Program for changing case

$ls
ascii-table     avg.txt               env-var.c      functions_ver1.c  mph-to-kph_v2.c
ascii-table.c   avg-with-garbage.txt  env-var-set.c  functions_ver2.c  output.c
ascii-table.md  case-changer.c        exist.sh       mph-to-kph.c
$gcc case-changer.c -o case-changer
$./case-changer 
g
G
v
V
FF
ff
$cat case-changer.c 
#include <stdio.h>
#include <string.h>

int main(void)
{
    char c[20] = { 0 };
    char newcase[20] = { 0 };
    int i;
    while(fgets(c, sizeof(c), stdin) != NULL)
    {
        for(i=0; i<=sizeof(c); i++)
        {
            /* Upper case to lower case */
            if ( (c[i] >= 65) && (c[i] <= 90) )
            {
                newcase[i] = c[i] + 32;
            }
            /* Lower case to upper case */
            if ( (c[i] >= 97 && c[i] <= 122) )
            {
                newcase[i] = c[i] - 32;
            }
        }
        printf("%s\n", newcase);
        /* zero out the arrays so there are no
           left-overs in the next run */
        memset(c, 0, sizeof(c));
        memset(newcase, 0, sizeof(newcase));
    }
    return 0;
}
$