Control flow statements determine the order in which program statements are executed. They allow you to make decisions, repeat code, and control program execution flow.
The most basic decision-making structure.
if (condition) {
// Code to execute if condition is true
}Example:
int age = 18;
if (age >= 18) {
cout << "You are an adult." << endl;
}Execute different code based on a condition.
if (condition) {
// Code if condition is true
} else {
// Code if condition is false
}Example:
int score = 85;
if (score >= 90) {
cout << "Grade: A" << endl;
} else {
cout << "Grade: B or lower" << endl;
}Handle multiple conditions in sequence.
if (condition1) {
// Code if condition1 is true
} else if (condition2) {
// Code if condition2 is true
} else if (condition3) {
// Code if condition3 is true
} else {
// Code if all conditions are false
}Example:
int score = 85;
if (score >= 90) {
cout << "Grade: A" << endl;
} else if (score >= 80) {
cout << "Grade: B" << endl;
} else if (score >= 70) {
cout << "Grade: C" << endl;
} else if (score >= 60) {
cout << "Grade: D" << endl;
} else {
cout << "Grade: F" << endl;
}If statements inside other if statements.
int age = 25;
bool hasLicense = true;
if (age >= 18) {
if (hasLicense) {
cout << "You can drive." << endl;
} else {
cout << "You need a license to drive." << endl;
}
} else {
cout << "You are too young to drive." << endl;
}Execute code while a condition is true.
while (condition) {
// Code to repeat
// Update condition to avoid infinite loop
}Example:
int count = 1;
while (count <= 5) {
cout << "Count: " << count << endl;
count++; // Important: update the condition
}Output:
Count: 1
Count: 2
Count: 3
Count: 4
Count: 5
Execute code at least once, then check condition.
do {
// Code to repeat
// Update condition
} while (condition);Example:
int number;
do {
cout << "Enter a positive number: ";
cin >> number;
} while (number <= 0);
cout << "You entered: " << number << endl;Most common loop with initialization, condition, and update.
for (initialization; condition; update) {
// Code to repeat
}Example:
for (int i = 0; i < 5; i++) {
cout << "Iteration " << i << endl;
}Output:
Iteration 0
Iteration 1
Iteration 2
Iteration 3
Iteration 4
Iterate over containers easily.
vector<int> numbers = {1, 2, 3, 4, 5};
// By value (copy)
for (int num : numbers) {
cout << num << " ";
}
// By reference (no copy)
for (int& num : numbers) {
num *= 2; // Modify original values
}
// By const reference (read-only)
for (const int& num : numbers) {
cout << num << " ";
}Alternative to long if-else chains for multiple discrete values.
switch (expression) {
case value1:
// Code for value1
break;
case value2:
// Code for value2
break;
case value3:
// Code for value3
break;
default:
// Code if no case matches
break;
}Example:
int day = 3;
switch (day) {
case 1:
cout << "Monday" << endl;
break;
case 2:
cout << "Tuesday" << endl;
break;
case 3:
cout << "Wednesday" << endl;
break;
case 4:
cout << "Thursday" << endl;
break;
case 5:
cout << "Friday" << endl;
break;
case 6:
case 7:
cout << "Weekend" << endl;
break;
default:
cout << "Invalid day" << endl;
break;
}Important Notes:
breakis crucial to prevent fall-through- Multiple cases can share the same code
defaulthandles unmatched values
Exit a loop immediately.
for (int i = 1; i <= 10; i++) {
if (i == 5) {
break; // Exit loop when i equals 5
}
cout << i << " ";
}
cout << "Loop ended." << endl;
// Output: 1 2 3 4 Loop ended.Skip current iteration and continue with next.
for (int i = 1; i <= 10; i++) {
if (i % 2 == 0) {
continue; // Skip even numbers
}
cout << i << " ";
}
// Output: 1 3 5 7 9for (int i = 1; i <= 3; i++) {
cout << "Outer loop iteration " << i << endl;
for (int j = 1; j <= 5; j++) {
if (j == 3) {
continue; // Skip j = 3
}
if (j == 5) {
break; // Exit inner loop when j = 5
}
cout << " Inner loop: j = " << j << endl;
}
}int getValidAge() {
int age;
while (true) {
cout << "Enter your age (1-120): ";
cin >> age;
if (age >= 1 && age <= 120) {
return age;
}
cout << "Invalid age. Please try again." << endl;
}
}void showMenu() {
cout << "=== Calculator Menu ===" << endl;
cout << "1. Addition" << endl;
cout << "2. Subtraction" << endl;
cout << "3. Multiplication" << endl;
cout << "4. Division" << endl;
cout << "5. Exit" << endl;
cout << "Choose an option: ";
}
void calculator() {
int choice;
do {
showMenu();
cin >> choice;
switch (choice) {
case 1:
cout << "Addition selected" << endl;
break;
case 2:
cout << "Subtraction selected" << endl;
break;
case 3:
cout << "Multiplication selected" << endl;
break;
case 4:
cout << "Division selected" << endl;
break;
case 5:
cout << "Goodbye!" << endl;
break;
default:
cout << "Invalid option. Try again." << endl;
}
} while (choice != 5);
}void countdown(int seconds) {
for (int i = seconds; i > 0; i--) {
cout << "Time remaining: " << i << " seconds" << endl;
// Simulate time passing (in real app, use sleep)
for (int j = 0; j < 100000000; j++) {
// Busy wait
}
}
cout << "Time's up!" << endl;
}// BAD: Missing update
int i = 0;
while (i < 5) {
cout << i << endl;
// i++; // Missing! Creates infinite loop
}
// GOOD: Proper update
int i = 0;
while (i < 5) {
cout << i << endl;
i++; // Update condition
}// BAD: Missing break causes fall-through
int day = 1;
switch (day) {
case 1:
cout << "Monday" << endl;
// Missing break!
case 2:
cout << "Tuesday" << endl;
break;
}
// Output: Monday Tuesday (unexpected!)
// GOOD: Proper break statements
int day = 1;
switch (day) {
case 1:
cout << "Monday" << endl;
break; // Prevents fall-through
case 2:
cout << "Tuesday" << endl;
break;
}// BAD: Off-by-one error
for (int i = 0; i <= 5; i++) { // Runs 6 times (0,1,2,3,4,5)
cout << i << " ";
}
// GOOD: Correct bounds
for (int i = 0; i < 5; i++) { // Runs 5 times (0,1,2,3,4)
cout << i << " ";
}#include <iostream>
#include <cstdlib>
#include <ctime>
using namespace std;
int main() {
srand(time(0)); // Seed random number generator
int secretNumber = rand() % 100 + 1; // 1 to 100
int guess;
int attempts = 0;
cout << "=== Number Guessing Game ===" << endl;
cout << "I'm thinking of a number between 1 and 100." << endl;
do {
cout << "Enter your guess: ";
cin >> guess;
attempts++;
if (guess < secretNumber) {
cout << "Too low! Try again." << endl;
} else if (guess > secretNumber) {
cout << "Too high! Try again." << endl;
} else {
cout << "Congratulations! You guessed it in " << attempts << " attempts!" << endl;
}
} while (guess != secretNumber);
return 0;
}#include <iostream>
using namespace std;
int main() {
int n;
cout << "Enter a number for multiplication table: ";
cin >> n;
cout << "Multiplication table for " << n << ":" << endl;
cout << "================================" << endl;
for (int i = 1; i <= 10; i++) {
cout << n << " 脳 " << i << " = " << (n * i) << endl;
}
return 0;
}#include <iostream>
using namespace std;
bool isPrime(int n) {
if (n <= 1) return false;
if (n <= 3) return true;
if (n % 2 == 0 || n % 3 == 0) return false;
for (int i = 5; i * i <= n; i += 6) {
if (n % i == 0 || n % (i + 2) == 0) {
return false;
}
}
return true;
}
int main() {
int number;
cout << "Enter a number to check if it's prime: ";
cin >> number;
if (isPrime(number)) {
cout << number << " is a prime number." << endl;
} else {
cout << number << " is not a prime number." << endl;
}
return 0;
}After mastering control flow, explore:
- Functions - Organizing code into reusable blocks
- Arrays & Vectors - Working with collections of data
- Strings - Text manipulation and processing
- Object-Oriented Programming - Classes and objects
- Use appropriate control structures for different scenarios
- Always update loop conditions to avoid infinite loops
- Include break statements in switch cases to prevent fall-through
- Test edge cases and boundary conditions
- Keep loops simple and readable
"The best programs are written for people to read, and only incidentally for machines to execute." - Harold Abelson