ของนิสานะ ^^

#include<stdio.h>

char char_twist(char c);
int main()
{
    char inchr;
    while(printf("ENTER: ") && scanf("\n%[a-zA-Z]", &inchr))
    {
        char_twist(inchr);
    }
    printf("End.\n");
    return 0;
}

char char_twist(char c)
{
    printf("=%c\n", (islower(c) ? toupper(c) : tolower(c)));
}