04 Control-Flow - 01 Conditional Branching

Control Flow Statements in Python

• if
• if...else
• if...elif...else

These are the primary structures for controlling the flow of execution based on conditions.

• while – Used to repeat a block of code based on a condition.
• for – Used to repeat a block of code a fixed number of times.
• else – Executes code when the preceding condition is not true.
• break – Exits the loop immediately.
• continue – Skips the rest of the current loop iteration and moves to the next iteration.
• pass – Does nothing; used as a placeholder.
• assert – Used for debugging purposes to test if a condition is true.
• return – Exits a function and optionally returns a value.

Note: Python does not have a switch case statement. but match case is introduced in python 3.10

Altering Control Flow

• if, elif, else – Conditional execution of code blocks based on boolean expressions.
• for loop – Used to repeat code a fixed number of times, often iterating over a sequence (like a list or range).
• while loop – Repeats a block of code as long as a specified condition is true.

Conditional Execution

Python allows you to track conditions efficiently with boolean expressions (True or False). Some examples include:

if m % n != 0:  # This executes only if m % n != 0 is True
(m, n) = (n, m % n)

Alternating Execution (using else):

if m % n != 0:
(m, n) = (n, m % n)
else:
gcd = n  # Optional else branch

In Python, certain values are treated as False in boolean contexts:

• Numeric value 0
• Empty sequences: "", []

For example, the expression if m % n will be True if there is a remainder, and False if the remainder is 0.

Checking Membership

You can check if a value is in or not in a sequence (like a list or string):

• Check if a value is in a list:

"Mushroom" in some_list

• Check if a value is not in a list:

if user not in banned_users:
  print(f"{user.title()}, you can post a response.")

Types of if Statements

• Simple if statement – A single condition with one action when the condition is true:

if condition:
  # action

• if...else statement – Takes one action if the condition is true, and a different one if it’s false:

if condition:
  # action 1
else:
  # action 2

• if...elif...else chain – Used when more than two possible outcomes need to be considered. Python checks each condition in order, and once one condition is true, it skips the rest:

if condition1:
  # action 1
elif condition2:
  # action 2
else:
  # action 3

• if...if...if chain – Checks all conditions, even if one of them is true. Each if is independent:

if condition1:
  # action 1
if condition2:
  # action 2
if condition3:
  # action 3

Multi-way Branching

When you have multiple conditions to check, nested if statements can become hard to read. Here’s an example with nested if statements:

if x == 1:
    y = f1(x)
else:
    if x == 2:
        y = f2(x)
    else:
        if x == 3:
            y = f3(x)
        else:
            y = f4(x)

This is difficult to follow. A cleaner way is using elif to avoid unnecessary nesting:

if x == 1:
    y = f1(x)
elif x == 2:
    y = f2(x)
elif x == 3:
    y = f3(x)
else:
    y = f4(x)

Using if Statements with Lists

You can check for special values and ensure that a list is not empty. When the name of a list is used in an if statement, Python returns True if the list contains at least one item.

request = []
if request:
    for _ in request:
        print(f"Adding {_}.")
else:
    print("You want something?")

Using Multiple Lists: Checking and Comparing Two Lists

You can compare two lists to check if items are present in both.

having = ["apple", "banana", "cherry"]
ordered = ["apple", "grape", "cherry"]

for item in ordered:
    if item in having:
        print(f"Adding {item}")
    else:
        print(f"Sorry, we don't have {item}")

CS50 Conditionals

Conditions allow you to take different paths based on boolean expressions (Yes or No).

Example 1: Basic Comparison

x = int(input("What is x? "))
y = int(input("What is y? "))

if x < y:
    print("x is less than y")
if x > y:
    print("x is greater than y")
if x == y:
    print("x is equal to y")

Even if the first condition is true, Python will execute all the lines asking three questions.

Example 2: Using elif to Avoid Unnecessary Checks

Using elif helps avoid executing unnecessary conditions after one is found to be true:

if x < y:
    print("x is less than y")
elif x > y:
    print("x is greater than y")
elif x == y:
    print("x is equal to y")

Using else with if

The else statement is used to handle the case where none of the previous conditions were true.

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")

Using or for Multiple Conditions

The or operator can be used to combine multiple conditions into one line:

if x > y or x < y:
    print("x is not equal to y")
else:
    print("x is equal to y")

Alternatively, you can use != to simplify the condition:

if x != y:
    print("x is not equal to y")
else:
    print("x is equal to y")

Using Multiple if Statements

When you use multiple if statements (instead of elif), each condition is checked independently, and all true conditions will result in executed code:

score = int(input("Score: "))

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

Checking for Even or Odd Numbers (Parity)

You can use the modulus operator (%) to determine if a number is even or odd. An even number divided by 2 leaves a remainder of 0.

Example:

if x % 2 == 0:
    print("x is even")
else:
    print("x is odd")

Using Boolean Functions

A boolean function returns True or False and can be used in if statements for conditions.

Example:

def is_even(n):
    if n % 2 == 0:
        return True
    else:
        return False

def main():
    x = int(input("What is x? "))
    if is_even(x):  # Calls the is_even function
        print("Even")
    else:
        print("Odd")

Alternatively, you can simplify the function to return the result directly:

def is_even(n):
    return n % 2 == 0

def main():
    x = int(input("What is x? "))
    if is_even(x):  # Directly checks if the result is True
        print("Even")
    else:
        print("Odd")

Simplifying Multiple if Conditions with or

Instead of checking for multiple conditions with multiple if statements, you can combine conditions into one line using or:

name = input("What is your name? ")
if name == "Harry" or name == "Hermione" or name == "Ron":
    print("Gryffindor")
elif name == "Draco":
    print("Slytherin")
else:
    print("Who?")