5.7.3 IF Constructs

IF constructs conditionally execute blocks (groups) of statements. An IF construct takes the following form:

[name:] IF (e) THEN
  block

[ELSE IF (e1) THEN
  block] . . .

[ELSE
  block]

END IF [name]

name (Alpha only)

Is the name of the IF construct.

e, e1, e2

Are logical expressions.

block

Is a sequence of zero or more complete Fortran statements (also called a statement block).

Rules and Behavior

On Alpha processors, if a construct name is specified in an IF statement, the same name must appear in the terminal END IF statement. If no construct name is specified in the IF statement, no name can appear in the terminal END IF statement. The same construct name must not be used for different named constructs in the same program unit; it must be unique.

Each statement in an IF construct, except the END IF statement, has an associated statement block. The statement block is conditionally executed based on the values of the logical expressions in the preceding IF construct statement.

The functions of individual statements in an IF construct are as follows:

• IF THEN: Begins an IF construct. The block following it is executed if the value of the logical expression in the IF THEN statement is true.

  ---

  Note

  No additional statement can be placed after the IF THEN statement in an IF construct. For example, the following statement is invalid:

  IF (e) THEN I = J
  

  This statement is translated as the following logical IF statement:

  IF (e) THENI = J
  

  ---

• ELSE IF THEN: An optional statement that specifies a statement block to be executed if no preceding statement block in the IF construct has been executed, and if the value of the logical expression in the ELSE IF THEN statement is true. An IF construct can contain any number of ELSE IF THEN statements.

• ELSE: Specifies a statement block to be executed if no preceding statement block in the IF construct has been executed. The ELSE statement is optional. However, if the ELSE statement is present, the ELSE statement block must be immediately followed by the END IF statement.

• END IF: Terminates the IF construct.

After the last statement in a statement block is executed, control passes to the next executable statement following the END IF statement. Consequently, no more than one statement block in an IF construct is executed each time the IF THEN statement is executed.

ELSE IF THEN and ELSE statements can have statement labels, but the labels cannot be referenced. The END IF statement can have a statement label to which control can be transferred from anywhere within the IF construct.

Figure 5-4 shows the flow of control for four examples of IF constructs.

Figure 5-4 Examples of IF Constructs