Lecture 5+6 — Control Flow

Source: sc++/ch05.md Duration: 2 x 75 minutes (lectures 5 and 6)

Chapter 5 is split across two lectures:

  • Lecture 5 — Decisions and while: if/else/else if/nested if, while loops
  • Lecture 6 — Loops, Jumps, and switch: do-while, break/continue, for (classic and range-based), switch with fall-through

Lecture 5 — Decisions and while

Learning Objectives

By the end of lecture 5, students should be able to:

  • Write if, if/else, and if/else if/else chains to make decisions
  • Recognize and avoid the = vs == trap in a condition
  • Write a while loop that terminates correctly
  • Trace a nested if and decide when to flatten it

Materials

  • Live coding terminal with g++ (-std=c++23 -Wall -Wextra -pedantic)
  • A text editor projected for the class
  • Copies of sc++/ch05.md for reference

0. Welcome and Review (5 min)

  • Review multiple choice (from lecture 4): What does this print?

    int a = 10;
int b = a++;
int c = ++a;
std::cout << a << " " << b << " " << c << "\n";
    • A. 10 10 11
    • B. 11 10 11
    • C. 12 10 12
    • D. 12 11 12
    • E. Ben got this wrong

    Answer: C

  • Up until now every program runs straight through. Today we learn how to branch and repeat.

1. if Statements (10 min) 

int score = 85;

if (score >= 90) {
    std::cout << "Excelente!\n";
}
  • The condition must be a boolean expression
  • If true, the body runs; if false, it is skipped entirely
  • Always use {} around the body — it costs you nothing and saves you from the dangling-if trap

else and else if 

if (score >= 90) {
    std::cout << "A\n";
} else if (score >= 80) {
    std::cout << "B\n";
} else if (score >= 70) {
    std::cout << "C\n";
} else {
    std::cout << "Try again\n";
}
  • Conditions are tested top to bottom
  • As soon as one matches, the rest are skipped
  • The final else catches anything that did not match

Trap: if (x = 5) assigns 5 to x; it does not compare. Use ==. -Wall will warn you.

2. Nested if (8 min) 

int age = 20;
bool has_ticket = true;

if (age >= 18) {
    if (has_ticket) {
        std::cout << "Welcome to the show\n";
    } else {
        std::cout << "You need a ticket\n";
    }
} else {
    std::cout << "Must be 18 or older\n";
}
  • Works, but deep nesting is hard to read
  • Flatten by combining conditions with &&/|| or by returning early from a function (we cover functions in chapter 6)

3. while Loops (15 min) 

int countdown = 5;

while (countdown > 0) {
    std::cout << countdown << "... ";
    countdown--;
}
std::cout << "Vamos!\n";
// 5... 4... 3... 2... 1... Vamos!
  • Condition is tested before each iteration
  • If the condition is false from the start, the body never runs at all
  • Make sure the condition will eventually become false, or you have an infinite loop

Trap: Forgetting to update the loop variable is a classic infinite loop:

int i = 0;
while (i < 10) {
    std::cout << i << "\n";
    // oops, forgot i++
}

Ctrl+C is your friend.

Input Validation With while 

int n;
std::cout << "Enter a positive number: ";
std::cin >> n;
while (n <= 0) {
    std::cout << "Try again: ";
    std::cin >> n;
}
  • Classic pattern: ask, test, ask again
  • Next lecture we will see that do-while makes this even cleaner

4. Try It — Live Demo (15 min)

Live-code a simple guessing game:

#include <iostream>

int main()
{
    int target = 42;
    int guess;
    std::cout << "Guess my number: ";
    std::cin >> guess;

    while (guess != target) {
        if (guess < target) {
            std::cout << "Higher! ";
        } else {
            std::cout << "Lower! ";
        }
        std::cin >> guess;
    }
    std::cout << "You got it!\n";
    return 0;
}
  • Ask the class for the smallest change that would make the loop infinite
  • Ask what happens if the user types non-numeric input (preview of chapter 9)

5. Wrap-up Quiz (4 min)

Q1. What does this print?

int x = 5;
if (x = 10) {
    std::cout << "A ";
}
std::cout << x << "\n";

A. A 5 B. A 10 C. 5 D. 10 E. Ben got this wrong

Answer: B — x = 10 assigns 10 and evaluates to 10 (truthy).

Q2. How many times does the body of this loop run?

int i = 10;
while (i > 10) {
    std::cout << "hi\n";
    i++;
}

A. 0 B. 1 C. 10 D. infinite E. Ben got this wrong

Answer: A — the condition is false from the start.

6. Assignment / Reading (3 min)

  • Read: chapter 5 of Gorgo Starting C++, sections on do-while, break/continue, for, and switch (the rest of the chapter)
  • Do: chapter 5 exercises 2, 3, 4, 6, 8 (loop control, for, switch, fall-through, day-of-week program)
  • Bring: questions about today’s guessing game if anything is unclear

Key Points to Reinforce

  • if / else if / else chains are tested top-to-bottom, first match wins
  • while tests the condition before each iteration
  • Make sure the loop variable changes — infinite loops are the #1 beginner bug
  • == compares, = assigns — do not confuse them

Lecture 6 — Loops, Jumps, and switch

Learning Objectives

By the end of lecture 6, students should be able to:

  • Use do-while for loops that must run at least once
  • Use break to exit a loop and continue to skip an iteration
  • Write classic for loops and range-based for loops
  • Use switch with break, fall-through, and default clauses

Materials

  • Live coding terminal with g++ (-std=c++23 -Wall -Wextra -pedantic)
  • A text editor projected for the class
  • Copies of sc++/ch05.md for reference

0. Welcome and Review (5 min)

  • Review multiple choice (from lecture 5): How many times does this loop run?

    int i = 10;
while (i > 10) {
    i++;
}
    • A. 0
    • B. 1
    • C. 10
    • D. infinite
    • E. Ben got this wrong

    Answer: A

  • Last lecture we saw if and while. Today we finish chapter 5 with more powerful looping constructs.

1. do-while Loops (10 min) 

#include <iostream>
#include <string>

int main()
{
    std::string input;

    do {
        std::cout << "Dime algo (or 'quit'): ";
        std::getline(std::cin, input);
        std::cout << "You said: " << input << "\n";
    } while (input != "quit");

    std::cout << "Adios!\n";
}
  • Condition is tested after the body — the body always runs at least once
  • Notice the semicolon after while(...) — required for do-while
  • Classic use cases: menu loops, input prompts, retry-until-valid

Tip: Use do-while when you would otherwise duplicate code to prime a while loop’s first test.

2. break and continue (10 min) 

// break: stop searching once we find what we want
std::string tracks[] = {
    "Losing My Religion",
    "Bitter Sweet Symphony",
    "Zombie"
};

for (int i = 0; i < 3; i++) {
    if (tracks[i] == "Zombie") {
        std::cout << "Found it at index " << i << "\n";
        break;
    }
}
  • break exits the nearest enclosing loop immediately
  • continue skips the rest of the current iteration and jumps to the next iteration
  • In a for loop, continue runs the update expression before testing the condition
// continue: print only odd numbers
for (int i = 1; i <= 10; i++) {
    if (i % 2 == 0) continue;
    std::cout << i << " ";
}
std::cout << "\n";
// 1 3 5 7 9

Trap: break and continue only affect the innermost loop. A break inside a nested loop does not escape the outer loop.

3. for Loops (15 min) 

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

Three parts of the header:

  1. init — runs once before the loop
  2. condition — tested before each iteration; if false, the loop ends
  3. update — runs after each iteration, before the next condition test

Iterating over an array with an index:

int scores[] = {90, 84, 77, 95, 88};
int n = sizeof(scores) / sizeof(scores[0]);

for (int i = 0; i < n; i++) {
    std::cout << "Score " << (i + 1) << ": " << scores[i] << "\n";
}

Any part of the header can be omitted. for (;;) is an infinite loop.

Tip: Always declare your loop variable inside the for: for (int i = 0; ...). That keeps i scoped to the loop.

Range-Based for 

int scores[] = {90, 84, 77, 95, 88};

for (int s : scores) {
    std::cout << s << " ";
}
// 90 84 77 95 88
  • Takes each value in turn — no index variable, no bounds worries
  • Essential for std::vector and std::array in chapter 8

Wut: A range-based for copies each element by default. To modify in place or avoid copying large objects, use a reference: for (int &s : scores) (we cover references in chapter 6).

4. switch Statements (15 min) 

int track = 2;

switch (track) {
case 1:
    std::cout << "Losing My Religion\n";
    break;
case 2:
    std::cout << "Bitter Sweet Symphony\n";
    break;
case 3:
    std::cout << "Zombie\n";
    break;
default:
    std::cout << "Unknown track\n";
    break;
}
  • Each case label must be a compile-time constant (no variables, no strings)
  • default runs when no other case matches — include one for safety

Fall-Through (Sometimes Useful, Often a Bug)

Intentional fall-through:

switch (grade) {
case 'A':
case 'B':
case 'C':
    std::cout << "Passing\n";
    break;
case 'D':
case 'F':
    std::cout << "Not passing\n";
    break;
}

Accidental fall-through:

switch (x) {
case 1:
    std::cout << "uno\n";
    // oops, no break!
case 2:
    std::cout << "dos\n";
    break;
}

If x is 1, this prints both “uno” and “dos”.

Trap: End every case with break unless you intentionally want fall-through. When fall-through is deliberate, add [[fallthrough]]; (C++17) or at least a comment so the reader knows you meant it.

5. Try It — Combined Demo (10 min)

Walk through the chapter’s Try It combined example that uses if, while, do-while, for, range-based for, and switch together. Ask the class to predict each block’s output.

for (int i = 1; i <= 100; i++) {
    if (i >= 20) break;
    if (i % 2 == 0) continue;
    std::cout << i << " ";
}
// 1 3 5 7 9 11 13 15 17 19

6. Wrap-up Quiz (5 min)

Q1. What does this print?

int x = 2;
switch (x) {
case 1:
    std::cout << "uno ";
case 2:
    std::cout << "dos ";
case 3:
    std::cout << "tres ";
    break;
default:
    std::cout << "other ";
}
std::cout << "\n";

A. uno B. dos C. dos tres D. uno dos tres E. Ben got this wrong

Answer: C — case 2 matches and falls through case 3.

Q2. How many times does the body run?

int count = 0;
int i = 10;
do {
    count++;
    i--;
} while (i > 10);

A. 0 B. 1 C. 9 D. 10 E. Ben got this wrong

Answer: B — do-while always runs the body once.

Q3. Where is the bug?

int total = 0;
for (int i = 0; i < 10; i++);
{
    total += i;
}

A. Stray ; after for(...) ends the loop body B. total needs to be double C. i is not in scope inside the braces D. Both A and C E. Ben got this wrong

Answer: D — the ; makes the loop body empty, and the block that follows cannot see i.

7. Assignment / Reading (3 min)

  • Read: chapter 6 of Gorgo Starting C++, sections on declarations, parameters, pass-by-value, pass-by-reference, const parameters, and default parameters (first half of the chapter)
  • Do: chapter 6 exercises 1, 2, 6, 11 (pass-by semantics, default params, ODR rule)
  • Bring: a for loop that surprises you if anything from today is unclear

Key Points to Reinforce

  • do-while runs at least once — use it for menus and input retries
  • break exits the nearest loop; continue skips to the next iteration
  • Classic for = init + condition + update; declare i inside
  • Range-based for is cleaner when you do not need the index; use & to avoid copies
  • switch requires a constant integer/enum; always break unless you mean to fall through