3.7 Nested Conditionals

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Lesson: Nested Conditionals
- What are Nested Conditionals?
- Nested Conditionals MC

Lesson: Nested Conditionals

What are Nested Conditionals?

A nested conditional is a conditional statement inside another conditional statement, or an if statement within an if statement.
This allows us to check multiple conditions in a structured way, which is useful when decisions depend on more than one factor

For example:

First, check if a main condition is true.
Then, only if it’s true, check an additional condition inside it.

The examples below uses if and else statments to form a nested conditional in both Python and Java.

# Example of a nested conditional in Python

age = 18 ##Sets age variable
has_id = True ##Sets has_id variable

if age >= 18: ##Checks if age is 18 or older
    if has_id: ##Checks if the person has an ID
        print("You are allowed to enter.") 
    else: ##If the person does not have an ID
        print("You need an ID to enter.")
else: ##If the person is younger than 18
    print("You are too young to enter.")

%%js

// Example of a nested conditional in JavaScript

let age = 18;
let hasID = true;

if (age >= 18) {
    if (hasID) {
        console.log("You are allowed to enter.");
    } else {
        console.log("You need an ID to enter.");
    }
} else {
    console.log("You are too young to enter.");
}

How the example works:

The program first checks if age >= 18.
If False, it immediately goes to the else branch.
If True, it goes inside and checks the nested condition if has_id.
Depending on if the if statement is true or false, a different final result is printed.

Snake Game Example

Heres an example of a nested conditional in our Snake Game Hack. In this example, the nested conditional is used to check whether or not the snake hits a wall, losing a life if they do.

// Wall Checker
            if(wall === 1){
                // Wall on, Game over test
                if (snake[0].x < 0 || snake[0].x === canvas.width / BLOCK || snake[0].y < 0 || snake[0].y === canvas.height / BLOCK){
                    loseLife();
                    return;
                }
            }else{
                // Wall Off, Circle around
                for(let i = 0, x = snake.length; i < x; i++){
                    if(snake[i].x < 0){
                        snake[i].x = snake[i].x + (canvas.width / BLOCK);
                    }
                    if(snake[i].x === canvas.width / BLOCK){
                        snake[i].x = snake[i].x - (canvas.width / BLOCK);
                    }
                    if(snake[i].y < 0){
                        snake[i].y = snake[i].y + (canvas.height / BLOCK);
                    }
                    if(snake[i].y === canvas.height / BLOCK){
                        snake[i].y = snake[i].y - (canvas.height / BLOCK);
                    }
                }
            }

Calculator Example

Heres another example of a nested conditional, this time in our calculator hack. In this example, multiple nested conditionals are used to do multiple things:

Distinguishing between digits and decimal points
Overwrting the display when inputing new numbers
prevent leading zeros
Allowing only 1 decimal per number

// Number action
function number (value) { // function to input numbers into the calculator
    if (value != ".") {
        if (nextReady == true) { // nextReady is used to tell the computer when the user is going to input a completely new number
            output.innerHTML = value;
            if (value != "0") { // if statement to ensure that there are no multiple leading zeroes
                nextReady = false;
            }
        } else {
            output.innerHTML = output.innerHTML + value; // concatenation is used to add the numbers to the end of the input
        }
    } else { // special case for adding a decimal; can't have two decimals
        if (output.innerHTML.indexOf(".") == -1) {
            output.innerHTML = output.innerHTML + value;
            nextReady = false;
        }
    }
}

Nested Conditionals MC

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Question 1

What is a nested conditional?

A) Two separate if statements next to each other
B) An if statement inside another if statement
C) A while loop inside an if statement
D) A conditional that always runs no matter what

Question 2

What will the following Python code print?

age = 18
has_id = False

if age >= 18:
    if has_id:
        print("You are allowed to enter.")
    else:
        print("You need an ID to enter.")
else:
    print("You are too young to enter.")

A) You are allowed to enter.
B) You need an ID to enter.
C) You are too young to enter.
D) Nothing will print.

Question 3

In the nested condition below, what condition must be true for “Access Granted” to print?

if username == "admin":
    if password == "1234":
        print("Access Granted")

A) Only username is correct
B) Only password is correct
C) Both username and password are correct
D) Either one is correct

Computer Science Principles