Modifying a dynamic 2D array in a function

Question

I've got a function that accepts a dynamic multidimensional array (which is initialized to 0) as a parameter, and I'm trying to modify certain values within the array in my function.

The function that accepts the array as a parameter is supposed to simulate the roll of two dice and output the frequency distribution to the array I made that's initialized to zero.

The code for it is as follows:

#include <iostream>
#include <cstdlib>

using namespace std;

int** rollDie(int numRolls, unsigned short seed, int** &rollarray)
{
srand(seed);
int side1, side2;
while (numRolls > 0)
{
side1 = 1 + rand() % 6;
side2 = 1 + rand() % 6;

rollarray[side1][side2]++;

numRolls--;
}
return rollarray;

}

int** initializeArray(void)
{
    int i, j;
    int** m = new int*[6];

    for (i = 0; i < 6; i++)
        m[i] = new int[6];

    for (i = 0; i < 6; i++)
        for (j = 0; j < 6; j++)
            m[i][j] = 0;

    return m;
}

int main()
{
int numRolls;
unsigned short seed;
int ** a = initializeArray();


cout << "rolls?\n";
cin >> numRolls;

cout << "seed?\n";
cin >> seed;
int ** b = rollDie(numRolls, seed, a);

int i,j;
for (i = 0; i < 6; i++) {
    for (j = 0; j < 6; j++) {
        cout << b[i][j];
    }
    cout << "\n";
    }

}

a is initialized in a function that makes a 6x6 array of zeroes. I tested that in main and was able to print out the array properly. — bc8787
– bc8787, Commented Jul 19, 2013 at 1:12
Sure, you're not passing it as a reference, you should use int** &rollarray instead, or make a copy and return the modified copy, or int &rollarray[][], for better reading — DGomez
– DGomez, Commented Jul 19, 2013 at 1:21
@bc8787 - you declared a and the initializeArray perfectly. as far as I can see the bug is only in the 1+rand()%6 line - just remove the 1+. Oh, and BTW - you don't need b at all! rollDie doesn't need to return anything - it changes the data pointed to by a, just print a instead of b after the rollDie\ — rabensky
– rabensky, Commented Jul 19, 2013 at 1:56

rabensky · Accepted Answer · 2013-07-19 01:34:27Z

1

Code works for me with just a few issues (I had to guess how you defined a. Next time add that too):

In the printing you should print a space after every number (minor)

In the random, you choose index as 1+rand()%6, so from 1 to 6, but when you print you take indexes from 0 to 5! So your first row and first column will be 0.

Other than that it seems to work.

answered Jul 19, 2013 at 1:34

rabensky

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Steve · Accepted Answer · 2013-07-19 02:01:04Z

0

Only when one goes and does something else does the answer come to mind. I suspect you declared a as:

int a[6][6];

which is an array of 36 integers. In your function, though, you're declaring rollarray to be a pointer to an array of pointers to integers. All you need to do is change the function signature to:

int* rollDie(int numRolls, unsigned short seed, int* rollarray)

As cluracan said, you also want to use array indices in the range 0 to 5.

This is a good case for either the judicious use of print statements or stepping through with a debugger to see what's really going on.

answered Jul 19, 2013 at 2:01

Steve

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2 Comments

rabensky Over a year ago

He edited his question to show he declared a as an array of arrays

Steve Over a year ago

Which, of course, leads to the question "Why??? Why make it so much more complicated than it needs to be?" :-)

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