Performance-Aware Loops

Early Exit Scan

Stop When Found

An early exit can reduce unnecessary loop work when a condition has already been satisfied.

Program

Play the program to choose the target and compare how many checks the scan performs.

early_exit_scan.f90 
program early_exit_scan_demo
    implicit none
    integer :: values(5)
    integer :: target
    integer :: i
    integer :: checks
    integer :: found_at

    values = [3, 6, 9, 12, 15]
    target = 
    checks = 0
    found_at = 0
    do i = 1, size(values)
        checks = checks + 1
        if (values(i) == target) then
            found_at = i
            exit
        end if
    end do
    print '(I0, 1X, I0)', found_at, checks
end program early_exit_scan_demo

program early_exit_scan_demo
    implicit none
    integer :: values(5)
    integer :: target
    integer :: i
    integer :: checks
    integer :: found_at

    values = [3, 6, 9, 12, 15]
    target = 
    checks = 0
    found_at = 0
    do i = 1, size(values)
        checks = checks + 1
        if (values(i) == target) then
            found_at = i
            exit
        end if
    end do
    print '(I0, 1X, I0)', found_at, checks
end program early_exit_scan_demo

program early_exit_scan_demo
    implicit none
    integer :: values(5)
    integer :: target
    integer :: i
    integer :: checks
    integer :: found_at

    values = [3, 6, 9, 12, 15]
    target = 
    checks = 0
    found_at = 0
    do i = 1, size(values)
        checks = checks + 1
        if (values(i) == target) then
            found_at = i
            exit
        end if
    end do
    print '(I0, 1X, I0)', found_at, checks
end program early_exit_scan_demo
early exit `exit` leaves the loop as soon as the target is found.
checks `checks` shows the amount of scanning work performed.
sentinel `found_at = 0` records the not-found case without raising an error.