# Some explanation on do loop **URL:** \ **Category:** Help\ **Created:** [April 9, 2024, 5:50pm UTC](https://fortran-lang.discourse.group/t/some-explanation-on-do-loop/7803 "2024-04-09T17:50:02Z")\ **Posts on this page:** 8\ **Page:** 1
**Author:** ![rmoortgat](https://yyz2.discourse-cdn.com/free1/user_avatar/fortran-lang.discourse.group/rmoortgat/32/3569_2.png) [@rmoortgat](https://fortran-lang.discourse.group/u/rmoortgat)\ **Post date:** [April 9, 2024, 5:50pm UTC](https://fortran-lang.discourse.group/t/some-explanation-on-do-loop/7803/1 "2024-04-09T17:50:03Z")
Good evening, Could someone explain the following do loop. I suppose this is someting very elementary but I don’t see it. I see an integer i used only once and I see two labels in the do loop. I tought labels were obsolete. ```auto program ch0505 implicit none real :: p = 0.4e-4, papprox = 0.41e-4 real :: abs_error, rel_error integer :: i do i = 1, 3 abs_error = abs(p-papprox) rel_error = abs(p-papprox)/abs(p) print 100, p, papprox 100 format (’p = ’, e11.4, /, & ’papprox = ’, e11.4) print 110, abs_error, rel_error 110 format (’abs error:’, 12x, e11.4, /, & ’rel error:’, 12x, e11.4, /) p = p*1.0e5 papprox = papprox*1.0e5 end do end program ch0505 ``` Thx Roger ---
**Author:** ![PierU](https://yyz2.discourse-cdn.com/free1/user_avatar/fortran-lang.discourse.group/pieru/32/1848_2.png) [@PierU](https://fortran-lang.discourse.group/u/PierU)\ **Post date:** [April 9, 2024, 6:05pm UTC](https://fortran-lang.discourse.group/t/some-explanation-on-do-loop/7803/2 "2024-04-09T18:05:39Z")
To me it just illustrates the difference between an absolute error and a relative error. And no, labels are not obsolete in such a context (they are used to refer to a `format` definition). Note that as pasted, this code contains ticks (`’` , which are invalid Fortran characters) instead of single quotes (`'`). Once corrected the output is: ```auto p = 0.4000E-04 papprox = 0.4100E-04 abs error: 0.1000E-05 rel error: 0.2500E-01 p = 0.4000E+01 papprox = 0.4100E+01 abs error: 0.1000E+00 rel error: 0.2500E-01 p = 0.4000E+06 papprox = 0.4100E+06 abs error: 0.1000E+05 rel error: 0.2500E-01 ``` ---
**Author:** ![rmoortgat](https://yyz2.discourse-cdn.com/free1/user_avatar/fortran-lang.discourse.group/rmoortgat/32/3569_2.png) [@rmoortgat](https://fortran-lang.discourse.group/u/rmoortgat)\ **Post date:** [April 9, 2024, 6:33pm UTC](https://fortran-lang.discourse.group/t/some-explanation-on-do-loop/7803/3 "2024-04-09T18:33:44Z")
Thank you, I’m aware of what the program is doing and it runs fine here. I guess I didn"t formulate my problem well. I just don’t understand what happens when you enter the do loop. I suppose the loop runs for i = 1, 2, 3. But where does i get incremented? I copied from a pdf so I suppose this is the reason of the invalid characters. Guess I’ll try to run line be line to see what happens within the do loop. Roger ---
**Author:** ![vmagnin](https://yyz2.discourse-cdn.com/free1/user_avatar/fortran-lang.discourse.group/vmagnin/32/28_2.png) [@vmagnin](https://fortran-lang.discourse.group/u/vmagnin)\ **Post date:** [April 9, 2024, 6:42pm UTC](https://fortran-lang.discourse.group/t/some-explanation-on-do-loop/7803/4 "2024-04-09T18:42:51Z")
> [@rmoortgat](#): > > But where does i get incremented? `i` is incremented automatically (+1 by default) each time you arrive at the `end do`, it is then compared with the right bound (3) and the loop continues if `i<=3`. In C, it would be: ```C for (int i=1 ; i<=3 ; i++) { } ``` ---
**Author:** ![vmagnin](https://yyz2.discourse-cdn.com/free1/user_avatar/fortran-lang.discourse.group/vmagnin/32/28_2.png) [@vmagnin](https://fortran-lang.discourse.group/u/vmagnin)\ **Post date:** [April 9, 2024, 6:51pm UTC](https://fortran-lang.discourse.group/t/some-explanation-on-do-loop/7803/5 "2024-04-09T18:51:23Z")
> [@rmoortgat](#): > > I tought labels were obsolete. Labels are not obsolete, but personnally I would rather write: ```fortran print '("p = ", e11.4, /, "papprox = ", e11.4)', p, papprox ``` If you use the format several times, you can even store it in a string: ```fortran character(*), parameter :: formatting = '("p = ", e11.4, /, "papprox = ", e11.4)' ... print formatting, p, papprox ``` ---
**Author:** ![wspector](https://avatars.discourse-cdn.com/v4/letter/w/47e85d/32.png) [@wspector](https://fortran-lang.discourse.group/u/wspector)\ **Post date:** [April 9, 2024, 8:31pm UTC](https://fortran-lang.discourse.group/t/some-explanation-on-do-loop/7803/6 "2024-04-09T20:31:32Z")
The number of trips through the loop is computed before the loop begins. The index variable, in this case ‘i’, is assigned a new value on each iteration. Note that zero trips through a loop is a possibility. For example: do, i=1, 0 … ! Nothing in here will be performed. end do ---
**Author:** ![RonShepard](https://avatars.discourse-cdn.com/v4/letter/r/a3d4f5/32.png) [@RonShepard](https://fortran-lang.discourse.group/u/RonShepard)\ **Post date:** [April 10, 2024, 3:47am UTC](https://fortran-lang.discourse.group/t/some-explanation-on-do-loop/7803/7 "2024-04-10T03:47:43Z")
> [@wspector](#): > > The number of trips through the loop is computed before the loop begins. The index variable, in this case ‘i’, is assigned a new value on each iteration. Normally that last sentence would be correct, but in this case the variable `i` is never referenced, so it is likely that it is never stored and/or incremented. The only way to tell if the compiler has done this would be to examine the machine code or to use a debugger and to monitor the storage location (if it exists) for` i`. If the code is modified to reference `i` somehow, then of course the compiler would not eliminate it. So the real magic for the do loop is the trip count. That is usually computed as a nonnegative value at the beginning of the loop and decremented by 1 each pass through the loop until it reaches 0. The test is typically at the beginning of the loop and the decrement is at the end of the loop, but a compiler is free to implement loops any way it wants with any available hardware instructions as long as the correct number of passes occurs. ---
**Author:** ![rmoortgat](https://yyz2.discourse-cdn.com/free1/user_avatar/fortran-lang.discourse.group/rmoortgat/32/3569_2.png) [@rmoortgat](https://fortran-lang.discourse.group/u/rmoortgat)\ **Post date:** [April 10, 2024, 7:35am UTC](https://fortran-lang.discourse.group/t/some-explanation-on-do-loop/7803/8 "2024-04-10T07:35:01Z")
Thank you all for your kind replies. Roger