How to make sure that results are always positive?

Hi, i have written a program which would give me a continuous outputs in the form of a histogram, using a set of for and while loops. I would like to know, how i can ensure that the results of the equations are always positive, ie, all the histogram columns lie in the positive axis? Here is the code i have written. All inputs are welcome and appreciated.

M = 1000.*ones(1,1000);
p = rand;
d = 1000;
n = randi(d);
m = randi(d);
for i = 1:1000
   while(M(1,n)>0||M(1,m)>0)
     n = randi(d);
     m = randi(d);
     if n==m % This loop ensures that the values of n and m remain distinct%
      while n==m  
          n = randi(d);
          m = randi(d);
          if ne(n,m)==1
              break;
          end
      end 
     end
   M(1,n) = M(1,n) - (p.*( M(1,n)+M(1,m)))./2;
   M(1,m) = M(1,m) + (p.*( M(1,n)+M(1,m)))./2;
       if (M(1,n)<0&&M(1,m)<0)
       break;
       end
   end
 end

Answer 1

[Stef van Haaren]

Use abs()