segmentation fault for reversing array using recursion

Question

I am getting segmentation fault for this code I don't know what is wrong. it is running fine if input is given already but fails when we try to take input from user.

`

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

void reverseArray(int arr[], int s,int e){
    if (s<e){
        swap(arr[s],arr[e]);
        reverseArray(arr,s++,e--);
    }
}

int main() {
    //code
    int t;
    cin>>t;
    while (t--)
    {
    int n;
    cin>>n;
    int arr[n];
    for(int i=0;i<n;i++)
        cin>>arr[i];
    reverseArray(arr,0,n-1);

    for(int i=0;i<n;i++)
        cout<<arr[i]<<" ";
   
    cout<<endl;
    }
}    `

Unrelated: once you fix this to use the proper pre vs post increment/decrement, challenge yourself and see if you can figure out how to do it recursively with only two arguments rather than three. And fyi, int arr[n]; isn't standard C++. Variable length arrays are a vendor-dependent extension, as is #include <bits/stdc++.h>, You should break that habit and not use either. — WhozCraig
– WhozCraig, Commented Sep 3, 2022 at 6:16

john · Accepted Answer · 2022-09-03 06:17:55Z

1

You meant

reverseArray(arr,s+1,e-1);

This would also work, (although it does unnecessary work by modifying the s and e variables).

reverseArray(arr,++s,--e);

The problem with s++ and e-- is that the value they return is the value of s and e, so you are passing unchanged values to your recursive call, leading eventually to a stack overflow.

Essentially your misunderstanding is about the difference between the return value of an operation and the side affect. The side effect of ++ and -- is to modify the variable. But this is irrelevant, what gets passed to the recursive call is the return value of the expression.

edited Sep 3, 2022 at 6:17

answered Sep 3, 2022 at 6:12

john

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