IF-THEN-END IF

The IF-THEN-END IF form is a simplification of the general IF-THEN-ELSE-END IF form with the ELSE part omitted:

IF (logical-expression) THEN
   statements
END IF

where statements is a sequence of executable statements, and logical-expression is a logical expression. The execution of this IF-THEN-ELSE-END IF statement goes as follows:

the logical-expression is evaluated, yielding a logical value
if the result is .TRUE., the statements in statements are executed, followed by the statement following the IF-THEN-END IF statement.
if the result is .FALSE., the statement following the IF-THEN-END IF is executed. In other words, if logical-expression is .FALSE., there is no action taken.

Examples

The following program segment computes the absolute value of X and saves the result into variable Absolute_X. Recall that the absolute value of x is x if x is non-negative; otherwise, the absolute value is -x. For example, the absolute value of 5 is 5 and the absolute value of -4 is 4=-(-4). Also note that the WRITE(*,*) statement has been intentionally broken into two lines with the continuation line symbol &. The trick is that the value of X is first saved to Absolute_X whose value is changed later only if the value of X is less than zero.
```
REAL  :: X, Absolute_X

X = .....
Absolute_X = X
IF (X < 0.0) THEN
   Absolute_X = -X
END IF
WRITE(*,*)  'The absolute value of ', x, &
            ' is ', Absolute_X
```
The following program segment reads in two integer values into a and b and finds the smaller one into Smaller. Note that the WRITE(*,*) has also been broken into two lines. This uses the same trick discussed in the previous example.
```
INTEGER  :: a, b, Smaller

READ(*,*)  a, b
Smaller = a
IF (a > b) THEN
   Smaller = b
END IF
Write(*,*)  'The smaller of ', a, ' and ', &
            b, ' is ', Smaller
```
In many cases, it is required to do something when certain condition is satisfied; otherwise, do nothing. This is exactly what we need the form of IF-THEN-END IF. In the following, an INTEGER variable Counter is used for counting something. When its value is a multiple of 10, a blank line is displayed.
```
INTEGER :: Counter

IF (MOD(Counter, 10) == 0) THEN
   WRITE(*,*)
END IF
```

A Useful Tip

The box trick can also be used with this IF-THEN-END IF form. Since there is no ELSE, you can leave the lower part empty like the following:

logical-expression	what you want to do when the logical expression is .TRUE.
	nothing is here!!!