Program runs fine, then crashes 5% of the time

Hi folks,

I'm writing a small simulation of bouncing hard spheres using the Metropolis algorithm. Now my problem is that the program runs fine almost always, also when I vary certain parameters. However, every 20th or so time I run the program it crashes after completion. I have something like 'cout << "Done.";' at the end, and even when the program crashes, this last line is still displayed. I don't even know where to begin looking for bugs since I cannot reliably reproduce the crash.

I'm working on an Acer notebook, Win7 with an i7 720, 4GB RAM.

Now I think I have narrowed the problem down to the following lines (describing a collision check between the spheres after one of them has been moved - if they overlap after the move, the boolean variable overlap prohibits this move):

Code:

` `

for (int j=0;j<N;j++)

{

if (j==n) continue;

if (dist(posbuff,0,pos,3*j)<=d) {overlap=true;Nacc-=1;break;}

//else overlap=false;

}

if (overlap==false) {buff2pos(n);}

cout << "\n" << Nacc << endl;

with the definitions:

Code:

`float *pos,posbuff[3];`

pos=(float *) malloc(3*N*sizeof(float));

void pos2buff(int n)

{posbuff[0]=pos[3*n];posbuff[1]=pos[3*n+1];posbuff[2]=pos[3*n+2];}

void buff2pos(int n)

{pos[3*n]=posbuff[0];pos[3*n+1]=posbuff[1];pos[3*n+2]=posbuff[2];}

float dist(float* a,int i,float* b,int j)

{return sqrt((a[i]-b[j])*(a[i]-b[j]) + (a[i+1]-b[j+1])*(a[i+1]-b[j+1]) + (a[i+2]-b[j+2])*(a[i+2]-b[j+2])) ;}

"n" in this case is the sphere that is being moved and hence is left out in the comparison. pos2buff and buff2pos just swap between a dummy variable, dist measures the distance between two spheres. Nacc counts the number of accepted moves.

I hope the above code suffices because the actual program is about 100 lines without much dangerous stuff (I believe).

Hope someone can help me here, or point me in a certain direction. The program is eventually supposed to run for hours, so a crash at the very end is not exactly what I'm looking for... ;)

Thanks in advance,

Matti

PS: Please show mercy on me - I'm still a beginner.