Contagion program, Matrices and Syntax issue - ocaml

Question

I am trying to do the following program:
Write a function onstepcontagion : bool array array -> bool array array = that given a rectangular bool matrix, where true represents an infected square and false represent non-infected square it calculates the next step of infection. Infected squares remain infected indefinitely, non-infected squares become infected if the they are vertically/horizontally adjacent to at least two other infected squares
My code so far:
let printmat matrix =
let n = Array.length matrix in
let n1 = Array.length matrix.(0) in
for i = 0 to n - 1 do
for j = 0 to n1 - 1 do
if matrix.(i).(j) == true then print_string"1"
else print_string"0";
done;
print_string "\n";
done;;
let onstepcontagion matrix =
let n = Array.length matrix in
let n1 = Array.length matrix.(0) in
for i = 0 to n - 1 do
for j = 0 to n1 - 1 do
if (j < n1-1) then
let right = if matrix.(i).(j+1) == true then 1 else 0 in
if (j > 0) then
let left = if matrix.(i).(j-1) == true then 1 else 0 in
if (i < n-1) then
let up = if matrix.(i-1).(j) == true then 1 else 0 in
if (i > 0) then
let down = if matrix.(i+1).(j) == true then 1 else 0 in
let sum = right + left + up + down in
if sum > 1 then matrix.(i).(j) = true
done;
print_string "\n";
done;;
printmat matrix
**Error: Error: This expression has type bool but an expression was expected of type
unit
Characters 1618-1639:
if sum > 1 then matrix.(i).(j) = true **
I have made a C version of the program I want to implement:
#include <stdio.h>
#include <stdlib.h>
void print_mat (int n, int m, int arr[n][m])
{
int i,j;
for (i = 0; i < n; i++){
for (j = 0; j < m; j++)
printf("%d ",arr[i][j]);
printf("\n");
}
printf("\n\n");
}
int main()
{
int n, m, i, j, tmp, changes = 1;
printf("input Mat len\n");
scanf("%d%d",&n,&m);
int arr[n][m];
int cpy[n][m];
for (i = 0; i < n; i++)
for (j = 0; j < m; j++){
scanf("%d",&tmp);
if (tmp == 0)
arr[i][j] = tmp;
else
arr[i][j] = 1;
}
while (changes){
changes = 0;
for (i = 0; i < n; i++){
for (j = 0; j < m; j++)
{
tmp = 0;
if (arr[i][j] != 1){
if (j < m-1)
if (arr[i][j+1] == 1)
tmp++;
if (j > 0)
if (arr[i][j-1] == 1)
tmp++;
if (i < n-1)
if (arr[i+1][j] == 1)
tmp++;
if (i > 0)
if (arr[i-1][j] == 1)
tmp++;
if (tmp > 1){
cpy[i][j] = 1;
changes = 1;
}
}
}
}
if (changes == 1){
for (i = 0; i < n; i++)
for (j = 0; j < m; j++)
if (arr[i][j] == 1 || cpy[i][j] == 1)
arr[i][j] = 1;
printf("\n");
print_mat(n,m, arr);
}
}
return 0;
}

Answer 1

= is the structural equality operator. To set an array element you need to use <-.
Also, you should almost never use == because it tests for physical equality, i.e. that references point to the same address in memory. For comparing bools it doesn't strictly matter, because they're not pointers, but you should get into the habit of using = instead to avoid surprises in the future.