Booleans

In the if-else lesson we saw a small surprise: C doesn't have a native boolean type. It uses integers, where 0 means false and anything else means true

#include <stdio.h>

int main(void) {
    int is_user_online = 1;   // "true"
    int is_hidden = 0;        // "false"

    if (is_user_online) {
        printf("online\n");
    }
    return 0;
}

This works, but reading int is_user_online = 1 is awkward. It looks like a counter, not a true/false value

The good news: starting with C99, the standard library gives us a real boolean type

#include <stdio.h>
#include <stdbool.h>

int main(void) {
    bool is_online = true;
    bool has_key = false;

    printf("%d\n", is_online);   // 1
    printf("%d\n", has_key);     // 0

    return 0;
}

After we #include <stdbool.h>, three new identifiers exist: bool, true, and false

Behind the scenes, bool is still essentially an int (true is 1, false is 0), but the names make our code much more readable. From now on, when something can only be true or false, prefer bool over int

There is no special format specifier for bool in printf: just use %d (it will print 0 or 1)

We can combine booleans with logical operators

• && → and (both must be true)
• || → or (at least one must be true)
• ! → not (flips the value)

A real example: in order to drive a car, you need to be at least 18 AND have a license

#include <stdio.h>
#include <stdbool.h>

int main(void) {
    int age = 20;
    bool has_license = true;

    if (age >= 18 && has_license) {
        printf("You can drive\n");
    } else {
        printf("Sorry, no driving today\n");
    }

    return 0;
}

|| is more relaxed. Just one of the conditions being true is enough

#include <stdio.h>
#include <stdbool.h>

int main(void) {
    bool is_vip = false;
    bool has_invitation = true;

    if (is_vip || has_invitation) {
        printf("Welcome to the club\n");
    } else {
        printf("Access denied\n");
    }
    return 0;
}

! flips a boolean. !true becomes false, and vice versa

#include <stdio.h>
#include <stdbool.h>

int main(void) {
    bool is_logged_in = false;
    if (!is_logged_in) {
        printf("Please log in first\n");
    }
    return 0;
}

Mission: Access to the Computer Room

The data center's computer room locks itself automatically. The gate runs a panel that decides who gets in: a person can pass if they are an employee AND it is a working day, OR if they are a guest WITH an invitation.

Read four integers (0 or 1), separated by space, in this order: is_employee, is_working_day, is_guest, has_invitation. Write an if / else using && and || that prints the correct verdict

Example

Input

1 1 0 0

Output

Access granted

Example

Input

0 0 1 0

Output

Access denied