Code works for smaller inputs but it is not working for large inputs. Why?

Question

Given a matrix of m * n elements (m rows, n columns), return all elements of the matrix in spiral order. Why is it working for smaller inputs but not for larger inputs? While this same code is working on other systems. The inputs and the outputs I used are given below.

#include<iostream>
using namespace std;
int main(){

    int n,m;
    cin>>n>>m;

    int a[n][m];

    for (int i = 0; i < n; i++)
    {
        for (int j = 0; j < n; j++)
            cin>>a[i][j];   
    }
    
    // spiral order print

    int row_start = 0, row_end = n-1, column_start = 0, column_end = m-1;
    while (row_start <= row_end && column_start <= column_end)
    {
        //for row start
        for (int col = column_start; col <= column_end; col++)
        {
            cout<<a[row_start][col]<<" ";
        }
        row_start++;

        //for column end
        for (int row = row_start; row <= row_end; row++)
        {
            cout<<a[row][column_end]<<" ";
        }
        column_end--;

        //for row end
        for (int col = column_end; col >= column_start; col--)
        {
            cout<<a[row_end][col]<<" ";
        }
        row_end--;

        //for column start
        for (int row = row_end; row >= row_start; row--)
        {
            cout<<a[row][column_start]<<" ";
        }
        column_start++;
    }
    
return 0;
}

INPUT:

5 6
1 5 7 9 10 11
6 10 12 13 20 21
9 25 29 30 32 41
15 55 59 63 68 70
40 70 79 81 95 105

OUTPUT:

1 5 7 9 10 1878030368 -1 -1 6422096 4200108 40 70 68 63 59 30 20 11 6 10 12 13 29 55 15 41 32 21 9 25 9

`int a[n][m];` is not standard C++. [Why aren't variable-length arrays part of the C++ standard?](https://stackoverflow.com/questions/1887097/why-arent-variable-length-arrays-part-of-the-c-standard). Its a non-stadnard compiler extension. If you want to know how it works or does not work you have to read your compilers manual. Anyhow, you cannot allocate arbitrary amounts of data on the stack. Better use `std::vector` for dynamic arrays — 463035818_is_not_an_ai, Jul 07 '21 at 10:22
`for (int j = 0; j < n; j++)` -> `for (int j = 0; j < m; j++)`. There may be other problems though — Jabberwocky, Jul 07 '21 at 10:32
The dimensions of the array `int a[n][m]` should be known at the compile time. If you want to use array size of which is to be disclosed at runtime, you could use dynamic arrays(the one declared with `new`) or `std::vector`. I'd recommend using `std::vector`. — Karen Baghdasaryan, Jul 07 '21 at 11:05
`int a[n][m];` can easily overflow the stack if m and n are large enough. — drescherjm, Jul 07 '21 at 12:40

score -1 · Answer 1 · answered Jul 07 '21 at 11:30

-1

Check your code:

for (int i = 0; i < n; i++)
{
    for (int j = 0; j < n; j++)
        cin>>a[i][j];   
}

The second condition should be: j < m

My guess is your code is accessing garbage memory.

answered Jul 07 '21 at 11:30

Yathaarth Rajiv Jamwal

1
2

Code works for smaller inputs but it is not working for large inputs. Why?

1 Answers1