if-statement - 即使满足条件，程序也不会在 Fortran 中进入 if 循环

Question

这是未按预期运行的代码部分。我必须包含一些老式的“暂停”语句来查明错误位置。

---

    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

---

输出如下：

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.

---

的值为heun_percent_tolerance0.01 和max_heun_number15。我希望执行进入第二个 if 循环进行更多迭代，直到达到最大限制 15，但似乎代码跳转到下一个x_next值 2。

我什至尝试将这两个条件结合起来If (cond1 .and. cond2)，但也没有用。

标签： if-statementfortrangfortranfortran95numerical-computing

Answer 1

您的代码中没有循环！IF不是循环！除非你放一个实际的循环，否则它不会重复执行。

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.