If else

If 是一個判別邏輯的程式，可以讓程式在某些情況下執行。

Example:

bool no_idea;

if (no_idea) {
    keep_learning();
} else {
    make_design();
}

if else statement 也可以是更複雜的結構：

int days_before_vacation;

if (days_before_vacation > 5) {
    printf("days_before_vacation is more than 5\n");
}
else if (days_before_vacation > 3) {
    printf("days_before_vacation is between 4 and 5\n");
}
else if (days_before_vacation > 1) {
    printf("days_before_vacation is between 2 and 3\n");
}
else {
    printf("days_before_vacation is 1 or less\n");
}