ของนิสาคับ :D

#include<stdio.h>

char char_twist(char c);
int main()
{
    char inchr;
    while(1)
    {
        printf("Char: ");
        scanf("\n%c", &inchr);
        if (char_twist(inchr))
        {
            break;
        }
    }
    printf("End.\n");
    return 0;
}

char char_twist(char c)
{
    if ((c >= 'A' && c <= 'Z')||(c >= 'a' && c <= 'z'))
    {
        if(islower(c))
        {
            printf("=%c\n", toupper(c));
        }else{
            printf("=%c\n", tolower(c));
        }
        return 0;
    }else
        return 1;
}