Question

I wanted to get familiar with 2D variable sized arrays in c++, so I wrote a little program, but it doesn’t work. Here is the code:

#include <iostream>

using namespace std;

int main(){
int a,i;
cin>>a; //the width of the array is variable
int **p2darray;
p2darray = new int*[2]; //the height is 2
for (i = 0; i < 2; i++){
    p2darray[i] = new int[a];
}
i=0;
while(i!=a){
    p2darray[0][i]=i; //filling some numbers in the array
    p2darray[1][i]=2*i;
    i++;
}
i=0;
while(i!=a){
    cout<<p2darray[0][i]<<endl;
    cout<<p2darray[1][i]<<endl;
    i++;
}
return 0;
}

So why doesn’t it work?

Answer 1

The main problem is that when you say p2darray[i][0], your indices are backwards because you set the second dimension to the size the user enters, but you’re incrementing the first dimension to that number instead. This would normally cause a segfault. It should be p2darray[0][i] in all four cases. You also didn’t set i back to 0 before entering the printing loop, so it’s skipping the entire printing process.

For a running program that illustrates the said corrections, see here.