#include <stdio.h>
#include <ctype.h>
int main()
{
char a[26];
int i;
printf("Enter the string: ");
fgets(a, 100, stdin); // safer than scanf()
i = 0;
while (a[i] && isalpha(a[i])) // check input is alphabet
{
if (isupper(a[i]))
a[i] = tolower(a[i]); // is this wat the OP wanted?
// no need to check bounds due to the nature of the prob stmt
if ((a[i] >= 'a') && (a[i] <= 'm')) 
{
a[i] += 13; 
}
// to other check req as this will always be greter than 'm' and less than equal to 'z'
else 
{
a[i] -= 13;
}
a[i] = toupper(a[i]); // is this wat the OP wanted?
i++; 
} 
fputs(a, stdout);
getchar();
return 0;
}