Logical IF

The logical IF is the simplest form. It has the following form:

IF (logical-expression)  one-statement

where one-statement is a executable statement which is not another IF, and logical-expression is a logical expression. The execution of this logical IF statement goes as follows:

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

Note that this logical IF does have its use although it looks not so powerful comparing with IF-THEN-ELSE-END IF and IF-THEN-END IF.

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)  Absolute_X = -X
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)  Smaller = b
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)  WRITE(*,*)
```
The following is wrong since the one-statement if a logical IF can not be another IF statement.
```
INTEGER :: a, b, c

IF (a < b) IF (b < c) WRITE(*,*)  a, b, c
```

From the above examples, one can easily see that if the THEN part has exactly one statement and there is no ELSE, the logical IF statement can save some space making a program a little shorter. But, my advice is that don't use it whenever it is possible.