Fortran, why do i suddenly get new errors?

snipertje

Sorry for the wrong title!

I currently have a subroutine which should return y like (the actual formula is a bit more complex): y=cos(t)

When i call the subroutine and then make a do-loop to test y for different t, it gives 1 value only. Does the subroutine give a value to y or am I doing something else wrong?

t doesn't have a value before the subroutine is called.

Mark44

Quote by snipertje

Sorry for the wrong title!

I currently have a subroutine which should return y like (the actual formula is a bit more complex): y=cos(t)

When i call the subroutine and then make a do-loop to test y for different t, it gives 1 value only. Does the subroutine give a value to y or am I doing something else wrong?

t doesn't have a value before the subroutine is called.

Fortran subroutines don't return values. By that, I mean that an expression that includes a subroutine call doesn't evaluate to anything.

Your routine should be a FUNCTION, which does return a value.

snipertje

Quote by Mark44

Fortran subroutines don't return values. By that, I mean that an expression that includes a subroutine call doesn't evaluate to anything.

Your routine should be a FUNCTION, which does return a value.

I have something like this, it's not about wether or not a subroutine is needed, I want to understand how it works.

Code:

call asd(y,t,a)
do i=1,10
t=i
test(i)=y
end do

subroutine asd(y,t,a)
y=a*cos(t)
end

For some reason test(i) gives the same value for every i, whereas the following works as i want it to, note that t doesn't have a value before the subroutine call.

Code:

do i=1,10
t=i
test(i)=a*cos(t)
end do

AlephZero

If your first code you only call the subroutine once. You need to call it inside the loop like this.

Code:

do i=1,10
    t=i
    call asd(y,t,a)
    test(i)=y
end do

You don't really need the variable y. You could do

Code:

do i=1,10
    t=i
    call asd(test(i),t,a)
end do

(Note, you DO need t, because by default t is a real variable and i is an integer, so
call asd(y,i,a) won't work. (It will probably compile, but give you the wrong answers).

snipertje

Quote by AlephZero

If your first code you only call the subroutine once. You need to call it inside the loop like this.

Code:

do i=1,10
    t=i
    call asd(y,t,a)
    test(i)=y
end do

You don't really need the variable y. You could do

Code:

do i=1,10
    t=i
    call asd(test(i),t,a)
end do

(Note, you DO need t, because by default t is a real variable and i is an integer, so
call asd(y,i,a) won't work. (It will probably compile, but give you the wrong answers).

Ok, thanks I think I get it now. As soon as y has a value it's no longer a*cos(t), so by taking the subroutine in the loop, y get's redeclared as a*cos(t) every single loop, after which y can be calculated with a new t again.

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