CS50 Introduction to Programming with Python - Conditionals

Presented by David Malan
Focus: Conditionals in Python, making decisions in code.

What are Conditionals?

Allow execution of different lines of code based on conditions.
Create logical forks in the code.

Comparison Operators in Python

Greater than: >
Greater than or equal to: >=
Less than: <
Less than or equal to: <=
Equality: == (two equal signs)
Not equal: !=

Using Conditionals in Python

Use the if keyword to ask questions.
Syntax:
```
if condition:
    # code to execute
```

Example of comparisons:

if X < Y:
    print("X is less than Y")
elif X > Y:
    print("X is greater than Y")
else:
    print("X is equal to Y")

Control Flow and Flowcharts

Code executes from top to bottom.
Use a flowchart to visualize logical flow:
- Start -> Question (Boolean expression) -> True/False branches -> End

Improving Conditional Logic

Avoid asking unnecessary questions.

Use elif to chain conditions logically:

if X < Y:
    # ...
elif X > Y:
    # ...
else:
    # ...

Using else provides a catch-all for when all previous conditions are false.

Example Program: Compare Values

Program compares user-input integers and prints results:

X = int(input("What's X? "))
Y = int(input("What's Y? "))
if X < Y:
    print("X is less than Y")
elif X > Y:
    print("X is greater than Y")
else:
    print("X is equal to Y")

Further Enhancements

Use logical operators and, or to combine conditions:

if (X > 90 and X <= 100):
    print("A")
elif (X > 80 and X < 90):
    print("B")

Grading Example

Implementing grading logic based on score ranges:

if score >= 90:
    print("A")
elif score >= 80:
    print("B")
elif score >= 70:
    print("C")
elif score >= 60:
    print("D")
else:
    print("F")

Using Functions

Create reusable functions for repeated logic:
```
def is_even(n):
    return n % 2 == 0
```
- Call functions within conditional statements.

Use of `match` Statement

New match statement as a more compact alternative to multiple if conditions:

match name:
    case "Harry":
        print("Gryffindor")
    case "Hermione":
        print("Gryffindor")
    case "Ron":
        print("Gryffindor")
    case "Draco":
        print("Slytherin")
    case _:  # catch-all
        print("Who?")

Summary

Conditionals allow for logical decision-making in programming.
Use if, elif, else, and now match to structure your code effectively.
Aim for readability and conciseness to improve code quality.

Understanding Conditionals in Python Programming