := TRUE;
b : INT := 0;

IF a THEN
    b := 1;   // a is true, b is assigned to 1
END_IF;

Elsif Clause

Additional branching if statements continue to test conditions in sequential order.

a : BOOL := TRUE;
b : INT := 0;

IF b > 0 THEN
    b := 1;   // Not executed, b is not greater than 0
ELSIF a THEN
    b := 2;   // a is true, b is assigned to 2
END_IF;

Else Clause

The else clause does not test a condition but rather provides a default pathway to execute statements when all other conditions have evaluated as false.

a : BOOL := TRUE;
b : INT := 0;

IF b > 0 THEN
    b := 1;   // Not executed, b is not greater than 0

ELSIF NOT a THEN
    b := 2;   // Not executed, a is true so NOT a is false
ELSE
    b := 3;   // None of the other conditions is true, b is assigned to 3
END_IF;

What happens when multiple clauses evaluate to true?

What is the value of b? Check it out in the editor!

a : BOOL := TRUE;
b : INT := 0;

IF b == 0 THEN
    b := 1;
ELSIF a THEN
    b := 2;
ELSE
    b := 3;
END_IF;