arrays not being assigned properly - c++

Question

void sort(int* A,int l)
{
int j;
int B[l];
for(int i=0;i<l;i++)
{
j = largest(A,l);
B[l-i-1] = A[j];
A[j] = -1;
}
A = B;
}
int main()
{
.
int C[3] = {x,y,z};
...
sort(C,3);
cout<<C[0]<<C[1];
}
output is coming to be -1-1
But if we assign A[0] = B[0] and so on, then we are getting the right answer.
PS: I've tried using *A = *B, which is only giving the first element to be correct.

Answer 1

When you assign A = B, you re-assign a local variable that holds a pointer to the first element of your array. This assignment will not change anything in main. In particular, the contents of A will not be affected.
You must copy all the elements from B to A after you have finished your sorting:
void sort(int *A, int l)
{
int j;
int B[l];
// sort into temporary array B
for (int i = 0; i < l; i++) {
j = largest(A, l);
B[l - i - 1] = A[j];
A[j] = -1;
}
// copy temporary array B to result array A
for (int i = 0; i < l; i++) A[i] = B[i];
}
But if you look at it, Amol Bavannavar was basically right: You don't have to check the whole array for the largest element each time. It is enough to check the remaining elements. So instead of assigning a low value to "used" elements, you could swap the largest elements to the end. When you do that, you'll see that the processed elements are at the end, the unprocessed elements are at the beginning. Then you can do your sorting in place without the need of a temporary array:
void sort2(int *A, int l)
{
while (l) {
int j = largest(A, l--);
int swap = A[j]; A[j] = A[l]; A[l] = swap;
}
}

Answer 2

There are many wrong uses of code in your example, for instance:
int B[l];
cannot be done, if you do it like this l must have a constant value.
A = B;
will perform a shallow copy instead of a deep copy.
You can see the diffrence here: What is the difference between a deep copy and a shallow copy?
cout<<C[0]<<C[1];
will print the numbers joined together without parsing.
As to how to fix this code one implementation you might be aiming towards can be:
#include <iostream>
using namespace std;
int largest(int* A, int l)
{
int big=-1;
int i;
int index=0;
for(i=0;i<l;i++)
{
if(A[i]>big)
{
big=A[i];
index=i;
}
}
return index;
}
void sort(int* A,int l)
{
int j;
int *B=new int[l];
for(int i=0;i<l;i++)
{
j = largest(A,l);
B[l-i-1] = A[j];
A[j] = -1;
}
for(int i=0;i<l;i++)
{
A[i]=B[i];
}
}
int main()
{
int C[3] = {2,5,1};
sort(C,3);
cout<<C[0]<<" "<<C[1];
return 1;
}