Determining the value of a loop control variable after the loop in Fortran

Question

I have the following program:

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

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?

Vladimir F Героям слава · Accepted Answer · 2017-04-04 15:55:19Z

3

"since variable i was never input or specified"

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

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.

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.

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.

edited Apr 4, 2017 at 15:55

answered Apr 4, 2017 at 15:32

Vladimir F Героям слава

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1 Comment

Vladimir F Героям слава Over a year ago

Fair enough, yes, the program as the whole becomes invalid. I have actually played with the code, but never enabled bounds checking thinking the code is too trivial.

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