exc/k&r/02-types-operators-and-exp/02-10.c

#include <stdio.h>

char lower(char c);

int main()
{
    int c;
    while ((c = getchar()) != EOF)
        putchar(lower(c));
    return 0;
}

char lower(char c)
{
    return (c >= 'A' && c <= 'Z') ? c + 'a' - 'A' : c;
}