Random Walk Simulation in 1D - Undesired output

This is a discussion on Random Walk Simulation in 1D - Undesired output within the C Programming forums, part of the General Programming Boards category; Hello Guys, I am struggling with the code for simulation of Random Walk in 1D. It compiles, but the output ...

  1. #1
    Registered User
    Join Date
    Dec 2010
    Posts
    20

    Question Random Walk Simulation in 1D - Undesired output

    Hello Guys, I am struggling with the code for simulation of Random Walk in 1D. It compiles, but the output to text file "Walk.txt" are all almost the same negative numbers. BTW I am a new to C programming, if you could look at the code and give your ideas about what is wrong with it. Here is the code itself:

    Code:
    #include <stdio.h>
    #include <stdlib.h>
    #include <math.h>
    #include <time.h>
    #define N 10
     
     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=x+1);
            }
                if (a>0.5) {
            (x=x-1);
            }  
            }
            return x;  
            }
             
    /*Main Function*/
         
    int main () {
        FILE *fout;
        int i,start,random[N],x;
        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(i=0; i<N; i++) {
    
            x=walk(start);       // Generates Random Number
            random[i]=x;     
            fprintf(fout, "%d\n", random[i]);   // Writes Number To File
            }
    
           double p = 0;
    	for (i=0; i<N; i++){
    		p+=(random[i]-start)*(random[i]-start);}
    	p=p/N;
    	fprintf(fout, "%lf\n",p);
    fclose(fout);
    return 0;
             
    }
    In this part, i've tried to calculate the mean square displacement (position):

    Code:
    double p = 0;
    	for (i=0; i<N; i++){
    		p+=(random[i]-start)*(random[i]-start);}
    	p=p/N;
    	fprintf(fout, "%lf\n",p);
    fclose(fout);
    return 0;
             
    }
    And the output in "Walk.txt" is :

    Code:
    -1079135414
    -1079135418
    -1079135418
    -1079135406
    -1079135418
    -1079135416
    -1079135418
    -1079135416
    -1079135414
    -1079135416
    12.400000

  2. #2
    - - - - - - - - oogabooga's Avatar
    Join Date
    Jan 2008
    Posts
    2,808
    (x=x+1);
    Do not put parens around a statement for no reason. In fact, you should be using the increment operator here.
    x++;
    Same for (x=x-1);, replace it with x--;

    In walk, use an if/else like this:
    Code:
    if (a<0.5)
        x++;
    else
        x--;
    But your program seems to essentially work for me. Here's the output I got:
    102
    102
    100
    94
    98
    100
    100
    98
    98
    110
    15.600000

  3. #3
    Registered User
    Join Date
    Dec 2010
    Posts
    20
    oogabooga, thank you very much, it did work now, I dont really get why it didnt before if you say it ran fine in urs. But apparently, something was wrong with my code. When I put your statement, it gave the desired output. thank you.

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