Control Flow - Loops

Repeating Actions

If you want to print “Hello” 10 times, you could write print("Hello") ten times, but that’s inefficient. Instead, we use loops.

Python provides two main types of loops: for loops and while loops.

The while Loop

A while loop repeats a block of code as long as a condition is True.

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count = 1

while count <= 5:
    print(f"Number {count}")
    count += 1  # Important: change the variable used in the condition!

The for Loop

A for loop is used to iterate over a sequence (like a list, a string, or a range of numbers).

Using range()

The range() function generates a sequence of numbers.

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for i in range(5):
    print(f"Iteration {i}")

• range(5) gives numbers: 0, 1, 2, 3, 4.
• range(1, 6) gives numbers: 1, 2, 3, 4, 5.
• range(0, 10, 2) gives even numbers: 0, 2, 4, 6, 8 (step of 2).

Loop Control: break and continue

Sometimes you need to exit a loop early or skip an iteration.

• break: Exits the loop immediately.
• continue: Skips the rest of the current block and starts the next iteration.

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for i in range(10):
    if i == 3:
        continue  # Skip 3
    if i == 7:
        break     # Stop at 7
    print(i)

Interactive Practice

Try writing a loop that calculates the sum of all numbers from 1 to 100.