111 lines
2.7 KiB
Python
111 lines
2.7 KiB
Python
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# Lesson 04: Conditionals
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#
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# Conditionals let your program make decisions: "if this is true, do that;
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# otherwise, do something else." The keywords are: if, elif, else.
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#
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# Indentation matters in Python! Everything inside an if block must be
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# indented by the same amount (typically 4 spaces).
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#
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# To run this file:
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# python 04_conditionals.py
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# --- Basic if statement ---
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temperature = 35
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if temperature > 30:
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print("It's hot outside!")
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# --- if / else ---
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# The else block runs when the if condition is False.
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is_raining = False
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if is_raining:
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print("Bring an umbrella.")
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else:
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print("No umbrella needed.")
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# --- if / elif / else ---
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# Use elif (short for "else if") to check multiple conditions in sequence.
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# Python checks each condition top to bottom and stops at the first True one.
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score = 78
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if score >= 90:
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print("Grade: A")
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elif score >= 80:
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print("Grade: B")
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elif score >= 70:
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print("Grade: C")
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elif score >= 60:
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print("Grade: D")
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else:
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print("Grade: F")
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# --- Comparison operators ---
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# These return True or False and are used inside conditions.
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#
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# == equal to
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# != not equal to
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# > greater than
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# < less than
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# >= greater than or equal to
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# <= less than or equal to
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x = 10
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print(x == 10) # True
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print(x != 5) # True
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print(x > 20) # False
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print(x <= 10) # True
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# --- Logical operators: and, or, not ---
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# Combine multiple conditions together.
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age = 25
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has_id = True
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# 'and' requires BOTH sides to be True
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if age >= 18 and has_id:
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print("Entry allowed.")
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# 'or' requires AT LEAST ONE side to be True
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is_weekend = False
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is_holiday = True
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if is_weekend or is_holiday:
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print("No work today!")
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# 'not' flips True to False and False to True
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is_logged_in = False
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if not is_logged_in:
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print("Please log in.")
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# --- Nested conditionals ---
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# You can place an if inside another if. Keep nesting shallow when possible.
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user_role = "admin"
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is_active = True
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if is_active:
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if user_role == "admin":
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print("Welcome, administrator.")
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else:
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print("Welcome, user.")
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else:
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print("Account is inactive.")
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# --- Checking membership with 'in' ---
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# 'in' tests whether a value exists inside a collection (like a list or string).
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allowed_colors = ["red", "green", "blue"]
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chosen_color = "green"
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if chosen_color in allowed_colors:
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print(f"{chosen_color} is a valid choice.")
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else:
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print(f"{chosen_color} is not allowed.")
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# --- Try it yourself ---
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# Write an if/elif/else block that:
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# - Takes a variable called 'hour' (an integer 0–23 representing the time of day)
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# - Prints "Good morning" if hour < 12
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# - Prints "Good afternoon" if 12 <= hour < 18
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# - Prints "Good evening" otherwise
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