🐝Control Flow

So far, our programs have executed line-by-line, step-by-step from top to bottom. In the real world however, we might want to

• make decisons based on a condition
• repeat certain actions
• organise logic into reusable pieces

This is where control flow comes in

Conditionals

Conditionals let our program make decisions.

main.cpp

int score = 85;

if (score == 100) {
    std::cout << "Perfect" << std::endl;
} else if (score >= 90) {
    std::cout << "Grade A" << std::endl;
} else if (score >= 80) {
    std::cout << "Grade B" << std::endl;
} else if (score >= 70) {
    std::cout << "Grade C" << std::endl;
} else {
    std::cout << "Needs improvement" << std::endl;
}

⚠️ Common Pitfall

= means assignment while == checks for equality

int x = 5;      // assignment
x == 5;         // comparison

Loops

Loops lets us repeat certain blocks of code

while loop

A while loop keeps running as long as the condition is fufilled.

Code	Output
int x = 1; while (x <= 5) { std::cout << x << std::endl; x++; }

for loop

A for loop is useful when we know how many times we want to repeat something.

main.cpp for (int i = 1; i <= 5; i++) { std::cout << i << std::endl; }

break / continue

break forces the loop to terminate prematurely.

continue skips the rest of the current iteration.

main.cpp for (int i = 1; i <= 5; i++) { if (i == 3) { break; } std::cout << i << std::endl; }
main.cpp for (int i = 1; i <= 5; i++) { if (i == 3) { continue; } std::cout << i << std::endl; }

Functions

Functions let us group code into reusable blocks.

Basic Usage

main.cpp

#include <iostream>

void greet() {
    std::cout << "Hello!" << std::endl;
}

int main() {
    greet();
}

Here:

• void means the function does not return anything
• greet is the function name
• greet() calls the function, causing Hello! to be printed.

Function Parameters

Our current greet function always print the same message (which isn't very flexible). Let's augment it by adding a parameter so that our greeting message can change depending on the person.

main.cpp

#include <iostream>

void greet(std::string name) {
    std::cout << "Hello, " << name << "!" << std::endl;
}

int main() {
    greet("Benn");
    greet("Bryan");
    greet("Anton Tim");
}

Functions Return Value

A return value is the value the function sends back to the caller after it's done executing. At the call site, you can use the result however you wish.

main.cpp

#include <iostream>

int add(int x, int y) {
    return x + y;
}

int main() {
    int result = add(3, 4);
    std::cout << result << std::endl;
}

Exercise(s)

⭐ Write a function countdown(int n) that takes in a number n and prints from n down to 1.

countdown(5)
5
4
3
2
1

⭐⭐ Write a program that hardcodes a secret number. It will repeatedly ask the user to guess. Depending on the user's guess, the program will feedback Too low, Too high or Correct.

Guess the number: 50
Too high

Guess the number: 25
Too low

Guess the number: 30
Too low

Guess the number: 42
Correct!