Convert one dimensional array to two dimensional array

Question

For my homework it is given one dimensional array and i have to convert it in a two dimensional array. The two dimensional array has 2 for the number of columns, because i have to represent the one dimensional array as pairs(the value of the number, the number of appearences in the array). This is what have tried. The error appears on the last 2 lines of code: access violation writing location 0xfdfdfdfd.

#include <iostream>
#include <stdlib.h>
using namespace std;

int main()
{
    const int NR=17;
    int arr[NR]={6,7,3,1,3,2,4,4,7,5,1,1,5,6,6,4,5};
    int **newArr;
    int count=0;
    int countLines=0;
    int searched;
    for(int i=0;i<NR;i++)
    {
            newArr=new int*[countLines];
        for(int i=0;i<countLines;i++)
        {
            newArr[i]=new int[2];
        }
        searched=arr[i];
        if(i>0)
        {
            for(int k=0;k<countLines;k++)
            {
                if(newArr[countLines][0] == searched)
                {
                    searched=arr[i]++;
                }

                for(int j=0;j<NR;j++)
                {
                    if(searched==arr[j])
                    {
                        count++;
                    }
                }
                countLines++;
            }
        }
        else
        {
            for(int j=0;j<NR;j++)
            {
                if(searched==arr[j])
                {
                    count++;
                }
            }
            countLines++;
        }

        newArr[countLines][0]=searched;
        newArr[countLines][1]=count;
    }
}

Answer 1

First you are using newArr in the first loop before allocating it any memory. You cannot dereference a pointer which owns no legal memory. It results in undefined behavior.

Secondly in the last part, you are allocating newArr a memory equal to countLines thus.

newArr = new int*[countLines] ;

It means that the indices in the first dimension of newArr are 0------>countLines-1 . Doing newArr[countLines][0] = searched ; is again undefined. Make it newArr[countLines - 1] .

Answer 2

I'm not going to bother with a line-by-line code analysis since (a) you're changing it while people are answering your question and (b) it would literally take too long. But here's a summary (non-exhaustive) of klunkers:

1. You are leaking memory (newArr) on each loop iteration starting with the second.
2. You're out-of-bounds on your array access multiple times.
3. You should not need to use a pointer array at all to solve this. A single array of dimension [N][2] where N is the number of unique values.

One (of countless many) way you can solve this problem is presented below:

#include <iostream>
#include <algorithm>

int main()
{
    // 0. Declare array and length
    int arr[]={6,7,3,1,3,2,4,4,7,5,1,1,5,6,6,4,5};
    const size_t NR = sizeof(arr)/sizeof(arr[0]);

    // 1. sort the input array
    std::sort(arr, arr+NR);

    /* alternaive sort. for this input size bubble-sort is
       more than adequate, in case your limited to not being
       allowed to use the standard library sort */
    /*
    for (size_t i=0;i<NR;++i)
        for (size_t j=i+1;j<NR;++j)
            if (arr[i] > arr[j])
            {
                arr[i] ^= arr[j];
                arr[j] ^= arr[i];
                arr[i] ^= arr[j];
            }
    */

    // 2. single scan to determine distinct values
    size_t unique = 1;
    for (size_t i=1;i<NR;++i)
        if (arr[i] != arr[i-1])
            unique++;

    // 3. Allocate a [unique][2] array
    int (*newArr)[2] = new int[unique][2];

    // 4. Walk array once more, accumulating counts
    size_t j=0;
    newArr[j][0] = arr[0];
    newArr[j][1] = 1;
    for (size_t i=1;i<NR;++i)
    {
        if (arr[i] != arr[i-1])
        {
            newArr[++j][0] = arr[i];
            newArr[j][1] = 0;
        }
        ++newArr[j][1];
    }

    // 5. Dump output
    for (size_t i=0;i<unique;++i)
        cout << newArr[i][0] << " : " << newArr[i][1] << endl;

    delete [] newArr;

    return EXIT_SUCCESS;
}

Output

1 : 3
2 : 1
3 : 2
4 : 3
5 : 3
6 : 3
7 : 2