Control Flow in C++: Conditionals (if, else, else if)

1Why use conditionals?

Not all programs follow a single, straight path. Often, you want your program to make decisions based on user input or changing values. That is where conditionals come in.

Here are some real-world examples of conditionals in action:

• If the user is logged in, show their dashboard. Otherwise, show the login screen.
• If the number is negative, print an error message.
• If the temperature is above a threshold, turn on a fan.

C++ gives you several tools to handle such decisions: if, else, and else if. These let your program choose which path to follow based on conditions that evaluate to either true or false.

2The if statement

The if statement runs a block of code only if a condition is true. If the condition is false, the block is skipped entirely.

Syntax:

if (condition) {
    // code runs if condition is true
}

Example:

int number = 10;

if (number > 0) {
    cout << "The number is positive." << endl;
}

3The else statement

The else block runs when the if condition is false. This gives you two paths: one for when the condition is true, and another for when it is false.

int number = -5;

if (number > 0) {
    cout << "Positive number." << endl;
} else {
    cout << "Not a positive number." << endl;
}

4The else if ladder

Use else if to check multiple conditions in order. The first condition that evaluates to true runs its block, and the rest are skipped.

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 {
    cout << "Grade: F" << endl;
}

5Relational operators

Relational operators compare two values and return true or false. These are the building blocks of conditional logic.

6Logical operators

Logical operators let you combine multiple conditions into a single expression.

Example:

int age = 25;
bool hasLicense = true;

if (age >= 18 && hasLicense) {
    cout << "You can drive." << endl;
} else {
    cout << "You cannot drive." << endl;
}

7Nested if statements

You can put an if statement inside another if statement. This is called nesting. It is useful for checking multiple related conditions.

int number = 8;

if (number > 0) {
    if (number % 2 == 0) {
        cout << "Positive even number." << endl;
    } else {
        cout << "Positive odd number." << endl;
    }
}

8Example: Simple login check

Here is a practical example that combines several concepts from this chapter: reading input, comparing strings, and using conditionals.

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

int main() {
    string username;
    string password;

    cout << "Enter username: ";
    cin >> username;
    cout << "Enter password: ";
    cin >> password;

    if (username == "admin" && password == "1234") {
        cout << "Login successful!" << endl;
    } else {
        cout << "Invalid credentials." << endl;
    }

    return 0;
}

9Common mistakes to avoid

10Mini exercises

11Quiz yourself