Before we start

What’s Happening Here:

  • The game is listening for a key press.
  • if checks which key was pressed.
  • Depending on the condition, the game runs a different action:
    • If O → start moving the pushers.
    • If Escape → stop them.
    • Otherwise → print a message.
  • The game also checks whether if the key’s direction is facing the lock and it is touching the lock
    • if yes, go to the next level
    • if no, continue with the game

Every time the player presses a key, the program runs this decision tree.
This is exactly what you’ll be learning to write in your own code today!

CSP 3.6 — Conditionals

Conditionals let a program choose what to do based on whether something is True or False.

If it rains → take an umbrella 🌧️; otherwise → wear sunglasses 😎.

🎯 Objectives

By the end, you will be able to

  • Explain selection and conditions.
  • Write if, if/else, and if/elif/else statements.
  • Build boolean expressions using comparisons and and / or / not.
  • Predict what branch of code will run given different inputs.

Key Vocabulary

  • Condition: a test that is either True or False.
  • Boolean: a value that is True or False.
  • Branch: the path your program takes depending on the condition.
  • Selection: choosing different actions based on conditions.

🧭 How selection flows

Start
  │
  ▼
[ Evaluate condition ]
   ├── True  → do Action A
   └── False → do Action B

1) Boolean expressions & comparisons

A boolean expression always gives True or False.

Common comparisons :

  • > greater than
  • < less than
  • >= greater or equal
  • <= less or equal
  • == equal to
  • != not equal to

Logical operators (single level for this lesson): and, or, not

# Try it: change the values and re-run to see True/False
x, y = 7, 10

print("x > y:", x > y)
print("x == 7:", x == 7)
print("x != y:", x != y)


print("x in [0,10] AND y in [0,10]:", (0 <= x <= 10) and (0 <= y <= 10))
print("x in [0,10] OR y in [0,10]:",  (0 <= x <= 10) or  (0 <= y <= 10))
print("NOT (x > y):", not (x > y))

x > y: False
x == 7: True
x != y: True
x in [0,10] AND y in [0,10]: True
x in [0,10] OR y in [0,10]: True
NOT (x > y): True

2) if — run a block only when the condition is True

# Example: driving check
age = 18
if age >= 16:
    print("You can drive!")  # this line only runs if condition is True

You can drive!

let age = 18;
if (age >= 16) {
  console.log("You can drive!") //this line only runs if condition is True
}

You can drive!

Think & Try:

  • Change age to 10, 16, 20. When does the message print?
  • Explain the role of the colon : and indentation in Python.

3) if / else — choose one of two branches

Exactly one branch runs.

# Example: simple gate
age = 14
if age >= 16:
    print("You can drive!")
else:
    print("You cannot drive yet.")

You cannot drive yet.

let age = 18;
if (age >= 16) {
  console.log("You can drive!");
} else {
  console.log("You cannot drive yet.");
}

You can drive!

Think & Try:

  • Flip the comparison to age > 15. Does it behave the same?
  • Reword messages to fit your style.

4) if / elif / else — multiple conditions

Python checks top to bottom and runs the first matching branch.

# Example: letter grade
score = 85
if score >= 90:
    print("A")
elif score >= 80:
    print("B")
elif score >= 70:
    print("C")
else:
    print("D or F")

B

let score = 85;

if (score >= 90) {
    console.log("A");
} else if (score >= 80) {
    console.log("B");
} else if (score >= 70) {
    console.log("C");
} else {
    console.log("D or F");
}


B

Think & Try:

  • Set score = 90, 80, 79, 70. Which branch triggers?
  • Why must elif score >= 80 come after the >= 90 check?

5) Combining conditions with and / or / not

We can combine simple comparisons into a single (non-nested) boolean expression.

# Example: number in range [0, 10]
if (n >= 0) and (n <= 10):  
    print("n is in [0,10]")
else:
    print("n is outside [0,10]")


//Javascript

let n = 7;
if (n >= 0 && n <= 10) {
  console.log("n is in [0,10]");
} else {
  console.log("n is outside [0,10]");
}

//Note: `&&` is JS AND, `||` is OR, `!` is NOT. In Python we write `and`, `or`, `not`.

<IPython.core.display.Javascript object>

Try

  • Changing n to -2, 0, 10, 11.
  • Replacing the “and” with “or”. Think of situations when you would use “and” & situations when you would use “or”

Mini Practice: Predict, then Run

Before you run each cell, think and predict what it will print. Then run and compare.

# 1) What prints?
temp = 60
if temp > 70:
    print("Warm")
else:
    print("Chilly")  # Predict before you run

Chilly

# 2) What prints?
water = "sparkling"
if water == "still":
    print("Calm")
elif water == "sparkling":
    print("Fizz")
else:
    print("Unknown")

Fizz

# 3) What prints?
x = 3
y = 12
if x > 0 and y < 10:
    print("Case A")
elif x > 0 and y >= 10:
    print("Case B")
else:
    print("Case C")

Case B

Common issues and solutions

  • Missing colon : after if, elif, or else. → Add it
  • Indentation: code inside a branch must be indented consistently (usually 4 spaces).
  • Using = vs ==: = assigns a value; == tests for equality.
  • Case sensitivity: "Yes" is not the same as "yes". Use .lower() for user inputs so that your program actually runs.
  • Exclusive vs inclusive: > (strict) vs >= (includes equality). Choose carefully based on your situation

Review

| Concept | Purpose | Example | |—|—|—| | if | Run only when condition True | if age >= 16: print("Drive") | | if/else | Choose between 2 paths | if x % 2 == 0: ... else: ... | | if/elif/else | Many options in order | grades ladder | | comparisons | build boolean tests | ==, !=, >, <, >=, <= | | logic | test multiple requirements | and, or, not |

Exit Ticket

Write a one-sentence answer in a new cell:

  • What does selection mean in programming?
  • Give a real-world example.