Calculate hypotenuse: example of subroutine

Hypotenuse can be calculated using following formula:

$$c = \sqrt{a^2 + b^2}$$

src/16_hypotenuse_subroutine.f90

! Compile the subroutine: gfortran -c 16_hypotenuse_subroutine.f90
SUBROUTINE calculate_hypotenuse(side_a, side_b, hypotenuse)
    IMPLICIT NONE

    ! declare the intent of dummy variables
    REAL, INTENT(IN) :: side_a
    REAL, INTENT(IN) :: side_b
    REAL, INTENT(OUT) :: hypotenuse

    REAL :: temp

    temp = side_a * side_a + side_b * side_b
    hypotenuse = SQRT(temp)
END SUBROUTINE calculate_hypotenuse

Compile the subroutine:

gfortran -c 16_hypotenuse_subroutine.f90

src/16_hypotenuse_program.f90

! subroutine without module
! gfortran -c 16_hypotenuse_subroutine.f90
! gfortran 16_hypotenuse_program.f90 16_hypotenuse_subroutine.o
PROGRAM hypotenuse
    IMPLICIT NONE
    REAL :: s1, s2, output

    WRITE (*, *) 'Program to calculate hypotenuse of a right triangle'
    WRITE (*, *) 'Enter length of side 1:'
    READ (*, *) s1
    WRITE (*, *) 'Enter length of side 2:'
    READ (*, *) s2

! notice subroutine call can update values in the calling environment
    CALL calculate_hypotenuse(s1, s2, output)

    WRITE (*, 99) output
99  FORMAT('Hypotenuse = ', F10.4)
END PROGRAM hypotenuse

Compile main program:

gfortran 16_hypotenuse_program.f90 16_hypotenuse_subroutine.o