The IF Statement

The IF statement allows us to execute some statement or block of code only if the result of the conditional expression is true. Each conditional expression is evaluated to .TRUE. or .FALSE. and the code is executed if the results is true

Relational Operators

FORTRAN	MATLAB	English
`.EQ.`	`==`	equal to
`.NE.`	`~=`	not equal to
`.LT.`	`<`	less than
`.LE.`	`<=`	less than or equal
`.GT.`	`>`	greater than
`.GE.`	`>=`	greater than or equal

Logical Operators

FORTRAN	MATLAB	English
`.NOT.`	~	True only if the operand is true
`.AND.`	&	True only if both operands are true
`.OR.`	\|	True if either operands are true

IF and IF-ENDIF

If there is only one short statement to execute, use the single line form of the IF statement.

    IF (newval .LT. minval) minval = newval

If there is more than one or a longer statement to execute, use the block form of the IF statement.

    IF (gradepercent >= 93) THEN
        lettergrade = 'A'
        Astudents = Astudents + 1
    ENDIF

IF-ELSE-ENDIF

Include an ELSE clause in your IF statement to provide an alternative code block to execute if the condition is false.

    IF (numscores > 0) THEN
        CALL computeAverage
    ELSE
        write (*,*) 'No scores to average'
    ENDIF

IF-ELSEIF-ELSE-ENDIF

    IF (x .EQ. 0) THEN
      WRITE(*,*) 'x equals 0'
    ELSEIF (x .GT. 0) THEN
      IF (x .LT. y) THEN
        WRITE(*,*) 'x > 0 and x < y'
      ELSE 
        WRITE(*,*) 'x > 0 and x >= y'
      ENDIF
    ELSE
      IF (x .LT. y) THEN
        WRITE(*,*) 'x <= 0 and x < y'
      ELSE 
        WRITE(*,*) 'x <= 0 and x >= y'
      ENDIF
    ENDIF

General Form

The words IF, THEN, ELSEIF, ELSE and ENDIF are reserved words,
The general form of an IF statement in FORTRAN is:

IF (condition) THEN
    statement(s)
ELSEIF (condition) THEN
    statement(s)
ELSEIF (condition) THEN
    statement(s)
    
    [more elseif conditions and blocks here]
    
ELSE
    statement(s)
ENDIF

where:
condition is a boolean expression and
statements is one or more FORTRAN command statements
that we want to execute if the condition evaluates to true.