Control Flow and Functions

Branching with if

if (temperature > 30) {
    std::cout << "Hot\n";
} else {
    std::cout << "Not hot\n";
}

Repetition with loops

for (int i = 1; i <= 5; ++i) {
    std::cout << i << '\n';
}

Extracting a function

int square(int x) {
    return x * x;
}

Combined example

#include <iostream>

bool is_even(int value) {
    return value % 2 == 0;
}

int main() {
    for (int i = 1; i <= 10; ++i) {
        if (is_even(i)) {
            std::cout << i << " is even\n";
        }
    }
}

Larger worked example

#include <iostream>
#include <vector>

bool passed(int score) {
    return score >= 50;
}

int count_passes(const std::vector<int>& scores) {
    int total = 0;
    for (int score : scores) {
        if (passed(score)) {
            ++total;
        }
    }
    return total;
}

void print_summary(const std::vector<int>& scores) {
    std::cout << "passed: " << count_passes(scores) << '\n';
}

int main() {
    std::vector<int> scores{48, 77, 90, 31, 64};
    print_summary(scores);
}

This is the point of functions: main() reads like a summary, while smaller helpers carry the details.

Key lessons

• Use functions to name behavior
• Keep each function focused on one job
• Prefer early returns to reduce nesting

Early return example

bool is_valid_score(int score) {
    if (score < 0) {
        return false;
    }
    if (score > 100) {
        return false;
    }
    return true;
}

This is usually easier to read than wrapping the whole function body in nested if blocks.

switch and enum class

enum class MenuChoice { add, remove, quit };

void handle(MenuChoice choice) {
    switch (choice) {
    case MenuChoice::add:
        add_item();
        break;
    case MenuChoice::remove:
        remove_item();
        break;
    case MenuChoice::quit:
        shutdown();
        break;
    }
}

This keeps control flow readable when the set of cases is closed and explicit.

Choosing parameter styles

• Pass small values like int and double by value.
• Pass large read-only objects by const&.
• Pass output targets by non-const reference only when returning a value directly is not practical.

When switch is better than if

Use switch when the set of cases is closed and tied to one value, such as a menu choice, token kind, or state enum. Use if chains when the conditions are ranges, combinations, or unrelated predicates.

Practice extension

Refactor a loop-heavy program into three functions: input, computation, and output. The goal is to make main() read like a summary of the program.