Booleans and selection: if, else, switch

True and false in C

• C has no separate true/false type by default. It uses numbers: 0 means false, and any other number means true.
• A test like 5 > 3 produces 1 (true). 5 < 3 produces 0 (false).
• (If you #include <stdbool.h> you can write bool, true, and false — but plain int works everywhere.)

Comparing and combining

• Compare values with == (equal), != (not equal), <, >, <=, >=.
• Be careful: == tests equality; a single = assigns a value. They are different!
• Combine tests with && (and), || (or), and ! (not). a && b is true only when both are true.

if / else if / else

• if (test) { ... } runs the block only when the test is true.
• Add else if (...) to check another test, and else for "none of the above".
• Only the first matching branch runs.

#include <stdio.h>

int main(void) {
    int score = 72;
    if (score >= 90) {
        printf("A\n");
    } else if (score >= 60) {
        printf("Pass\n");      // this one runs
    } else {
        printf("Fail\n");
    }
    return 0;
}

switch: choosing among many values

• A switch picks a branch based on one value (an int or char).
• Each case ends with break; so it does not fall into the next one. default: catches everything else.
• switch is neat when you compare the same variable against many fixed values.

Common mistakes

• Use == to compare; = assigns (and if (x = 5) is always true).
• Each switch case needs a break;.

Now you try

• Use comparisons and && / || to build your tests. Remember 0 is false, anything else is true.
• For the switch task, divide the score by 10 first, then switch on the result.
• Do not write a main — the checker provides one.