Random Walk Simulation in 1D for 10 or more particles

[SalamuB]

Hello,

I have written a code for Random Walk in 1D for 1 particle. And I calculated its Mean square Density for 1 particle. Now I am trying to calculate the MSD for 10 particles, but I get something wrong. The value of MSD should increase when the particles are added. But unfortunately, I get almost the same numbers. Can anyone please have a look and see what is wrong with the code? here is the code:

#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#include <time.h>
#define N 100  /*flipping 100 times*/
#define T 10 /*for 10 particles*/


double rndm(void)
{
	return(double)rand()/RAND_MAX;
}

/*Function to simulate random walk in 1D*/

double walk(int start)
{
	
    double a;
    int i,x;
	
    x=start;
	
    for(i=0; i<N; i++)
	{
		
        a=rndm();
        if(a<0.5)    /*Choses which direction to move in*/
			x++;
        
		else 
			x--;
		
	}
	return x; 
}

/*Main Function*/

int main () {
    FILE *fout;
    int i,j,start,random[N],x;
    double m;
    srand(time(NULL));
    fout=fopen("Walk.txt", "w");   //Open The File
	
	
    printf("What would you like the initial x position to be?\n:");  
    scanf("%d", &start);                        //Define Start position for x
	
    for(j=1;j<=T;j++){
    	for(i=0; i<N; i++) {

        x=walk(start);       // Generates Random Number
        random[i]=x;
	  
        
	fprintf(fout, "%d\n", random[i]); // Writes Number To File
	}
	
	m+=(random[i]-start)*(random[i]-start); /*MSD for 10 particles*/
	m=m/T;
	fprintf(fout, "The MSD for %d particle(s) is: %lf\n",j,m);
	
}


	double p = 0;
    for (i=0; i<N; i++){
	p+=(random[i]-start)*(random[i]-start);}
    p=p/N;
    fprintf(fout, "The MSD for 1 particle: %lf\n",p); /*Here I calculate MSD for 1 particle*/
	fclose(fout);
	return 0;  
}

[Salem]

Perhaps you're starting with garbage...

$ gcc -Wall -Wextra -O2 bar.c
bar.c: In function ‘main’:
bar.c:49: warning: ignoring return value of ‘scanf’, declared with attribute warn_unused_result
bar.c:61: warning: array subscript is above array bounds
bar.c:43: warning: ‘m’ may be used uninitialized in this function

The MSD for 10 particle(s) is: -171906769.316143
The MSD for 1 particle: 101.160000