Having trouble sorting my array. Why won't my array sort properly?

Question

My program needs to read in the size of an array from the command line, it then should fill the array with random numbers. It should then display the sorted and unsorted contents of the array.

Created one for loop in order to read random numbers into the array and to display unsorted contents. Created a second for loop nested in the first one in order to sort the contents within the array.

#include <time.h>
#include <iostream>
#include <iomanip>
#include <cstdlib>


using namespace std;

int main(int argc, char * argv[], char **env)
{

    int SIZE = atoi(argv[1]);
    int *array = new int[SIZE];



    for(int i = 0; i < SIZE; i++)
    {
        array[i] = rand()% 1000;
        cout << i << ": " << array[i] << "\n";

        for(int j = 0; j < SIZE; j++)
        {
            if(array[j] > array[j + 1])
            {
                swap(array[j], array[j +1]);
            }

            cout << i << ": " << array[i] << "\n";
        }


    }


    return 0;
    }



I expect an output like

0: 350
1: 264
2: 897

0:264
1:350
2:897

I'm getting an output like
0:41
0:41
0:41
0:41
1:46
1:46
1:0
1:0
2:334
2:334
2:334
2:334

Answer 1

Because you are attempting to continually sort the array before it's initialized. Also, you have undefined behavior when j == SIZE-1 because it's illegal to compare array[j] > array[j+1] at that point. array[j+1] is an invalid index.

Instead of this:

for(int i = 0; i < SIZE; i++)
{
    array[i] = rand()% 1000;
    cout << i << ": " << array[i] << "\n";

    for(int j = 0; j < SIZE; j++)
    {
        if(array[j] > array[j + 1])
        {
            swap(array[j], array[j +1]);
        }

        cout << i << ": " << array[i] << "\n";
    }
}

This instead:

   // randomly initialize the array
   for(int i = 0; i < SIZE; i++)
   {
        array[i] = rand()% 1000;
        cout << i << ": " << array[i] << "\n";
   }

   // sort
   for (int i = 0; i < SIZE; i++)
   {
       for(int j = i+1; j < SIZE; j++)
       {
            if(array[i] > array[j])
            {
                swap(array[i], array[j]);
            }
       }
   }

   // print results
   cout << "finished sorting" << endl;
   for(int i = 0; i < SIZE; i++)
   {
        cout << i << ": " << array[i] << "\n";
   }

Answer 2

The immediate issue is that your code is attempting to sort the array even before it's been fully initialized.

You need to split your main loop into three:

• the first one to initialize the array;
• the second one to sort the array (this would include an inner loop used by bubble sort);
• the third one to print the results.

Separately, you also need to think carefully about the boundary conditions of your loops.