Question

For some reason my outer for loop does not seem to be doing anything, I have checked all the paranthesis and everything looks ok but it is still not looping.
With this program I want to sum 50 random numbers (the numbers can either be 1 or -1…it’s for a computational physics problem) and print the magnitude, which the program does. BUT I want to go further and do this 10 times, and calculate the average magnitude.
I know what I need to do I’m just having a problem with this loop.

#include<stdio.h>
#include<math.h>
#include<stdlib.h>
#include<time.h> //Neeed this to seed the random generator with time, or else it      will always generate the same numbers.

//This is a program to calculate the magnitude of the displacement of a particle after    random collisions.
#define RAND_MAX 1
int main()
{   
    //using ints because the particle can only move one unit on x-axis at a time.
    int i, x, displacement, sum = 0, avg;
    int total_disp=0, mag_disp;
    srand(time(NULL));


    //Each collision causes the particle to advance or retreat by one unit on the x axis.

    for(i=0; i<10; i++)
    {   
        for (i=0; i<50; i++)    //for 50 collisions
        {

            x = rand() % 2;  //generates random numbers between 0 and 1.
            if (x==0)   //if x is 0 then it was a displacement in the minus x direction
            {
                displacement = -1;
            }
            else {  //if x is 1 it is a displacement in the minus x direction
                displacement = 1;
            }
            printf("the disp is : %d\n", displacement);
            total_disp = total_disp + displacement;     //sum the displacements

        }

        if (total_disp < 0) {
            mag_disp = total_disp * -1;
        }

        else{ mag_disp = total_disp; }

        printf("The total displacement is: %d\n", mag_disp);
        sum = sum + mag_disp;   //sum of the displacement magnitudes, there should be ten of them in this case
        avg = sum / i; //average displacement is the sum of all the magnitudes divded by the number of times this is performed.
        printf("The average displacement for i = %d particles is: %d", i, avg);
    }
    return 0;
}

Answer 1

You cannot use the same iteration variable in both loops.

for(i=0; i<10; i++)
    for (i=0; i<50; i++)

increments i to 50 in the first iteration of the outer loop.

for(i=0; i<10; i++)
    for (j=0; j<50; j++)

will work.