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Logical expression and control

OperatorsMeaning
.lt.less than
.gt.greater than
.le.less than or equal to
.ge.greater than or equal to
.eq.equals
.ne.not equal to
.and.and
.or.or
.not.not
.xor."exclusive" or
.eqv.equivalent
.neqv.not equivalent

F90 introduced following simplified notation for the relational operators:

OperatorsMeaning
<less than
>greater than
<=less than or equal to
>=greater than or equal to
==equal to
/=not equal to

IF condition​

Calculate square root of real positive numbers:

src/06_sq_root.f90
! Program : Calculate square root

PROGRAM sq_root
  IMPLICIT NONE

  REAL :: input, output

  PRINT *, "Input = "
  READ *, input

  IF (input > 0) THEN
    output = SQRT(input)
    PRINT *, "Square root of ", input, "= ", output
  ELSE
    PRINT *, "Input must be positive real."
  ENDIF

END PROGRAM sq_root

Multi-way IFs​

src/06_multi_way_if.f90
PROGRAM age_group
  IMPLICIT NONE

  INTEGER :: age
  PRINT "(a,$)", "Enter your age: "
  READ *, age

  IF (age <= 0) THEN
    PRINT *, "Error: please enter a positive integer age."
    STOP
  ELSE IF (age < 18) THEN
    PRINT *, "You are not adult."
  ELSE IF (age >= 18 .AND. age < 65) THEN
    PRINT *, "You are adult."
  ELSE
    PRINT *, "You are senior."
  END IF
END PROGRAM age_group

Notice also the STOP keyword in above example.

Select case​

src/06_select_case.f90
PROGRAM age_select
  IMPLICIT NONE

  INTEGER :: age
  PRINT "(a,$)", "Enter your age: "
  READ *, age

  SELECT CASE (age)
  CASE DEFAULT
    PRINT *, "You are not adult."
  CASE(18:64)
    PRINT *, "You are adult."
  CASE(65:)
    PRINT *, "You are senior."
  END SELECT
END PROGRAM age_select