CODEWITHSUNDEEP
if-statementfortrangfortranfortran95numerical-computing
vikash
vikash

Reputation: 23

Program not entering an if-loop in Fortran even when condition is satisfied

This is the section of the code that is not running as expected. I had to include some old-style "pause" statements to pin-point the error location.

    iteration_index = 1
    y_refined = y_current + (0.5d0*dx*(dydx_predictor + dydx_refined_corrector))               ! Solution corresponds to i = 2, i.e., the refined Heun's method.

    relative_percent_error = 100.d0*(abs((y_refined - y_next)/y_refined))                         ! Calculation of relative percentage error. This is NOT true error.

    if (relative_percent_error > heun_percent_tolerance) then

        iteration_index = iteration_index + 1

        print*, 'first loop enter', x_next, relative_percent_error, iteration_index
        pause

        if (iteration_index < max_heun_number) then

            y_next = y_refined
            call dydx(x_next, y_next, dydx_refined_corrector)
            y_refined = y_current + (0.5d0*dx*(dydx_predictor + dydx_refined_corrector))
            relative_percent_error = 100.d0*(abs((y_refined - y_next)/y_refined))

            print*, 'second loop enter', x_next, relative_percent_error, iteration_index
            pause

        end if

    end if

Output is seen as follows:

first loop enter   1.0000000000000000        6.7763423346068707          2

PAUSE

To resume execution, type go.  Other input will terminate the job.

go

RESUMED

second loop enter   1.0000000000000000        1.6658644147581094        2

PAUSE 

To resume execution, type go.  Other input will terminate the job.

go

RESUMED

first loop enter   2.0000000000000000        6.6615482639252761         2

PAUSE 

To resume execution, type go.  Other input will terminate the job.

The values of heun_percent_tolerance is 0.01 and max_heun_number is 15. I expect the execution to enter the second if-loop for more iterations until the max limit of 15 is reached, but it seems that the code jumps to the next x_next value of 2.

I have even tried to combine the two conditions as If (cond1 .and. cond2), that did not work either.

Upvotes: 0

Views: 474

Answers (1)

Vladimir F Героям слава
Vladimir F Героям слава

Reputation: 60008

There is NO loop in your code! IF is not a loop! It will NOT be executed repeatedly unless you put there an actual loop.

If you want to loop some code while a condition is still valid, use a DO WHILE loop or a DO loop with EXIT.

do while (relative_percent_error > heun_percent_tolerance) 

    iteration_index = iteration_index + 1

    print*, 'first loop enter', x_next, relative_percent_error, iteration_index
    pause

    if (iteration_index < max_heun_number) then

        y_next = y_refined
        call dydx(x_next, y_next, dydx_refined_corrector)
        y_refined = y_current + (0.5d0*dx*(dydx_predictor + dydx_refined_corrector))
        relative_percent_error = 100.d0*(abs((y_refined - y_next)/y_refined))

        print*, 'second loop enter', x_next, relative_percent_error, iteration_index
        pause

    end if

end do

Note the code is still likely incorrect, you will have to restructure it, but the point is that you need an actual DO loop and not just an IF condition. An IF is not a loop.

Upvotes: 3

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