Question

In the program below, when the program goes into the while loop, it stops working. When I tried to assisgn values without the loop, it woked fine.
Can anybody tell me what the problem is?
I am using Visual Studio 2010.

#include <stdio.h>
#include<iostream.h>
#include <conio.h>

using namespace std;
int random(int min,int max);

struct pts
{
    int x;
    int y;
};

int main()
{
    struct pts *p;
    int w = 600,h=400;
    int nmax,kmax,k=0,n=0;

    while(k<5)
    { 
        p[0].x = random(0,h-1);
        p[1].y = random(0,w-1);
        cout << p[0].x << "   " << p[1].y << "\n";
        k++;
     }
     getch();
     return 0;
}

int random(int min,int max)
{
    int n=0;
    n=(rand()%(max-min+1))+min;
    return n;
}

Answer 1

The while loop itself is working fine. It’s just that you’re corrupting memory. Your problem is with:

struct pts *p;
p[0].x = whatever;

with no intervening setting of p to a valid memory block. In other words, you are using an unitialised pointer, hence undefined behaviour, hence all bets are off.

Since you (currently) only seem to be using p[0] and p[1], you can probably change:

struct pts *p;

to:

struct pts p[2];

Of course, if you want a variable structure, you can use:

struct pts *p = new pts[500];

replacing 500 with whatever size you want, of course.

---

You might also want to consider upgrading to a more recent compiler, iostream.h and conio.h are anachronisms.

Here’s a complete program that shows how to do it, at least until you decide to store things a little more “sanely” inside the loop:

#include <iostream>
#include <stdlib.h>

struct pts
{
    int x;
    int y;
};

int random(int min,int max)
{
    int n=0;
    n=(rand()%(max-min+1))+min;
    return n;
}

int main()
{
    struct pts p[2];
    int w = 600,h=400;
    int k=0;

    while(k<5)
    {
        p[0].x = random(0,h-1);
        p[1].y = random(0,w-1);
        std::cout << p[0].x << "   " << p[1].y << "\n";
        k++;
     }
     return 0;
}

A sample run of that gives:

183   286
377   115
193   535
186   492
249   421

(and, in fact, will probably always give that sequence since you don’t call srand to set the seed – your numbers may well be different to mine but they’ll give you the same sequence every time).

---

If you want a better baseline to start with, see:

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

struct pts { int x; int y; };

int random (int min, int max) {
    return (rand() % (max - min + 1)) + min;
}

int main (void) {
    pts *p = new pts[5];
    int w = 600, h = 400;
    int k = 0;

    srand (time (0));
    while (k < 5) {
        p[k].x = random (0, h - 1);
        p[k].y = random (0, w - 1);
        std::cout << p[k].x << "   " << p[k].y << "\n";
        k++;
    }
    delete[] p;
    return 0;
}

This shortens the code a bit by removing unnecessary stuff, and gets rid of your undefined-behaviour problem.

It also initialises the random number generator and populates the array properly.