C++ selecting a number of random items without repeating - c++

Question

Write a program that randomly selects from a bag of eight objects.
Each object can be red, blue, orange, or green, and it can be a ball or a cube.
Assume that the bag contains one object for each combination (one red ball, one
red cube, one orange ball, one orange cube, and so on). Write code similar to
Example 5.3, using two string arrays—one to identify colors and the other to
identify shapes.
I am trying to write a program to carry out the above exercise - the problem I am having is the same object can be selected more than once each time.
This is the code so far
#include "stdafx.h"
#include <iostream>
#include <cstdlib>
#include <ctime>
#include <cmath>
using namespace std;
int rand_0toN1(int n);
void choose_object();
char *colour[4] =
{"Red", "Blue", "Orange", "Green"};
char *object[2] =
{"Ball", "Cube"};
int main()
{
int n, i;
srand(time(NULL)); // Set seed for randomizing.
while (1) {
cout << "Enter no. of objects to draw ";
cout << "(0 to exit): ";
cin >> n;
if (n == 0)
break;
for (i = 1; i <= n; i++)
choose_object();
}
return 0;
}
void choose_object() {
int c; // Random index (0 thru 4) into
// colours array
int o; // Random index (0 thru 2) into
// object array
c = rand_0toN1(4);
o = rand_0toN1(2);
cout << colour[c] << "," << object[o] << endl;
}
int rand_0toN1(int n) {
return rand() % n;
}

Answer 1

Let's try to solve this by making a real world analogy:
Let's say you have a massive jar of marbles, of the colors listed above. It's so massive (infinite size!) that you always have the same chance to draw a marble of a given color, always 1/4 each time.
How would you do this in real life? Would you just keep picking randomly, chucking the marble away as you draw it? Or would you maybe keep a little list of things you've drawn already?
Or maybe you only have one of each in the jar... You wouldn't put it back in would you? Because that's kind of what you're doing here.
Each of these thought paths will lead you to a good solution. I don't want to provide a code or anything because this kind of assignment is one that teaches you how to think like a computer.

Answer 2

Since this is homework, I'm not going to give an exact answer, but describe what you could do:
Keep a list of objects you've already chosen.
After you choose an object, compare that object to the list of objects you've already chosen. If it's in the list, choose another object. If it's not in the list, add it to the list.
Make sure that you don't try to choose more than 8 objects, or else you'll end up in an infinite loop in part 2.
These would go in your choose_object() subroutine. You could do it in a while() loop, something like:
int seen_before = 0;
while(!seen_before) {
pick your random numbers
if(numbers not in list) {
add to list
break
}
}