#include <stdio.h>

#define MAX_LEN 20

void lower(char* source, char* dest);

int main() {
    char* s = "This Is A Test.";
    char lower_s[MAX_LEN];
    lower(s, lower_s);
    printf("%s -> %s\n", s, lower_s);
    return 0;
}

void lower(char* source, char* dest) {
    int i;

    for (i = 0; source[i] != '\0'; i++)
        dest[i] = source[i] >= 'A' && source[i] <= 'Z' ? source[i] - 'A' + 'a' : source[i];

    dest[i] = '\0';
}