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Week 2: Control Flow - Conditionals and While Loops

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Week 2: Control Flow - Conditionals and While Loops

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Course: Jan 2026 - Python Difficulty: Intermediate Focus: Logic branching, relational operators, and indefinite loops.

1. Relational and Logical Operators

To make decisions in code, we need to compare values. These operations always result in a Boolean (True or False).

1.1 Relational Operators

== (Equal to)
!= (Not equal to)
> (Greater than)
< (Less than)
>= (Greater than or equal to)
<= (Less than or equal to)

1.2 Logical Operators

Used to combine multiple conditions:

and: True only if both operands are True.
or: True if at least one operand is True.
not: Reverses the Boolean value.

2. Conditional Statements (`if`, `elif`, `else`)

Conditional statements allow your program to execute different blocks of code based on conditions. Python uses indentation (usually 4 spaces) to define blocks.

2.1 Syntax


python
score = 85

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

2.2 Nested Conditionals

You can place if statements inside other if statements.


python
num = 10
if num > 0:
    if num % 2 == 0:
        print("Positive Even")
    else:
        print("Positive Odd")

3. String Basics

Strings are ordered sequences of characters.

3.1 Indexing

Characters can be accessed using their position (index), starting from 0. Negative indexing starts from the end (-1 is the last character).


python
word = "Python"
print(word[0])   # Output: 'P'
print(word[-1])  # Output: 'n'

3.2 Basic String Operations

Concatenation (+): "Hello" + "World" results in "HelloWorld"
Repetition (*): "A" * 3 results in "AAA"
Length: len("Python") returns 6

4. The `while` Loop

A while loop repeatedly executes a block of code as long as a specified condition evaluates to True.

4.1 Structure


python
count = 0
while count < 3:
    print("Count is:", count)
    count = count + 1  # Crucial to avoid an infinite loop!

Output:


Count is: 0
Count is: 1
Count is: 2

4.2 Common Use Case: Input Validation

while loops are excellent when you don't know beforehand how many times the loop will run.


python
password = ""
while password != "secret123":
    password = input("Enter password: ")
print("Access Granted!")

Caution

Infinite Loops: If the condition in a while loop never becomes False (e.g., you forget to increment a counter), the loop will run forever, crashing your program.

📘 Textbook Notes: Week 2 Notes
📝 Assignment: Week 2 Assignment
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