Error Handling Patterns

Guarded Divide

Return a Status

A guard checks the risky condition before doing the work. The caller can inspect a status value and keep control flow explicit.

Program

Play the program to choose a divisor and watch the guard choose a result and status.

guarded_divide.f90 
program guarded_divide_demo
    implicit none
    integer :: numerator, divisor
    integer :: quotient, status

    numerator = 20
    divisor = 
    if (divisor == 0) then
        quotient = 0
        status = 1
    else
        quotient = numerator / divisor
        status = 0
    end if
    print '(I0, 1X, I0)', quotient, status
end program guarded_divide_demo

program guarded_divide_demo
    implicit none
    integer :: numerator, divisor
    integer :: quotient, status

    numerator = 20
    divisor = 
    if (divisor == 0) then
        quotient = 0
        status = 1
    else
        quotient = numerator / divisor
        status = 0
    end if
    print '(I0, 1X, I0)', quotient, status
end program guarded_divide_demo

program guarded_divide_demo
    implicit none
    integer :: numerator, divisor
    integer :: quotient, status

    numerator = 20
    divisor = 
    if (divisor == 0) then
        quotient = 0
        status = 1
    else
        quotient = numerator / divisor
        status = 0
    end if
    print '(I0, 1X, I0)', quotient, status
end program guarded_divide_demo
guard The `if` condition checks for the unsafe divisor before division.
status `status = 1` records the failure path without stopping the program.
normal path When the guard is false, the program computes the quotient normally.