Re: 3/4 = 0 ???

[Rick Riolo]

This is because numOthers and numNeighbors are int's and
when C (and so objectiveC) divides int's (or unsigned int's)
it truncates, not rounds.

- r

Rick Riolo                           address@hidden
Center for Study of Complex Systems (CSCS)
4477 Randall Lab                
University of Michigan         Ann Arbor MI 48109-1120
Phone: 734 763 3323                  Fax: 734 763 9267
http://www.pscs.umich.edu/PEOPLE/rlr-home.html

On Wed, 21 Jul 1999, DARREN MATTHEW SCHREIBER wrote:

> Date: Wed, 21 Jul 1999 12:12:40 -0700 (PDT)
> From: DARREN MATTHEW SCHREIBER <address@hidden>
> Reply-To: address@hidden
> To: address@hidden
> Subject: 3/4 = 0 ???
> 
> 
> 
> I am trying to debug this simple method from a Schelling model.  The
> function is to look at its neighbors, see their color, and report the
> fraction of neighbors that are of a different color.  The result of the
> internal print statements look correct except that the math is wrong.
> 
> Fraction is defined as Others/Neighbors, but if it outputs others=3 and
> neighbors=4 then fraction comes out as 0... why?
> 
> Any number divided by itself gives one, but any other division results in
> 0.000.  This just doesn't make any sense to me and I am wondering if there
> is something about float that I need to know?
> 
>       Darren
> 
> 
> 
> -(float)getFractionOf: (int) otherColor {
>       float fraction=0.0;
>       int xvals[4]={0,0,1,-1};
>       int yvals[4]={-1,1,0,0};
>       int i,j;
>       int numNeighbors=0; //added for Version 2.5
>       int numOthers=0;    //added for Version 2.5
>               Person * neighbor;
> 
>       if(myType==1) 
>         {
>           // This enforces a VonNeuman neighborhood
>           
>           for(i=0;i<4;i++)
>             {
>               //printf(" %d : %d, %d ",i, myX+xvals[i], myY+yvals[i]);
>               if ((((myX + xvals[i]) < worldSize) && ((myX + xvals[i])
> >=0)) 
>                   &&   (((myY + yvals[i]) < worldSize) && ((myY +
> yvals[i]) >= 0)))
>                 //keeps us on a 2d grid
>                 {
>                   numNeighbors=numNeighbors + 1;
>                   if ((neighbor=[myWorld getObjectAtX: myX + xvals[i] Y:
> myY+yvals[i]]))
>                     if ([neighbor getColor]==otherColor)
>                       numOthers=numOthers+1;
>                 }  
>               printf("X %d , Y %d ",myX, myY);
>               printf("Others %d/Neighbors%d    ", numOthers,
> numNeighbors);
>               if (numNeighbors != 0)
>                 fraction=(numOthers/numNeighbors);
>               else
>                 fraction=100.0;  // This is a warning flag for division
> by zero
>               printf("Fraction   %3.3f \n", fraction);
>             }
>         } 
>       else
>         {
>         // This enforces a Moore neighborhood
>           printf("Moore");
>           for(i=-1;i<2;i++) 
>             for(j=-1;j<2;j++) 
>               if (((myX+xvals[i]<worldSize) && (myY+yvals[i]<worldSize)) 
>                   && (!(myX+xvals[i]<0) && !(myY+yvals[i]<0))) //keeps
> us on a 2d grid
>                 {
>                   numNeighbors=numNeighbors + 1;
>                   if ((neighbor=[myWorld getObjectAtX: myX + xvals[i] Y:
> myY+yvals[i]]))
>                     if ([neighbor getColor]==otherColor)
>                       numOthers=numOthers+1;
>                 }
>         }
>      
>        
>       return fraction;
> }
> 
> 
> 
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