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FORTRAN 90+: IF-THEN-ELSE

The IF-THEN-ELSE conditional statements are used to control the program to perform certain operations depending on the value of one or multiple variables. Once an IF is .TRUE. the code skips any following ELSE or ELSE IF statements.

The simplest example is the IF statement. Here, the general format is

IF (true or false statement) arithmetic operation

For example, if a > c, then set a = b. This would appear as

IF (a > c) a = b

If a is less than or equal to c, then no operation is performed. Only one arithmetic operation can be performed per simple IF statement.

A more powerful tool is the block IF-THEN-ELSE set of statements. Its general format is

IF (true or false statement) THEN
   Code to do if .TRUE.
ELSE 
   Code to do if .FALSE.
END IF

END IF can appear as two words or as one word, ENDIF, depending on the programmer’s fancy.

The block IF statement can be used in place of the simple IF statement:

IF (a > c) THEN
   a = b
ENDIF

Additional statements can be added here as well:

IF (a > c) THEN
   a = b
   WRITE(*,*) ’a is greater than c’
ENDIF

Good programming practice is to indent the operations within the block IF for readability. In addition, using capital letters for the intrinsic operations and small letters for the programmer’s operations makes for easier reading.

Multiple true or false questions to give the program more options other than the two allowed in the above example can be accomplished using one of two possible procedures.

Nesting can be used to produce multiple true false questions as well as to make multiple options for a .TRUE. answer. Nesting is when IF statements reside within other IF statements. Using partial pseudo-code this is illustrated as
```
IF (a > b) THEN
   IF (a > c) THEN
      output= ’Arnold wins’
   ELSE
      output= ’Chad wins’
   END IF
ELSE
   IF (b>c) THEN
      output= ’Bobby wins’
   ELSE
      output= ’Chad wins’
   END IF
END IF
```
Nesting can get messy very quickly so a better method may be to use the ELSE IF command. This statements tells the computer to check a series of questions looking for the first .TRUE. statement. The code above could be written as follows using ELSE IFs.
```
IF (a > b .AND. a > c) THEN
   output= ’Arnold wins’
ELSE IF (b > a .AND. b > c) THEN
   output= ’Bobby wins’
ELSE
   output= ’Chad wins’
END IF
```

ELSE IF (or ELSEIF) statements must come before any ELSE statements, and only one END IF statement is needed for the entire block.

For long block IF statements, it is good programming practice to name it. The name assigned to the statement is stated at the beginning and end of the block statement and the name must be identical. The example above can be named as ’winner’:

winner:  IF (a > b .AND. a > c) THEN
   output= ’Arnold wins’
ELSE IF (b > a .AND. b > c) THEN
   output= ’Bobby wins’
ELSE
   output= ’Chad wins’
END IF winner

IF-THEN statements are generally very intuitive but are also quite useful. A useful programming tip is to write down and check the logical statements for complex problems prior to programming them.