在Fortran中确定循环后的循环控制变量的值 [英] Determining the value of a loop control variable after the loop in Fortran

问题描述

我有以下程序:

program example
implicit none
integer::i, x1
real::x(10)=0
do i=10,1,-2
  x(10-i)=2*i+1
enddo
x1=i*2-1
end program example

我的问题是确定 x1 的值，但是由于从未输入或指定变量 i ，Fortran如何确定 i 用于计算 x1 ?

My problem is to determine the value of x1 but since variable i was never input or specified, how does Fortran determine the value of i used in the calculation of x1?

推荐答案

因为从未输入或指定变量 i "

这个假设是错误的.在循环内， i 取精确定义的值10、8、6、4、2.

This assumption is wrong. Inside the loop i takes exactly defined values 10, 8, 6, 4, 2.

循环结束后，Fortran标准指定 i 的值应为循环计数器的下一个值，就像循环仍在继续一样，因此 i 将为 0 .

After the loop finishes, the Fortran standard specifies that the value of i shall be the next value of the loop counter as if the loop still continued, so the value of i will be 0.

因此 x1 的值将为0 * 2-1-= -1，可以通过在程序末尾添加 print *，x1 轻松地进行验证

So the value of x1 will be 0*2 - 1 = -1 and it can be easily verified by adding print *, x1 at the end of the program.

但是由于francescalus发现了您的程序存在另一个问题.在第一次迭代中， i 的值为10，并且您正在访问 x(0)的 x(10-i).该元素不存在， x 仅具有从 x(1)到 x(10)的元素，因此访问 x(0)是非法的，并且在此之后整个程序的行为是不确定的.

But there is a different problem with your program as francescalus found out. On the first iteration the value of i is 10 and you are accessing x(10-i) which is x(0). This element does not exist, x has only elements from x(1) to x(10) so accessing x(0) is illegal and the behaviour of the whole program after that point is undefined.

这篇关于在Fortran中确定循环后的循环控制变量的值的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！