Making choices with if and switch

Learning objectives

• Choose between independent if statements and an if/else if/else chain.
• Use switch for a set of discrete integral or enumeration values.
• Trace a decision and predict which statements execute.

Prerequisites: variables, input/output, comparison operators, and Boolean values. Review Comparison and logical operators first.

How a condition controls a branch

An if statement evaluates its condition. If the result is true, its controlled statement runs; otherwise it is skipped. In an if/else if/else chain, conditions are checked in order and at most one branch is selected. Use braces to make each branch boundary clear.

#include <iostream>

int main() {
    int marks{72};

    if (marks >= 90) {
        std::cout << "Excellent\n";
    } else if (marks >= 40) {
        std::cout << "Pass\n";
    } else {
        std::cout << "Try again\n";
    }
}

Trace the decision

Conditions are contextual Boolean expressions: in C++, a scalar value can be contextually converted to bool (zero is false; nonzero is true). Prefer an explicit comparison when it communicates the rule more clearly.

When switch is a better fit

Use switch when one integral or enumeration expression is compared against several constant case labels. It is not a general replacement for ranges such as “marks at least 40”. A break exits the switch; without it, execution can fall through into the next case. A default label handles values not matched by another case.

#include <iostream>

int main() {
    int choice{2};

    switch (choice) {
        case 1:
            std::cout << "Start\n";
            break;
        case 2:
            std::cout << "Settings\n";
            break;
        case 3:
            std::cout << "Quit\n";
            break;
        default:
            std::cout << "Unknown option\n";
            break;
    }
}

Common mistakes

• Use == for equality; = assigns a value.
• Order range checks from the most specific/highest threshold when later branches depend on earlier ones.
• Remember that a missing break can cause switch fall-through.
• Do not expect switch to accept arbitrary run-time ranges or strings as case labels in C++17.

Practice

1. For marks = 39, which branch runs in the first program?
2. For choice = 7, what does the switch program print?
3. Why is an if/else if chain more suitable than switch for classifying a numeric range?
4. What happens if the break after case 2 is removed?

Worked answers

Summary

Use if for Boolean conditions and ranges; use switch for discrete integral or enumeration choices. Trace conditions in order and make branch boundaries explicit with braces and deliberate break statements.

References

• cppreference: if statement
• cppreference: switch statement