Make basic merge sort algorithm dynamic so the user can input his own numbers

Question

Trying to make this code dynamic, but I am having errors. I have a basic merge sort algorithm that I want to make dynamic (so the user can input his own numbers) and I am sure it would be like a 5min work but I am unable to do it.

#include <iostream>

using namespace std;
//merginimas



void merge(int arr[], int l, int m, int r)
{
    int i = l; //kaires
    int j = m + 1; //desines
    int k = l; //laikinasis (i kuri sueina)

    int size = (r - 1) + 1;
    int temp[size]; // laikinas
    
    while (i<= m && j<=r)
    {
        if (arr[i] <= arr[j]) //lygina kaires ir desines array skaicius
        {
            temp[k] = arr[i];  // arr[i] mazesnis uz arr[j]
            i++;
            k++;
        }
        else
        {
            temp[k] = arr[j];  // arr[j] mazesnis uz arr[i]
            j++;
            k++;
        }
    }
    while (i<=m)
    {
        temp[k] = arr[i]; //kopijuoja skaicius is kaires i laikina
        i++;
        k++;
    }
    while (j <= r)
    {
        temp[k] = arr[j]; //kopijuoja skaicius is desines i laikina
        i++;
        k++;
    }
    for (int x = l; x <= r; x++)
    {
        arr[x] = temp[x];
    }
}

//sortinimas

void mergeSort(int arr[], int l, int r)
{
    if (l < r)
    {
        int m = (l + r) / 2;
        mergeSort(arr, 0, m);
        mergeSort(arr, m+1, r);
        merge(arr, l, m, r);
    }
}



int main()
{
    int size;
    int* ptr;

    cout << "Kiek skaiciu noresite surusiuoti? " << endl;//enter size of the array
    cin >> size;

    ptr = new int[size];
    

    cout << "Iveskite " << size << " skaicius kuriuos noresite surusiuoti: " << endl;//enter the numbers that you want to sort
    int myarr[size];
    for (int i = 0; i < size; i++)
    {
        cin >> myarr[i];
    }
    //int myarr[size];

    

    cout << "Before merge sort: " << endl;
    for (int i = 0; i < size; i++)
    {
        cout << myarr[i] << " ";
    }

    // mergesort function

    mergeSort(myarr, 0, (size - 1));

    cout << "After merge sort: " << endl;
    for (int i = 0; i < size; i++)
    {
        cout << myarr[i] << " ";
    }


    return 0;
}

I suggest that you use a [`std::vector`](https://en.cppreference.com/w/cpp/container/vector) to store the data. A `vector` can grow dynamically. Also, there is already a [`std::merge`](https://en.cppreference.com/w/cpp/algorithm/merge) function template that merges sorted ranges. I suggest that you use that one. — Ted Lyngmo, May 22 '22 at 16:55
i want to make dynamic (so the user can input his own numbers) — saim kazmi, May 22 '22 at 16:56
[C++ doesn't have variable-length arrays](https://stackoverflow.com/questions/1887097/why-arent-variable-length-arrays-part-of-the-c-standard). Use `std::vector` for all your "dynamic array" needs. — Some programmer dude, May 22 '22 at 16:57
variable length arrays are non standard in c++ `int temp[size]` etc. use std::vector instead. — pm100, May 22 '22 at 16:58
***kindly fix the errors*** Except for `int temp[size]` which is illegal in standard c++ we don't know what errors you have. — drescherjm, May 22 '22 at 16:58
Usage note: Always include the error messages you receive in plain text. It helps direct answerers to the specific problem you are dealing with. — user4581301, May 22 '22 at 16:59
Please take some time to read [the help pages](http://stackoverflow.com/help), take the SO [tour], read [ask], as well as [this question checklist](https://codeblog.jonskeet.uk/2012/11/24/stack-overflow-question-checklist/). Then **[edit]** your question to improve it, like copy-pasting any possible build errors (as text) into your question, and adding comments on the lines where you get the errors. — Some programmer dude, May 22 '22 at 16:59
How is it not dynamic? `cin >> myarr[i]` lets the user input his own numbers. — David G, May 22 '22 at 17:12

score 1 · Answer 1 · answered May 22 '22 at 17:08

this loops forever

while (j <= r)
{
    temp[k] = arr[j]; //kopijuoja skaicius is desines i laikina
    i++;
    k++;
}

and runs k to values outside the bounds of temp (once I changed temp to not be a variable lenghth array)

You never change j or r in the loop, so it just keeps going

Make basic merge sort algorithm dynamic so the user can input his own numbers

1 Answers1