Why does this merge sort code keeps giving me this weird output?

Question

I wrote the count sort code but it is showing me weird output, can you tell where I am wrong ??

#include <bits/stdc++.h>
using namespace std;

void CountSort(int a[], int n, int k)
{
    int count[k + 1]={0};
    int b[n];

    for (int i = 0; i < n; i++)
    {
        ++count[a[i]];
    }
    for (int i = 1; i <= k; i++)
    {
        count[i] += count[i - 1];
    }
    for (int i = 0; i >= 0; i--)
    {
        b[count[a[i]]-1] = a[i];
        --count[a[i]];
    }
    for (int i = 0; i < n; i++)
    {
        a[i] = b[i];
    }
}

int main()
{
    int a[] = {2, 1, 1, 0, 2, 5, 4, 0, 2, 8, 7, 7, 9, 2, 0, 1, 9};
    CountSort(a, 17, 9);
    cout<<"The sorted array is  ->  "<<a;
    return 0;
}

It gives output like this -

The sorted array is  ->  0x7bfdd0

ScreenShot of the code and the output

who told you `cout << a;` would be the way to print elements of an array? — 463035818_is_not_an_ai, Sep 15 '21 at 13:50
There is no universally accepted "correct way" to print the contents of an array. See [Printing an array in C++?](https://stackoverflow.com/questions/1370323/printing-an-array-in-c) — Drew Dormann, Sep 15 '21 at 13:53
Side note: `int b[n]` is a Variable Length Array, which is not valid C++. It may be supported as a non-standard extension on some compilers, but you should unlearn it. Also `using namespace std` and include bits/stdc++.h are considered things you should avoid. — , Sep 15 '21 at 13:57
Why title says "merge sort" and code says "CountSort" (this is invalid implementation of count sort)? Do you know what are you suppose to do? — Marek R, Sep 15 '21 at 14:38

score 2 · Answer 1 · answered Sep 15 '21 at 13:54

When trying to print the array, you cannot just use std::cout << a. At the moment your code is printing the memory address of the array a, which is not what you want.

To fix the problem, print all elements of the array, one by one. These loops can be helpful:

for (const auto& elem : a)
    std::cout << elem << " ";

Or

for (int i = 0; i < sizeof(a) / sizeof(int); i++)
    std::cout << a[i] << " ";

sangyun Lee · Accepted Answer · 2021-09-15T14:08:29.957

Your code have two fault.

print array method is wrong.
third loop is wrong in CountSort. this loop working only once.

There is fix result.

void CountSort(int a[], int n, int k)
{
    int count[k + 1]={0};
    int b[n];

    for (int i = 0; i < n; i++)
    {
        ++count[a[i]];
    }
    for (int i = 1; i <= k; i++)
    {
        count[i] += count[i - 1];
    }
    for (int i = 0; i < n; i++)
    {
        b[count[a[i]]-1] = a[i];
        --count[a[i]];
    }
    for (int i = 0; i < n; i++)
    {
        a[i] = b[i];
    }
}

int main()
{
    int a[] = {2, 1, 1, 0, 2, 5, 4, 0, 2, 8, 7, 7, 9, 2, 0, 1, 9};
    CountSort(a, 17, 9);
    cout<<"The sorted array is  ->  ";
    for (int i = 0; i < 17; ++i) {
        cout << a[i] << ' ';
    }
    cout << endl;
   
    return 0;
}

result:

The sorted array is  ->  0 0 0 1 1 1 2 2 2 2 4 5 7 7 8 9 9

Why does this merge sort code keeps giving me this weird output?

2 Answers2