Do Loops

Basic Do Loops

Fortran uses do loops instead of C-style for loops.

This is a minimal example to show "Do Loops".

program do_counter
    implicit none
    integer :: i

    print *, "i -> 1..5"
    do i = 1, 5
        print *, i
    end do

    print *, "i -> 10..8..0"
    do i = 10, 0, -2
        print *, i
    end do
end program do_counter

Let's see the result.

i -> 1..5
1
2
3
4
5
i -> 10..8..0
10
8
6
4
2
0

So basically, these two blocks are the same.

    do i = 1, 5
        print *, i
    end do

    do i = 1, 5, 1
        print *, i
    end do

This means Fortran 2018 implicitly assumes the third argument of do loops to be 1.

While Loops

program while_loops
    implicit none
    integer :: n = 1

    do while (n < 5)
        print *, "while n =", n
        n = n + 1
    end do

    n = 1
    do
        if (n >= 5) exit
        print *, "while n =", n
        n = n + 1
    end do
end program while_loops

exit breaks out of a loop.

Those two loops print the same output.

while n = 1
while n = 2
while n = 3
while n = 4

When a condition is complex enough, you can use if branches to break out of a loop.

But when the condition is simple enough to use do while, don't use if branches.

Then, how can you continue to the next cycle without executing the remaining statements?

program loop_control
    implicit none
    integer :: i

    do i = 1, 10
        ! Continue and skip to the next cycle when i is even
        if (mod(i, 2) == 0) cycle

        ! Exit the loop when i is greater than 7
        if (i > 7) exit

        print *, "Odd number:", i
    end do
    ! Print 1, skip 2, print 3, skip 4, print 5, skip 6, print 7
    ! The result is 1, 3, 5, 7
end program loop_control

When i is even, it continues to the next cycle. As a result, print function is not executed since the cycle was skipped.

When i is bigger than 7, exit procedure breaks the loop.

However, when i is 8 exit is not executed since its location is below cycle.

As a result, when i is 9, comparison i > 7 is interpreted as 9 > 7.

9 is bigger than 7. This expression is true. The program executes exit procedure.