Conditions & Loops — C++ for Beginners Part 3 | webencher

C++ for Beginners Part 3: Control Flow — Conditions and Loops

Learn how to make decisions in C++ with if/else and switch statements, and how to repeat code with for, while, and do-while loops. Covers nested loops, break, continue, and the ternary operator, with practical examples including a grade calculator and FizzBuzz.

Making decisions: `if` and `else`

The if statement runs a block of code only when a condition is true:

C++

#include <iostream>

int main() {
    int age = 17;

    if (age >= 18) {
        std::cout << "You may vote.
";
    } else {
        std::cout << "You cannot vote yet.
";
    }
}

`else if` — multiple branches

C++

int score = 74;

if (score >= 90) {
    std::cout << "Grade: A
";
} else if (score >= 80) {
    std::cout << "Grade: B
";
} else if (score >= 70) {
    std::cout << "Grade: C
";
} else if (score >= 60) {
    std::cout << "Grade: D
";
} else {
    std::cout << "Grade: F
";
}
// Output: Grade: C

Branches are evaluated top-to-bottom. The first condition that's true runs; the rest are skipped.

Omitting braces (be careful)

C++

if (score > 50)
    std::cout << "Pass
";   // Only this line is in the if
    std::cout << "Done
";   // This ALWAYS runs — not part of the if!

Always use braces {} even for single-line bodies. It prevents this class of bug.

Ternary operator — compact if/else

C++

int x = 10;
std::string result = (x % 2 == 0) ? "even" : "odd";
std::cout << result;  // even

Read it as: condition ? value-if-true : value-if-false

Good for simple assignments; avoid nesting ternaries — readability drops fast.

`switch` — matching one value against many cases

When you're comparing one variable against several exact values, switch is cleaner than a chain of else if:

C++

int day = 3;

switch (day) {
    case 1:
        std::cout << "Monday
";
        break;
    case 2:
        std::cout << "Tuesday
";
        break;
    case 3:
        std::cout << "Wednesday
";
        break;
    case 4:
        std::cout << "Thursday
";
        break;
    case 5:
        std::cout << "Friday
";
        break;
    default:
        std::cout << "Weekend
";
}
// Output: Wednesday

The `break` — don't forget it

Without break, execution falls through to the next case:

C++

int n = 1;
switch (n) {
    case 1:
        std::cout << "One
";
        // no break — falls through!
    case 2:
        std::cout << "Two
";
        break;
}
// Output:
// One
// Two

Fallthrough is occasionally useful (grouping cases), but almost always a bug if unintentional.

Grouping cases

C++

switch (day) {
    case 1:
    case 2:
    case 3:
    case 4:
    case 5:
        std::cout << "Weekday
";
        break;
    case 6:
    case 7:
        std::cout << "Weekend
";
        break;
}

switch only works with integer-like types (int, char, enum) — not float, double, or std::string.

Loops: repeating code

`for` loop — when you know how many iterations

C++

for (int i = 0; i < 5; i++) {
    std::cout << i << " ";
}
// Output: 0 1 2 3 4

The three parts of the for header:

Initializer — runs once before the loop: int i = 0
Condition — checked before each iteration: i < 5
Increment — runs after each iteration: i++

Counting down

C++

for (int i = 10; i >= 1; i--) {
    std::cout << i << " ";
}
// Output: 10 9 8 7 6 5 4 3 2 1

Stepping by 2

C++

for (int i = 0; i <= 20; i += 2) {
    std::cout << i << " ";
}
// Output: 0 2 4 6 8 10 12 14 16 18 20

`while` loop — repeat while a condition holds

C++

int count = 0;
while (count < 5) {
    std::cout << count << " ";
    count++;
}
// Output: 0 1 2 3 4

The condition is checked before each iteration. If it's false from the start, the body never runs.

Reading input until valid

C++

int number;
std::cout << "Enter a positive number: ";
std::cin >> number;

while (number <= 0) {
    std::cout << "Invalid. Try again: ";
    std::cin >> number;
}
std::cout << "You entered: " << number << "
";

`do-while` loop — run at least once

C++

int choice;
do {
    std::cout << "Enter 1 to continue, 0 to quit: ";
    std::cin >> choice;
} while (choice != 0 && choice != 1);

The body runs first, then the condition is checked. Useful for menus and input validation where you always need at least one execution.

`break` and `continue`

`break` — exit the loop immediately

C++

for (int i = 0; i < 100; i++) {
    if (i == 5) break;
    std::cout << i << " ";
}
// Output: 0 1 2 3 4

`continue` — skip to the next iteration

C++

for (int i = 0; i < 10; i++) {
    if (i % 2 == 0) continue;   // skip even numbers
    std::cout << i << " ";
}
// Output: 1 3 5 7 9

Nested loops

A loop inside another loop. The inner loop completes fully for each iteration of the outer:

C++

for (int row = 1; row <= 3; row++) {
    for (int col = 1; col <= 4; col++) {
        std::cout << row << "×" << col << "=" << row*col << "  ";
    }
    std::cout << "
";
}
// 1×1=1  1×2=2  1×3=3  1×4=4
// 2×1=2  2×2=4  2×3=6  2×4=8
// 3×1=3  3×2=6  3×3=9  3×4=12

Range-based `for` loop (C++11)

When iterating over a collection, the range-based for is much cleaner:

C++

#include <vector>

std::vector<int> scores = {85, 92, 78, 96, 88};

for (int s : scores) {
    std::cout << s << " ";
}
// Output: 85 92 78 96 88

We'll cover vectors in detail in Part 5; for now, think of them as resizable arrays.

Practical example: FizzBuzz

A classic programming exercise — print numbers 1–100, but replace multiples of 3 with "Fizz", multiples of 5 with "Buzz", and multiples of both with "FizzBuzz":

C++

#include <iostream>

int main() {
    for (int i = 1; i <= 100; i++) {
        if (i % 15 == 0) {
            std::cout << "FizzBuzz
";
        } else if (i % 3 == 0) {
            std::cout << "Fizz
";
        } else if (i % 5 == 0) {
            std::cout << "Buzz
";
        } else {
            std::cout << i << "
";
        }
    }
}

Note the order: check i % 15 first. If you check % 3 first, 15 would print "Fizz" instead of "FizzBuzz".

Practical example: grade calculator

C++

#include <iostream>

int main() {
    int numStudents;
    std::cout << "How many students? ";
    std::cin >> numStudents;

    double total = 0;
    for (int i = 1; i <= numStudents; i++) {
        double score;
        std::cout << "Score for student " << i << ": ";
        std::cin >> score;
        total += score;
    }

    double average = total / numStudents;
    std::cout << "Class average: " << average << "
";

    std::string grade;
    if      (average >= 90) grade = "A";
    else if (average >= 80) grade = "B";
    else if (average >= 70) grade = "C";
    else if (average >= 60) grade = "D";
    else                    grade = "F";

    std::cout << "Class grade: " << grade << "
";
}

Key takeaways

if/else if/else handles multiple conditions; always use {} braces.
switch is cleaner when matching one variable against exact values — don't forget break.
for is for a known count; while for an unknown count; do-while for at least one execution.
break exits a loop; continue jumps to the next iteration.
The range-based for loop is the idiomatic C++ way to iterate collections.

Next up: functions — how to organise code into reusable, named blocks.

Making decisions: if and else

else if — multiple branches

Omitting braces (be careful)

Ternary operator — compact if/else

switch — matching one value against many cases

The break — don't forget it