Lesson 4 • Beginner

Control Flow

By the end of this lesson you'll be able to make your C++ programs decide what to do — branching with if/else if/else, choosing values with the ternary ?:, matching exact values with switch, and combining conditions with &&, || and !.

What You'll Learn

Branch your code with if, else if and else
Pick a value in one line with the ternary ?: operator
Match exact values with switch / case / break / default
Use fall-through deliberately (and avoid it by accident)
Nest conditions and combine them with &&, || and !
Scope a variable to one test with if (init; condition) (C++17)

Before you start: make sure you've finished Lesson 3: Operators. You should be comfortable with comparison operators (>, <, ==) and printing with cout. We build directly on those here.

💡 Real-World Analogy

Control flow is the signpost at a fork in the road. Your program walks up to a junction, reads a condition ("is it raining?"), and takes the matching path. An if/else if/else chain is a row of signs checked in order — you follow the first one that points your way and ignore the rest. A switch is the departures board at a station: it looks up your exact destination and sends you straight to the right platform. Without these signposts, the program can only ever walk in one straight line.

1. if / else if / else

An if statement runs a block of code only when a condition is true. The condition goes in (parentheses) and must evaluate to true or false. Add else for a fallback, and chain else if for extra options. The chain is checked top to bottom and the first true branch wins — every branch below it is skipped — so put your most specific conditions first. Read this worked example, run it, then you'll write your own.

Worked example: if / else if / else

Read every comment, run it, and check which branch fires.

Try it Yourself »

C++

#include <iostream>
using namespace std;

int main() {
    int score = 82;

    // An if statement runs its block ONLY when the condition is true.
    // The condition lives in (parentheses) and must be true or false.
    if (score >= 50) {
        cout << "You passed!" << endl;   // runs, because 82 >= 50 is true
    }

    // if / else — one of the two blocks ALWAYS runs.
    if (score >= 90) {
        cout << "Top of the class." << endl;
    } else {
        cout << "Not quite top, but well d
...

⚠️ Always use braces {}. Without them, only the single line after the if is conditional — the next line runs every time, which is a classic, silent bug.

2. The Ternary Operator `?:`

The ternary is a compact if/else that returns a value. The shape is condition ? valueIfTrue : valueIfFalse. Because it produces a value, you can assign it directly (string s = ok ? "yes" : "no";) or drop it inside a bigger expression. Reach for it on short either/or choices; if a branch needs several statements, a normal if/else reads more clearly.

Worked example: the ternary operator

See how ?: returns a value you can assign or print.

Try it Yourself »

C++

#include <iostream>
#include <string>
using namespace std;

int main() {
    // The ternary operator is a one-line if/else that RETURNS a value:
    //     condition ? valueIfTrue : valueIfFalse
    int age = 20;
    string status = (age >= 18) ? "Adult" : "Minor";
    cout << "Status: " << status << endl;   // Status: Adult

    // Great for picking the bigger of two numbers without a temp if-block.
    int a = 42, b = 17;
    int largest = (a > b) ? a : b;
    cout << "Largest: " << largest <<
...

Your turn. Fill in the two ___ blanks using the hints in the comments, then run it.

🎯 Your turn: write two ternaries

Fill in the ___ blanks, then check your output against the expected lines.

Try it Yourself »

C++

#include <iostream>
#include <string>
using namespace std;

int main() {
    // 🎯 YOUR TURN — replace each ___ then press "Try it Yourself".

    int temperature = 30;

    // 1) Use a ternary to set "advice" to "Hot" when temperature >= 25,
    //    otherwise "Mild".
    string advice = ___;   // 👉 (temperature >= 25) ? "Hot" : "Mild"

    // 2) Use a ternary to store the absolute value of -8 in "abs".
    int value = -8;
    int abs = ___;         // 👉 (value >= 0) ? value : -value

    co
...

3. switch / case / break / default

When you're comparing one value against many exact options, a switch is cleaner than a long else if chain. It works with int, char and enum (not strings or ranges). Each case ends with break to jump out of the switch; default is the catch-all, like else. If you omit break, execution falls through into the next case — handy when you deliberately stack cases (case 6: case 7: sharing one block), but a bug when you forget it.

switch (variable) {
    case value1:
        // code for value1
        break;          // stop — without this it falls through
    case value2:
    case value3:        // value2 and value3 share the next block
        // code for value2 OR value3
        break;
    default:            // optional — runs when nothing matched
        // fallback code
}

Worked example: switch, break, default & fall-through

Match days and a grade letter, and see deliberate fall-through.

Try it Yourself »

C++

#include <iostream>
using namespace std;

int main() {
    // switch compares ONE value against many exact cases.
    // It works with int, char and enum — NOT with strings or ranges.
    int day = 3;

    switch (day) {
        case 1:
            cout << "Monday" << endl;
            break;          // break = stop here, jump out of the switch
        case 2:
            cout << "Tuesday" << endl;
            break;
        case 3:
            cout << "Wednesday" << endl;  // matches -> this r
...

Now you try. This vending machine is missing the keyword that stops fall-through and the catch-all label. Fill in the two blanks:

🎯 Your turn: finish the switch

Add the missing break and default, then check the output.

Try it Yourself »

C++

#include <iostream>
using namespace std;

int main() {
    // 🎯 YOUR TURN — a tiny vending machine. Fill in the blanks.
    int choice = 2;   // 1 = Water, 2 = Cola, anything else = unknown

    switch (choice) {
        case 1:
            cout << "Dispensing Water" << endl;
            ___          // 👉 add the keyword that stops fall-through here
        case 2:
            cout << "Dispensing Cola" << endl;
            break;
        ___:             // 👉 the catch-all label for unmatched
...

4. Nesting, Logic & `if`-with-Initializer

Real decisions often depend on several things at once. Combine conditions with the logical operators: && (AND — both must be true), || (OR — either is enough), and ! (NOT — flips true and false). You can also nest an if inside another to make a second decision only when the first passed. Finally, C++17's if-with-initializer lets you declare and test a variable together — if (int x = f(); x > 0) — keeping x scoped to just that if/else.

Worked example: &&, ||, nesting & if (init; condition)

Combine conditions, nest a decision, and scope a variable to one test.

Try it Yourself »

C++

#include <iostream>
#include <string>
using namespace std;

// Returns how many items are left in stock for an order id.
int stockFor(int id) {
    return (id == 7) ? 3 : 0;   // only product 7 is in stock (3 left)
}

int main() {
    int age = 22;
    bool hasTicket = true;
    bool isVip = false;

    // Logical operators combine conditions:
    //   &&  AND  -> true only if BOTH sides are true
    //   ||  OR   -> true if EITHER side is true
    //   !   NOT  -> flips true <-> false
    cout 
...

Common Errors (and the fix)

Missing break (accidental fall-through): if a case has no break, its code and the next case's both run. Add break; to every case unless you mean to fall through.
= instead of ==: if (x = 5) assigns 5 to x and is always true. Use == to compare; writing if (5 == x) turns the typo into a compile error.
No default case: if no case matches and there's no default, the switch silently does nothing. Add a default: to catch unexpected values.
Dangling else: an else binds to the nearest unmatched if, not the one you indented it under. Use braces {} on every nested if so the else pairs with the right one.
"error: 'x' was not declared in this scope": a variable from an if-with-initializer or a case block only lives inside that block. Declare it outside if you need it afterwards.

Pro Tips

💡 Handle edge cases first: deal with the unusual inputs early with simple ifs, then write the normal path without deep nesting.
💡 Use switch for menus: it's ideal for processing numbered choices (1, 2, 3…) in a console app.
💡 Flatten deep nesting: 4+ levels of indentation is a smell — pull the inner logic into a small function.
💡 Scope tightly: prefer if (auto v = lookup(); v) so short-lived variables can't leak into the rest of your function.

📋 Quick Reference

Concept	Syntax
If	if (cond) { ... }
If / else	if (cond) { ... } else { ... }
Else if chain	if (...) {} else if (...) {} else {}
Ternary	result = cond ? a : b;
Switch	switch (x) { case 1: ...; break; default: ...; }
Logical AND / OR / NOT	a && b \| a \|\| b \| !a
If with initializer (C++17)	if (int x = f(); x > 0) { ... }

Frequently Asked Questions

Q: When should I use a switch instead of an if/else chain?

Use switch when you are comparing ONE variable against many exact, constant values (like menu numbers or a char grade). It is cleaner and shows your intent. Use if/else when you need ranges (score >= 80), combined conditions with && or ||, or anything other than an int, char or enum.

Q: What is fall-through in a switch?

If a case has no break, execution continues into the next case. Usually that is a bug — you forget the break and two messages print. But it is also a deliberate trick: stacking case 6: case 7: with one block lets several values share the same code.

Q: Why does if (x = 5) always run?

A single = is assignment, not comparison. if (x = 5) stores 5 in x and then tests that value, and any non-zero number counts as true in C++, so the branch always runs. Use == to compare. Putting the constant first — if (5 == x) — makes a typo a compile error.

Q: Is the ternary operator just a shortcut for if/else?

Almost. The key difference is that the ternary RETURNS a value, so you can assign it: string s = ok ? "yes" : "no";. A plain if/else does not return anything. Keep the ternary for short either/or choices; if a branch needs several statements, a normal if/else reads better.

Q: What is an if-with-initializer and why use it?

Since C++17 you can write if (int x = f(); x > 0) — declare x and test it in one line. The variable only exists inside the if/else, so it cannot leak into the rest of your function. It keeps short-lived variables tightly scoped and your code tidier.

Mini-Challenge: Ticket Price Calculator

No blanks this time — just a brief and a blank canvas (with an outline to keep you on track). Build it, run it, and check your output against the example in the comments. This combines an if/else if chain with a ternary, exactly like real code.

🎯 Mini-Challenge: build a ticket price calculator

Write the if/else chain and the ternary yourself.

Try it Yourself »

C++

#include <iostream>
#include <string>
using namespace std;

int main() {
    // 🎯 MINI-CHALLENGE: Ticket price calculator
    // 1. Make an int "age" and a bool "isStudent".
    // 2. Decide the price with if / else if / else:
    //       under 5            -> 0   (free)
    //       65 and over        -> 8   (senior)
    //       isStudent is true  -> 10  (student)
    //       everyone else      -> 15  (standard)
    // 3. Print:  "Price: $X"
    // 4. BONUS: use a ternary to print "Child" w
...

🎉 Lesson Complete

✅ if / else if / else run the first branch whose condition is true
✅ The ternary cond ? a : b returns a value you can assign or print
✅ switch matches exact int/char/enum values; break stops fall-through; default is the catch-all
✅ Combine conditions with &&, || and !, and nest ifs for layered decisions
✅ if (init; condition) (C++17) scopes a variable to just that test
✅ Next lesson: Loops — repeat code efficiently with for, while and do-while

Control Flow

What You'll Learn

💡 Real-World Analogy

1. if / else if / else

Worked example: if / else if / else

2. The Ternary Operator ?:

Worked example: the ternary operator

🎯 Your turn: write two ternaries

3. switch / case / break / default

Worked example: switch, break, default & fall-through

🎯 Your turn: finish the switch

4. Nesting, Logic & if-with-Initializer

Worked example: &&, ||, nesting & if (init; condition)

Common Errors (and the fix)

Pro Tips

📋 Quick Reference

Frequently Asked Questions

Mini-Challenge: Ticket Price Calculator

🎯 Mini-Challenge: build a ticket price calculator

🎉 Lesson Complete

Cookie & Privacy Settings

2. The Ternary Operator `?:`

4. Nesting, Logic & `if`-with-Initializer