> Source URL: /demos/cs1-path/lectures/lecture-02.slides --- lecture_number: 2 title: Control Flow outcomes: [2] date: Week 2 --- # Lecture 2: Control Flow ## Overview Today we'll learn how to make programs that make decisions. --- ## The Story So Far Programs we've written execute **top to bottom**, every line, every time. ```python name = input("Name: ") print("Hello,", name) # Always prints, no matter what ``` But what if we want different behavior in different situations? --- ## Making Decisions Real programs need to respond to different conditions: - "If the password is correct, log in" - "If the temperature is below freezing, show a warning" - "If the grade is above 90, assign an A" This is called **control flow**—controlling which code runs. --- ## Boolean Values Before we make decisions, we need **True** and **False**: ```python is_raining = True is_sunny = False print(is_raining) # True print(type(True)) # ``` Named after mathematician George Boole. --- ## Comparison Operators Create boolean values by comparing things: | Operator | Meaning | Example | | -------- | --------------------- | ------------------ | | `==` | Equal to | `5 == 5` → `True` | | `!=` | Not equal to | `5 != 3` → `True` | | `<` | Less than | `3 < 5` → `True` | | `>` | Greater than | `5 > 3` → `True` | | `<=` | Less than or equal | `5 <= 5` → `True` | | `>=` | Greater than or equal | `3 >= 5` → `False` | --- ## Common Mistake: = vs == `=` is **assignment** (storing a value) `==` is **comparison** (checking equality) ```python x = 5 # Assigns 5 to x x == 5 # Checks if x equals 5 (True) x == 3 # Checks if x equals 3 (False) ``` This is a **very common** source of bugs! --- ## The if Statement Run code only when a condition is True: ```python temperature = 75 if temperature > 70: print("It's warm outside!") ``` If `temperature > 70` is `True`, the indented code runs. If `False`, Python skips it. --- ## Indentation Matters! Python uses **indentation** to know what's inside the `if`: ```python temperature = 75 if temperature > 70: print("It's warm!") # Inside the if print("Wear shorts!") # Also inside print("Have a nice day!") # Outside - always runs ``` Use **4 spaces** for each level of indentation. --- ## if-else Do one thing or another: ```python temperature = 65 if temperature > 70: print("It's warm!") else: print("It's cool!") ``` Exactly **one** of these will run—never both, never neither. --- ## Multiple Conditions: elif Check several conditions in order: ```python score = 85 if score >= 90: grade = "A" elif score >= 80: grade = "B" elif score >= 70: grade = "C" elif score >= 60: grade = "D" else: grade = "F" print("Your grade:", grade) ``` --- ## How elif Works Python checks conditions **top to bottom** and stops at the first `True`: ```python score = 85 if score >= 90: # False, skip grade = "A" elif score >= 80: # True! Run this, then done grade = "B" elif score >= 70: # Never checked grade = "C" # ... ``` Order matters! --- ## Combining Conditions: and Both conditions must be True: ```python age = 25 has_license = True if age >= 16 and has_license: print("You can drive!") ``` | `age >= 16` | `has_license` | Result | | ----------- | ------------- | -------- | | True | True | **True** | | True | False | False | | False | True | False | | False | False | False | --- ## Combining Conditions: or At least one condition must be True: ```python is_weekend = False is_holiday = True if is_weekend or is_holiday: print("No class today!") ``` | `is_weekend` | `is_holiday` | Result | | ------------ | ------------ | -------- | | True | True | **True** | | True | False | **True** | | False | True | **True** | | False | False | False | --- ## Negation: not Flips True to False and vice versa: ```python is_raining = False if not is_raining: print("Leave the umbrella home!") ``` | `is_raining` | `not is_raining` | | ------------ | ---------------- | | True | False | | False | **True** | --- ## Nested Conditionals You can put `if` statements inside other `if` statements: ```python has_ticket = True age = 15 if has_ticket: if age >= 18: print("Welcome to the R-rated movie!") else: print("Sorry, you need to be 18+") else: print("Please buy a ticket first") ``` But often you can simplify with `and`: ```python if has_ticket and age >= 18: print("Welcome!") ``` --- ## Comparing Strings Works just like numbers: ```python password = input("Enter password: ") if password == "secret123": print("Access granted!") else: print("Wrong password!") ``` String comparison is **case-sensitive**: - `"Hello" == "hello"` → `False` - `"Hello" == "Hello"` → `True` --- ## Common Patterns **Validating input:** ```python age = int(input("Enter age: ")) if age < 0: print("Age can't be negative!") elif age > 150: print("That seems unlikely...") else: print("Your age is", age) ``` --- ## Common Patterns **Checking ranges:** ```python grade = 85 if 80 <= grade <= 89: print("That's a B!") ``` This is Python shorthand for: ```python if grade >= 80 and grade <= 89: print("That's a B!") ``` --- ## Common Mistakes **Forgetting the colon:** ```python if temperature > 70 # Error! Missing : print("Warm!") ``` **Wrong indentation:** ```python if temperature > 70: print("Warm!") # Error! Must be indented ``` **Using = instead of ==:** ```python if password = "secret": # Error! Use == print("Correct!") ``` --- ## Key Takeaways 1. **Booleans** are `True` or `False` 2. **Comparison operators**: `==`, `!=`, `<`, `>`, `<=`, `>=` 3. **if** runs code when condition is True 4. **elif** checks additional conditions 5. **else** runs when all conditions are False 6. **and**, **or**, **not** combine conditions 7. **Indentation** defines what's inside the if --- ## Lab Preview In [Lab 2](../labs/lab-02.spec.md), you'll: - Practice with boolean expressions - Build programs that make decisions - Create a number guessing game - Combine conditions with `and`/`or` --- ## Questions? --- ## Backlinks The following sources link to this document: - [Lecture 2: Control Flow](/demos/cs1-path/index.path.llm.md) - [Lecture 2: Control Flow](/demos/cs1-path/labs/lab-02.spec.llm.md) - [Lecture 2: Control Flow](/demos/cs1-path/course.schedule.llm.md)