Friendly introduction
Have you ever made a choice like "If it's raining, I'll take an umbrella, else I won't"? Computers do the same thing using conditional statements. In Python, we use if, elif, and else to help the computer decide what to do based on conditions.
1) What is a condition?
- A condition is a question that's either True or False.
- Example conditions:
- 10 > 5 is True
- name == "Alex" is True only if name is exactly "Alex"
- age >= 13 is True if age is 13 or more
2) Step 1: The if statement
Use if to run code only when a condition is True.
Example:
x = 10
if x > 5:
print("x is bigger than 5")What happens: Since x > 5 is True, it prints the message.
3) Step 2: Adding else
Use else for what happens when the if condition is False.
Example:
password = "cat"
guess = "dog"
if guess == password:
print("Welcome!")
else:
print("Try again!")What happens: guess == password is False, so it prints "Try again!"
4) Step 3: More choices with elif
elif means "else if." Use it when you have more than two choices. Python checks from top to bottom and runs the first True block.
Example: score to grade
score = int(input("Enter your score (0-100): "))
if score >= 90:
print("Grade: A")
elif score >= 75:
print("Grade: B")
elif score >= 60:
print("Grade: C")
else:
print("Grade: D (Keep practicing!)")Tip: The order matters! Put the highest conditions first (like 90 before 75).
5) Step 4: Comparison and logical operators
Comparison operators:
- == equal to
- != not equal to
- >, <, >=, <= greater/less than (or equal)
Logical operators:
- and both conditions must be True
- or at least one condition is True
- not flips True to False (and vice versa)
Examples:
age = 14
has_ticket = True
if age >= 13 and has_ticket:
print("You can watch the movie.")
color = "red"
if color == "red" or color == "blue":
print("Primary color!")
is_raining = False
if not is_raining:
print("Enjoy the sunshine!")6) Step 5: Indentation matters
- The code "inside" if/elif/else must be indented (spaces at the start of the line).
- Python typically uses 4 spaces. Be consistent.
Example:
number = 3
if number % 2 == 1:
print("Odd number")
print("Done")In this example, "Odd number" prints only if the condition is True. "Done" prints every time because it's not indented under the if.
7) Common beginner tips
- Use = to assign a value, and == to compare values.
- input() gives you text (a string). Use int(input(...)) if you need a number.
8) Mini exercise: Weather helper
Goal: Ask the user for the temperature and print a simple message.
Instructions:
- Ask the user for the temperature in Celsius.
- Convert it to an integer.
- Use if/elif/else to print:
- 30 or more: "It's hot!"
- 20 to 29: "It's warm."
- 10 to 19: "It's cool."
- Below 10: "It's cold."
Starter code:
temp = int(input("Temperature in °C: "))
if temp >= 30:
print("It's hot!")
elif temp >= 20:
print("It's warm.")
elif temp >= 10:
print("It's cool.")
else:
print("It's cold.")Try changing the numbers to test different paths.
Bonus (optional):
- Ask the user if it's raining ("yes" or "no"). If it's raining and temp < 20, print "Bring a jacket!"
9) Short summary
- if checks a condition and runs code if it's True.
- elif adds more conditions, checked in order.
- else runs when none of the above conditions are True.
- Use comparison (==, >, <=, etc.) and logical operators (and, or, not) to build smarter decisions.
- Indentation shows which lines belong to the if/elif/else blocks.
You're now ready to make your programs think and choose like you do!