Operators and decisions: if, else and switch

Why decisions?

Real programs must choose: pass or fail, free delivery or not, child fare or adult fare. A condition is an expression that is either true or false. if runs a block of code only when its condition is true.

Comparison and logical operators

#include <iostream>

int main() {
    int marks = 72;
    if (marks >= 90) {
        std::cout << "Grade A\n";
    } else if (marks >= 60) {
        std::cout << "Grade B\n";
    } else if (marks >= 33) {
        std::cout << "Grade C\n";
    } else {
        std::cout << "Fail\n";
    }
}

Short-circuit evaluation

&& stops as soon as the left side is false, and || stops as soon as the left side is true. This is called short-circuiting, and it is useful for safety checks: in if (count != 0 && total / count > 50), the division only happens when count is not zero.

The ternary operator

int age = 70;
double fare = (age >= 60) ? 600.0 : 1000.0;   // senior fare

switch for fixed choices

char choice = 'b';
switch (choice) {
    case 'a':
        std::cout << "Check balance\n";
        break;
    case 'b':
        std::cout << "Send money\n";
        break;
    default:
        std::cout << "Invalid option\n";
}

switch works only with integral types (int, char, enum and similar), not with std::string or double. For text, use if / else if.

The = versus == trap

int seats = 0;
if (seats = 5) {          // BUG: assigns 5, which is "true"
    std::cout << "Always runs!\n";
}
if (seats == 5) { }       // correct comparison

Common mistakes

• A semicolon right after the condition: if (x > 5); { ... } runs the block always, because the ; is an empty statement.
• Leaving out braces and adding a second line later: only the first line belongs to the if. Always use braces.
• Writing 10 < x < 20 to test a range. It means (10 < x) < 20, which is always true. Write x > 10 && x < 20.
• Comparing doubles with ==: 0.1 + 0.2 == 0.3 is false. Compare with a small tolerance instead.