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What are if statements?

If statements let your program make decisions. They check if something is true or false, then act accordingly. Real-life logic:
  • IF it’s raining THEN take umbrella
  • IF battery < 20% THEN charge phone
  • IF password correct THEN allow access

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Basic if statement

age = 18

if age >= 18:
    print("You can vote!")
    print("You're an adult")
How it works:
  1. Check the condition (age >= 18)
  2. If True, run the indented code
  3. If False, skip it
The colon : and indentation are required! This is how Python knows what code belongs to the if statement.

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If-else statements

Handle both True and False cases: 
temperature = 25

if temperature > 30:
    print("It's hot!")
else:
    print("Nice weather!")

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If-elif-else chains

For multiple conditions: 
score = 85

if score >= 90:
    print("A - Excellent!")
elif score >= 80:
    print("B - Good job!")
elif score >= 70:
    print("C - Keep it up!")
else:
    print("F - Need improvement")
Python checks each condition in order and runs the first True one.
Why elif instead of multiple if statements? With elif, Python stops checking once it finds a true condition. This is more efficient and prevents multiple blocks from running. The order matters - always put more specific conditions first!

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Multiple conditions

Combine conditions with and, or, not: 
age = 25
has_license = True

# Both must be True
if age >= 18 and has_license:
    print("You can drive!")

# At least one must be True
if weekend or holiday:
    print("No work today!")

# Reverse the condition
if not raining:
    print("Let's go outside!")

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Nested if statements

Put if statements inside other if statements: 
has_ticket = True
age = 15

if has_ticket:
    if age >= 18:
        print("Enjoy the movie!")
    else:
        print("Need adult supervision")
else:
    print("Buy a ticket first")

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Common mistakes

# Wrong
if x > 5
    print("Big")

# Right
if x > 5:
    print("Big")

# Wrong (assignment)
if x = 5:
    print("Five")

# Right (comparison)
if x == 5:
    print("Five")

# Wrong
if True:
print("Hello")  # IndentationError

# Right
if True:
    print("Hello")

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What’s next?

Now let’s learn about loops to repeat code efficiently!

Loops

Repeat code without copying