Random walk simulation in one dimension using random number generator

Question

I am trying to develop a random walk simulation in one dimension. I am modelling a particle along a line with 10 discrete positions that particle can occupy. The particle can only move left or right one space each time it 'hops'. In this simulation I get the code to account for 20 hops. A random number generator produces a 0 - for left and 1 - for right before each 'hop' is executed to tell the "particle" to go left or right. Please see code below and further comments.

#include <stdio.h> 
#include <stdlib.h>
#include <time.h>

int randomInt( int max); // declaration of function //

int main() 
{ // declaration of variables //
  int i, j = 0;
  int totalHops=20; 
  int seed;
  int sites[10] = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10}; 
  int *location;
  location = &sites[j];

  seed = time(NULL); 

  printf("The seed value for this 'random walk' is: %d.\n\n", seed); //keeping track of the seed used// 

  // setup the random number generator // 
  srandom(seed);  // random path sequence //
  printf("Initial location of the particle is at site %d.\n\n", *location);
  for ( i=1; i<totalHops+1; i++ ) 
  {

    int direction = randomInt(2); // Two possible directions for particle to move, 0 = left, 1 = right // 

    if (direction == 1) {
        location+= 1; // pointer moves right one space on array //
    }
    else {
        location-= 1; // pointer moves left one space on array // 
    }

    printf("After %2.d hop(s), the particle is at site %d.\n", i, (*location)%10);  // I  would prefer to be printing the entry of my array rather than relying on the fact the array is lablled with each entry the positon I have changed the pointer to //
  }
  printf("\n");
}

// function definition //
int randomInt(int max)
{
    return (random() % max);
}

My output does not follow each time the sort of patterns I am expecting. It appears to output that the particle is in postion 0 in one iteration and in the next all of a sudden be in postion 4, as an example. I would prefer that I were printing the entry of the sites[] array instead of inputing the postion into each entry and printing the value of the pointer.

I would be most grateful for anyones help here. I am new to pointers so any help would be much appreciated.

Please rephrase your question so it's minimal and more generic, so it'd be easier to answer AND will benefit future generations — GalAbra, Apr 13 '20 at 11:09
The return type of `time()` is `time_t`. Your `int` may be too small. It also means: _turn warnings of your compiler on!_ — Paul Ogilvie, Apr 13 '20 at 11:21

ravivasani75 · Answer 1 · 2020-04-13T11:42:31.220

Consider following part:

if (direction == 1) {
        location+= 1; // pointer moves right one space on array //
    }
else {
    location-= 1; // pointer moves left one space on array // 
}

Now, consider when the for loop executing first iteration. What if direction is 0 ? The location will become &sites[0] - 1, off course it will out of array. So, you should put conditions for boundary checking for array sites.

score 0 · Answer 2 · answered Apr 14 '20 at 11:40

This fixes the probelm I was having. I decided to automate the array that indicates the position of the particle by filling up the elements with an integer corresponding to the position via a for loop. If there are any recommendatons on how to make this program more efficient or easier to read dont hesitate to comment.

#include <stdio.h> 
#include <stdlib.h>
#include <time.h>
int randomInt( int max);
int main() 
{ 
  int i, j = 0;
  const int L = 10;
  const int totalHops=20; 
  int sites[L] = {};
  for (int k=0; k<L; k++) 
  {sites[k]=k+1;} 
  int *location;
  location = &sites[j];
  srandom(time(NULL));  
  printf("\nThe seed value for this 'random walk' is: %ld.\n\n", time(NULL)); 
  printf("Initial location of the particle is at site %d.\n\n", *location);
    tracker[0] = sites[0];
  for ( i=1; i<=totalHops; i++ ) 
  { 
    int direction = randomInt(2); 
    // Two possible directions for particle to move, 0 = left, 1 = right // 
    if (direction == 1) {
        location+= 1; // pointer moves right one space on array //
        if (location == &sites[L-1]+1) // takes particle from site 10 to 1 //
        {
            location = &sites[0];
        }
    }   else if (direction == 0) {
        location-= 1; // pointer moves left one space on array //
        if (location == &sites[0] - 1) // takes particle from site 1 to 10 //
        {
            location = &sites[L-1]; 
        }  
    }
    printf("After %2.d hop(s), the particle is at site %d.\n", i, (*location));
  } printf("\n");
}
// function definition //
int randomInt(int max)
{
    return (random() % max);
}

Random walk simulation in one dimension using random number generator

2 Answers2