Conditional Statements and Loops

# CHAPTER 5

Conditional Statements and Loops #

1. Chapter Introduction #

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

The if statement evaluates a condition (using the comparison operators from Chapter 4). If the condition evaluates to True, the indented block of code runs.

age = 22

# Simple IF
if age >= 18:
    print("You are an adult.")

For more complex logic, we add elif (Else If) and else.

score = 85

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

*How it works:* Python evaluates from top to bottom. The moment it finds a True condition, it executes that block and skips the rest.

3. The for Loop #

A for loop iterates over a sequence (like a list, string, or a range of numbers) and executes a block of code for each item. This is how you automate repetitive data processing.

1. Looping over a List:

fruits = ["Apple", "Banana", "Cherry"]

for item in fruits:
    print(f"I like {item}s")

2. Looping over a Range of Numbers: The range(start, stop) function generates a sequence of numbers. (Note: It stops *before* the stop number).

# Prints 0, 1, 2, 3, 4
for i in range(5):
    print(i)

4. The while Loop #

A while loop continues to execute *as long as* a specific condition remains True.

count = 0

while count < 3:
    print(f"Count is {count}")
    count += 1  # Equivalent to: count = count + 1

*Warning:* If you forget to update the condition (like forgetting count += 1), the loop will run forever (an Infinite Loop) and crash your program!

5. Loop Control: break and continue #

Sometimes you need to interrupt a loop before it finishes naturally.

break: Stops the entire loop immediately.

# Find the first even number and stop
numbers = [1, 3, 5, 8, 9]

for num in numbers:
    if num % 2 == 0:
        print(f"Found even number: {num}")
        break  # Loop terminates here

continue: Skips the rest of the *current* iteration and moves to the next item.

# Print only odd numbers
for i in range(5):
    if i % 2 == 0:
        continue # Skip the print statement below for even numbers
    print(i)

6. Mini Project: Student Grade Calculator #

Let's build a program that processes a list of student scores, calculates their letter grade, and counts how many students passed.

scores = [95, 42, 78, 88, 55, 91]
passed_count = 0

for score in scores:
    # 1. Determine Grade
    if score >= 90:
        grade = "A"
    elif score >= 80:
        grade = "B"
    elif score >= 70:
        grade = "C"
    else:
        grade = "F"
        
    # 2. Count passes
    if score >= 70:
        passed_count += 1
        status = "Passed"
    else:
        status = "Failed"
        
    print(f"Score: {score} | Grade: {grade} | Status: {status}")

print("-" * 20)
print(f"Total students passed: {passed_count} out of {len(scores)}")

7. Common Mistakes #

• Forgetting the colon : Every if, elif, else, for, and while statement MUST end with a colon.

• Indentation Errors: The code inside the loop or if-statement MUST be indented. Code that is not indented is considered outside the loop.

• Infinite While Loops: Creating a while True: loop without a break statement inside it will freeze your computer.

8. MCQs #

9. Interview Questions #

• Q: Explain the difference between break and continue in a loop. Give a data-cleaning scenario where you would use each.

• Q: Why is the order of if / elif statements important? What happens if you check if score > 50 before checking if score > 90?

11. Summary #

12. Next Chapter Recommendation #