# Homework Help: Fortran variable value changing randomly

1. Sep 22, 2016

### cpburris

This isn't really a question for which the template is useful..
Can anyone explain to me why the value of the variable 'seed' in location 1 is different from the value in location 2? The only thing happening in between is the defining of the varaible v(1) using the real function. I don't see where in that 'seed' is given a new value. I've tried google searching 'Fortran seed' and such and can't find any answer.

Code (Text):

JRDM = 83721563
call baysprep(JRDM)
! -----------------------------------------
subroutine baysprep(first)
integer first,seed
common /tobays/ v(53),oldbays,seed
save /tobays/
seed = abs(first)
if (seed.gt.2147483647) seed = 24287

do i = 1,53
!Location 1
print*,'seed into do loop iteration=',seed
v(1) = congmult(seed)
!Location 2
print*,'seed coming out of do loop iteration=',seed
end do
!
real function congmult(ix)
!
integer a,p,ix,b15,b16,xhi,xalo,leftlo,fhi,k
data a,b15,b16,p/16807,32768,65536,2147483647/
! assigns variables these values
xhi = ix / b16
xalo = a*(ix - xhi*b16)
leftlo = xalo / b16
fhi = xhi*a + leftlo
k = fhi / b15
ix = (((xalo-leftlo*b16) - p) + (fhi-k*b15)*b16) + k
if (ix.lt.0) ix = ix + p
congmult = real(ix)/2.147483647e+09
end

Last edited by a moderator: Sep 22, 2016
2. Sep 22, 2016

### Staff: Mentor

Hi cpburris. Please use the template for posts to the homework forums regardless of whether you believe it is "suitable" or not. It's just one of those rules we're picky about

Check to see if your function parameters are call-by-reference or call-by-value. I see that seed is passed to function congmult which takes takes parameter ix and modifies it, so try printing out its value from inside that function just before it returns. See if the value shown matches that of 'seed coming out of do loop iteration'.

3. Sep 22, 2016

### cpburris

Ah, yes you were correct. The modification of the value by the function was being passed back. Thank you very much.

4. Sep 22, 2016

### Staff: Mentor

You'e welcome. Glad I could help.

5. Sep 22, 2016

### Staff: Mentor

The latter doesn't exist in Fortran. All variables are passed by reference.

@cpburris: Note that this is how this RNG keeps track of where it is in the sequence. You give it an initial seed, but then it modifies it every time a new random number is generated. It is important not to change the value of seed between calls to congmult.

6. Sep 22, 2016

### Staff: Mentor

It's something that programmers need to be aware of for whatever language they are using. Discovering the hard way is one path to enlightenment