Conditionals

In life, we make decisions all the time: "If it is raining, I will take an umbrella. Otherwise, I will wear sunglasses." Programs also need to make decisions to perform different actions based on different situations. This is called conditional logic.

In C++, we use conditional statements to control which blocks of code get executed.

The if Statement

The if statement is the most fundamental tool for decision-making. It runs a block of code only if a certain condition is true. Its basic structure looks like this:

if (condition) {
    // Code to execute if the condition is true
}

Here is a simple example:

#include <bits/stdc++.h>
using namespace std;

int main() {
    int score = 100;

    if (score > 60) {
        cout << "You passed!" << endl;
    }

    return 0;
}

Let's break down how it works:

• The if keyword starts the statement.
• The condition part is the question being asked. In our example, the condition is score > 60. This expression evaluates to either true or false.
• The { ... } block contains the code that will only run if the condition is true.

What is a Condition?

A condition is an expression that results in a boolean value (true or false). The most common way to create conditions is by using relational operators to compare values.

Warning: A very common bug is to mix up the assignment operator (=) with the equality operator (==).

• if (x = 5) assigns the value 5 to x.
• if (x == 5) compares x to 5 and checks if they are equal.

Combining Conditions

You can combine multiple conditions using logical operators.

• && (AND) is true only if both conditions are true.
• || (OR) is true if at least one condition is true.
• ! (NOT) inverts the value (true becomes false, false becomes true).

int age = 25;
if (age >= 18 && age <= 60) {
    cout << "You are of working age." << endl;
}

The else Statement

The else statement provides an alternative block of code to run when the if condition is false.

int score = 32;

if (score >= 33) {
    // Above condition fails, so this line is skipped and execution jumps to the 'else' block
    cout << "You passed!" << endl;
} else {
    cout << "You did not pass. Better luck next time!" << endl;  // This is printed on screen
}

The else if Statement

To check a multiple conditions one after another, you can use else if. As soon as one condition in the chain is met, its code block runs, and the rest of the chain is skipped.

int score = 79;

if (score >= 80) {                // Condition fails, jump to the next condition check
    cout << "Grade: A+" << endl;  // Skipped
} else if (score >= 70) {         // Condition succeeds, run the code
    cout << "Grade: A" << endl;   // This will be printed, and jump to the end of last "else" block
} else if (score >= 60) {         // Skipped...
    cout << "Grade: A-" << endl;
} else {
    cout << "Grade: Too Bad, Barely Pass" << endl;
}
// Jumps to here after any code block is executed

Nested Conditionals

You can put any kind of code inside a conditional statement, including another conditional statements. This is called nesting.

Nesting is very useful for checking a second condition only if the first condition is already true.

#include <bits/stdc++.h>
using namespace std;

int main() {
    int num = 10;

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

        // This is a nested if statement
        if (num % 2 == 0) {
            cout << "And it is also even." << endl;
        } else {
            cout << "And it is also odd." << endl;
        }

    } else if (num < 0) {
        cout << "The number is negative." << endl;
    } else {
        cout << "The number is zero." << endl;
    }

    return 0;
}

In this example, the check for whether the number is even or odd (the inner if-else) only happens after we have already established that the number is positive (the outer if).

The bool Type: Storing true and false

As you have learned, the result of any condition (like age >= 18) is a special value that is either true or false, called a boolean value. In C++, there is a specific builtin data type for storing these values: bool.

The bool type can only hold one of two values: true or false. These are called boolean literals.

You can declare a bool variable and assign a value to it just like any other builtin type:

bool is_raining = true;       // Initialize with a boolean literal
bool is_sunny = !is_raining;  // Initialize with a condition

You can store the result of a condition in a bool variable to reuse it, and also to make your if statements cleaner and more readable. For example:

int age = 20;
bool can_vote = (age >= 18); // The condition (age >= 18) evaluates to true, so can_vote becomes true

if (can_vote) {
    cout << "You are eligible to vote. Vote for your favorite Akib!" << endl;
} else {
    cout << "You are not yet eligible to vote. But soon you'll be!" << endl;
}

if (can_vote && age == 18) {
    cout << "Woah, new voter? Feeling excited? You should be!" << endl;
}