Random Walk Simulation in 1D for 10 or more particles

This is a discussion on Random Walk Simulation in 1D for 10 or more particles within the C Programming forums, part of the General Programming Boards category; Hello, I have written a code for Random Walk in 1D for 1 particle. And I calculated its Mean square ...

  1. #1
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    Dec 2010
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    Question Random Walk Simulation in 1D for 10 or more particles

    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:

    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;  
    }

  2. #2
    and the hat of wrongness Salem's Avatar
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    Perhaps you're starting with garbage...
    Code:
    $ 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
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
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