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Lesson 7: Loops

Back to the main roadmap | Previous lesson: Functions & Built-in Functions

What is a Loop?

A loop lets you repeat a block of code without writing it over and over again.

Instead of writing:

print("Hi")
print("Hi")
print("Hi")
print("Hi")
print("Hi")

You can simply tell Python:

Repeat this 5 times.

Real-Life Example

Imagine washing 10 dishes.

You don't invent a new set of instructions for every dish.

You simply repeat the same steps:

  • Pick up the dish
  • Scrub it
  • Rinse it
  • Put it away

A loop works the same way.

Types of Loops

Python has two main types of loops:

  • for loop
  • while loop

for Loop

A for loop goes through items one at a time.

Example

fruits = ["apple", "banana", "cherry"]

for fruit in fruits:
    print(fruit)

Output

apple
banana
cherry

How It Works

Python reads it like this:

  • Take the first item → "apple"
  • Store it in fruit
  • Run the code inside the loop
  • Repeat for every remaining item

After the last item, the loop ends automatically.

Looping Through a String

Strings are collections of characters.

Python can loop through each character.

Example

for letter in "cat":
    print(letter)

Output

c
a
t

Using range()

The range() function generates numbers.

Example

for i in range(5):
    print(i)

Output

0
1
2
3
4

Notice that 5 is not included.

range(5) produces five numbers, starting from 0.

Different Forms of range()

Example

range(5)

Output

0 1 2 3 4

range(2, 6)

Output

2 3 4 5

range(0, 10, 2)

Output

0 2 4 6 8

The third value is called the step.

Using the Loop Variable

The variable inside the loop changes every iteration.

Example

for i in range(5):
    print("Round:", i)

Output

Round: 0
Round: 1
Round: 2
Round: 3
Round: 4

Using range(len())

Sometimes you need the position (index) instead of the item.

Example

fruits = ["apple", "banana", "cherry"]

for i in range(len(fruits)):
    print(i, "-", fruits[i])

Output

0 - apple
1 - banana
2 - cherry

Here:

  • i is the index.
  • fruits[i] gets the item at that position.

Using enumerate()

enumerate() gives both the index and the item.

Example

fruits = ["apple", "banana", "cherry"]

for i, fruit in enumerate(fruits):
    print(i, "-", fruit)

Output

0 - apple
1 - banana
2 - cherry

This is cleaner than using range(len()).

while Loop

A while loop keeps running as long as a condition is True.

Example

count = 0

while count < 5:
    print(count)
    count += 1

Output

0
1
2
3
4

How It Works

  • Check if count < 5
  • If True, run the loop
  • Increase count
  • Check the condition again
  • Stop when the condition becomes False

Infinite Loops

If you forget to update the condition, the loop never ends.

Incorrect Example

count = 0

while count < 5:
    print(count)

Since count never changes, the condition is always True.

The loop runs forever.

To stop an infinite loop in the terminal, press:

Ctrl + C

while True

Sometimes you intentionally want a loop to run forever until the user decides to stop.

Example

while True:
    answer = input("Type 'quit' to stop: ")

    if answer == "quit":
        break

    print("You typed:", answer)

The loop stops only when break is executed.

break

break immediately exits the loop.

Example

for i in range(10):
    if i == 5:
        break

    print(i)

Output

0
1
2
3
4

When i becomes 5, the loop ends immediately.

continue

continue skips the current iteration and moves to the next one.

Example

for i in range(5):
    if i == 2:
        continue

    print(i)

Output

0
1
3
4

The number 2 is skipped.

break vs continue

Keyword What It Does
break Stops the loop completely
continue Skips only the current iteration

Nested Loops

A nested loop is a loop inside another loop.

Example

for i in range(3):
    for j in range(2):
        print(i, j)

Output

0 0
0 1
1 0
1 1
2 0
2 1

The inner loop runs completely for every iteration of the outer loop.

else with Loops

A loop can also have an else block.

The else block executes only if the loop finishes normally (without break).

Example

for i in range(5):
    print(i)

else:
    print("Loop finished normally.")

Output

0
1
2
3
4
Loop finished normally.

Practice Program

Create a file named lesson_loops.py.

# 1. Basic for loop
fruits = ["apple", "banana", "cherry"]

for fruit in fruits:
    print(fruit)

# 2. range()
for i in range(1, 6):
    print("Count:", i)

# 3. enumerate()
for i, fruit in enumerate(fruits):
    print(i, "-", fruit)

# 4. while loop
count = 0

while count < 5:
    print("While count:", count)
    count += 1

# 5. break and continue
for i in range(10):

    if i == 7:
        break

    if i % 2 == 0:
        continue

    print("Odd number:", i)

# 6. Nested loop
for i in range(1, 4):
    for j in range(1, 4):
        print(f"{i} x {j} = {i*j}")

Output

apple
banana
cherry

Count: 1
Count: 2
Count: 3
Count: 4
Count: 5

0 - apple
1 - banana
2 - cherry

While count: 0
While count: 1
While count: 2
While count: 3
While count: 4

Odd number: 1
Odd number: 3
Odd number: 5

1 x 1 = 1
1 x 2 = 2
1 x 3 = 3
2 x 1 = 2
2 x 2 = 4
2 x 3 = 6
3 x 1 = 3
3 x 2 = 6
3 x 3 = 9

Key Takeaways

  • Loops repeat code automatically.
  • Python has two loop types: for and while.
  • Use for when looping through a sequence.
  • Use while when repeating until a condition becomes False.
  • range() generates a sequence of numbers.
  • enumerate() provides both the index and the value.
  • break exits the loop immediately.
  • continue skips the current iteration.
  • Nested loops are loops inside other loops.
  • A loop's else block runs only if the loop completes without break.

Next Lesson: Lists, Tuples, and Sets