Tolower function for array of strings in C

Question

I have an array of strings and I'm trying to convert all characters to lower case.

void make_lower(char **array)
{   
int i = 0;
while (array[i] != NULL){
       array[i] = tolower(array[i]);
       i++;
}
}

I know that tolower function reads characters one at a time, not the whole string at once. That's why I thought I had to use a loop like this, but still I get warnings and the function doesn't work:

passing argument 1 of ‘tolower’ makes integer from pointer without
a cast [-Werror]
note: expected ‘int’ but argument is of type ‘char *’
assignment makes pointer from integer without a cast [-Werror]

I would really appreciate your help.

Answer 1

You need a pair of nested loops, one for the string, one for the chars within it.

#include <stdio.h>
#include <ctype.h>

void make_lower(char **array)
{   
    int i = 0, j;
    while (array[i] != NULL){
        j = 0;
        while (array[i][j] != '\0') {
             array[i][j] = tolower(array[i][j]);
             j++;
        }
        i++;
    }
}    

int main(void) {
    char s1[]="ONE", s2[]="tWo", s3[]="thREE";
    char *array[] = {s1, s2, s3, NULL };
    make_lower(array);
    printf ("%s\n", array[0]);
    printf ("%s\n", array[1]);
    printf ("%s\n", array[2]);
    return 0;
}

Program outout:

one
two
three