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lizi_n

Feb 7th, 2008 5:09 PM

random numbers fortran 90

I need to randomly generate an equal number of +,- 1's. I have used [call random_number] and made every number >0.5=1 and any number <0.5=-1. It's in a loop from 1..n but I can't work out how to make sure that there are exactly the same amount of each. Please help!
Lizi

Sane	Feb 7th, 2008 6:09 PM

Re: random numbers fortran 90

You want an equal amount, but you want it random? A little strange, but okay. I'd suggest the following.

If you want there to be 10 numbers, then, make an array of 5 ones and 5 negative ones.

{+1, +1, +1, +1, +1, -1, -1, -1, -1, -1}

Loop through each index. Each time, generate a random number between (0 - 9). Use that number to determine which position the current element should be swapped with. Swapping 10 elements 10 times, will effectively produce a random array of perfectly uniform distribution.

0th index

Random Number: 9

Swap 9th element with 0th element

{-1, +1, +1, +1, +1, -1, -1, -1, -1, +1}



1st index

Random Number: 2

Swap 2nd element with 1st element

{-1, +1, +1, +1, +1, -1, -1, -1, -1, +1}



... etc ...



9th index

Random Number: 5

Swap 9th element with 5th element

{+1, +1, -1, -1, +1, +1, -1, -1, +1, -1}

In Pseudocode:

n = 10



Make array X [Size n]

X[0 .. n/2-1] = {1}

X[n/2 .. n-1] = {-1} 



for i : 0 ... n-1

    j = random number (0, n-1)

    swap(X[i], X[j])



output X

lizi_n

Feb 11th, 2008 3:09 PM

Re: random numbers fortran 90

Thank you that's awesome. Could you just tell me what I write in order to swap the elements. At the moment I think I'm just replacing them by typing

x(i)=x(j)
x(j)=x(i)

so I end up with an uneven amount of +1 and -1.
Thank you!

lizi_n

Feb 11th, 2008 3:18 PM

Re: random numbers fortran 90

This is what I've got so far if it helps.

integer:: n

parameter(n=10)



integer:: i,t,h

real:: s

integer, dimension(n) :: x



do i=1,n/2

x(i)=1

end do



do i=n/2+1,n

x(i)=-1

end do



print '(10i2)',x



do i=1,n



call random_number(s)

s=n*s



do h=0,n-1

if(s.gt.h.and.s.lt.h+1) t=h+1

end do





x(i)=x(t)

x(t)=x(i)



print*,x(i)





end do

Sane	Feb 11th, 2008 3:30 PM

Re: random numbers fortran 90

x(i)=x(j)

x(j)=x(i)

That is close, but examine what happens when you swap two elements. The value of the first element is overwritten, by the time you assign it a new value.

E.G. Let x(i) be 5, and x(j) be 6.



x(i) becomes x(j) = 6

x(j) becomes x(i) = 6 as well

The solution to this is to use a temporary variable, let's say temp, to remember the first value, before the swapping occurs.

temp = x(i)

x(i) = x(j)

x(j) = temp

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