Find max element in a matrix [duplicate] - c

Question

This question already has answers here:
How to pass 2D array (matrix) in a function in C?
(4 answers)
Closed 6 years ago.
I've got this homework. Basically, what I have to do is complete the following code that returns the maximum element of a bidimensional array of 'r' rows and 'n' columns.
#include <stdio.h>
int max_element(int **A, int r, int n) {
// complete the code
int max;
max = a[0][0];
for (int i = 0; i < r; i++) {
for (int j = 0; j < n; j++) {
if (A[i][j] > max)
max = A[i][j];
}
}
return max; }
// implement a main() function to test the algorithm
int main() {
int A[2][3] = { {1, 0, 4}, {10, 3, 1} };
printf("%d\n", max_element(&A, 2, 3));
return 0; }
I have 1 warning:
passing argument 1 of 'max_element' from incompatible pointer type [-Wincompatible-pointer-types]
The console stopped working: a problem caused the program to stop working correctly...

Answer 1

Your max_element function is defined as such:
int max_element(int **A, int r, int n);
It takes a pointer to a pointer to int (int**) and you are feeding it this:
int A[2][3];
max_element(&A, 2, 3);
Do you expect the expression &A to yield a result of type int**? It will not. It will in fact yield a result of type int(*)[2][3]. That will not bind to int**. This is where the compiler warning kicks in. Those are incompatible pointers!!
You have a wider problem though. A 2D array is not int**. It has type int[][COLS]. You must specify the second number.
Change your function to be:
const int COLS = 3;
int max_element(int A[][COLS], int r, int n);
and then call as:
max_element(A, 2, 3);

Answer 2

Change the function prototype of max_element from:
int max_element(int **A, int r, int n)
To
int max_element(int A[][3], int r, int n)
This C-Faq thoroughly explains why. The gist of it is that arrays decay into pointers once, it doesn't happen recursively. An array of arrays decays into a pointer to an array, not into a pointer to a pointer.
And also, you should call max_element by max_element(A, 2, 3) instead of max_element(&A, 2, 3).
If a function is already declared as accepting a pointer to a pointer (as in your case), it is almost certainly meaningless to pass a two-dimensional array directly to it. An intermediate pointer would have to be used when attempting to call it with a two-dimensional array:
int max_element(int **A, int r, int n);
int *ip = &A[0][0];
max_element(&ip, 2, 3); /* PROBABLY WRONG */
but this usage is misleading and almost certainly incorrect, since the array has been flattened (its shape has been lost).