How to generate random integer that are random "enough"?

Question

I'm trying to solve the 280th problem in Project Euler, and for this I have written the following simulation;

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

/* Directions

    1
2       3
    4
*/

int grid[5][5] = {
    {0, 0, 0, 0, 2},
    {0, 0, 0, 0, 2},
    {0, 0, 0, 0, 2},
    {0, 0, 0, 0, 2},
    {0, 0, 0, 0, 2}
};

int InitPos[2] = {2, 2};

int MaxExp = 5000000;

bool Success = false;
int StepCount = 0;
int ExpNumber = 1;
int AntsBag = 0;
void Init();
void CarryFood(int * pos);
void LeftFood(int * pos);
bool checkMovability(int * pos, int direction);
bool moveToDirection(int pos[2], int direction);
bool checkSuccess();
void ShowResult();


int main(int argc, char const *argv[])
{   
    timeval curTime;
    gettimeofday(&curTime, NULL);
    int milli = curTime.tv_usec / 1000;


    time_t t;
    srand((unsigned)time(&t));

    //timeTData*.txt corresponds to using "time(&t)" above 
    //milliData.txt corresponds to using "milli" variable above
    //timeTUnsigData*.txt corresponds to using "(unsigned)time(&t)" above

    printf("%% Experiment Number : %d \n", MaxExp);
    while(ExpNumber <= MaxExp)
    {
        Init();
        int pos[2];
        pos[0] = InitPos[0];
        pos[1] = InitPos[1];
        do{
            int direction = (rand() % 4) + 1;
            if (moveToDirection(pos, direction))
            {
                StepCount++;    
            }
            if (pos[1] == 4&&grid[pos[0]][4]==2&&AntsBag==0)
            {
                CarryFood(pos);
            }
            if (pos[1] == 0&&grid[pos[0]][0]==0&&AntsBag==2)
            {
                LeftFood(pos);
            }
            checkSuccess();
        }
        while(!Success);

        ShowResult();
        ExpNumber++;
    }   

    return 0;
}

void Init()
{
    Success = false;
    StepCount = 0;
    AntsBag = 0;
    int gridInit[5][5] = {
        {0, 0, 0, 0, 2},
        {0, 0, 0, 0, 2},
        {0, 0, 0, 0, 2},
        {0, 0, 0, 0, 2},
        {0, 0, 0, 0, 2}
    };
    for (int i = 0; i < 5; ++i)
    {
        for (int j = 0; j < 5; ++j)
        {
            grid[i][j] = gridInit[i][j];
        }
    }
}

void ShowResult()
{
    /*
    for (int i = 0; i < 5; ++i)
    {
        printf("\n");
        for (int j = 0; j < 5; ++j)
        {
            printf("%d ", grid[i][j]);
        }
    }
    */
    printf("%d %d\n", StepCount, ExpNumber);
}

void CarryFood(int * pos)
{
        AntsBag = 2;
        grid[pos[0]][4] = 0;
}

void LeftFood(int * pos)
{
        AntsBag = 0;
        grid[pos[0]][0] = 2;
}

bool checkMovability(int * pos, int direction)
{
    switch(direction)
    {
        case 1:
        {
            if(pos[1]==0){
                return false;
            }
            break;
        }
        case 2:
        {
            if (pos[0]==0)
            {
                return false;
            }
            break;
        }
        case 3:
        {
            if (pos[0]==4)
            {
                return false;
            }
            break;
        }
        case 4:
        {
            if (pos[1]==4)
            {
                return false;
            }
            break;
        }
        default:
        {
            printf("Wrong direction input is given!!\n");
            return false;
            break;
        }
    }
    return true;

}

bool moveToDirection(int * pos, int direction)
{
    if ( !checkMovability(pos, direction) )
    {
        return false;
    }

    switch(direction){
        case 1:
        {
            pos[1] -= 1;
            break;
        }
        case 2:
        {
            pos[0] -= 1;
            break;
        }
        case 3:
        {
            pos[0] += 1;
            break;
        }
        case 4:
        {
            pos[1] += 1;
            break;
        }
        default:
        {
            printf("I'm stunned!\n");
            return false;
            break;
        }
    }
    return true;
}

bool checkSuccess()
{
    for (int i = 0; i < 5; ++i)
    {
        if (grid[i][0] != 2)
        {
            return false;
        }
    }
    //printf("Success!\n");
    Success = true;
    return true;
}

And the redirected the output to a *.txt file and find the expected value of the number of steps with the following octave code;

clear
load data.txt
n = data(:,1);

output_precision(15);

mean(n)

%% The actual data

%% milliData1 -> 430.038224000000
%% milliData2 -> 430.031745000000

%% timeTData1 -> 430.029882400000
%% timeTData2 -> 430.019626400000

%% timeUnsigData1 -> 430.028159000000
%% timeUnsigData2 -> 430.009509000000

However, even I run the exact same code twice I get different results, as you can see from the above results.(Note that, I have tried this with different srand(..) inputs for the reason I'm going to explain).

I thought that the reason for this is because how I generate a random integer between 1-4 for the random directions of the ant, because as far as I have been though, the probability distribution of this experiment should be the same as long as I repeat the experiment large number of time (in this particular case 5000000 times).

So my first question is that is it really the problem with the method of how I generate random integers ? If so, how can we overcome this problem, I mean how can we generate integer random enough so that when we repeat the same experiment large number of times, the expected value between those is smaller than these result that I have got ?

Answer 1

You can use something like

std::mt19937 gen{std::random_device{}()};
std::uniform_int_distribution<int> dist{1, 4};

int randNumber = dist(gen);

This generates a more uniform distribution of values.