Loops: Making Your Computer Do Repetitive Work

The Thousand Envelopes Problem

Imagine you run a small business and you need to send a letter to 1,000 customers. Each envelope needs the same return address stamped in the top-left corner. You could write it by hand — once, twice, three times... by envelope 50, your hand is cramping. By envelope 200, you've made spelling mistakes. By envelope 500, you've given up entirely.

Smart businesses use a rubber stamp. You ink it once, press it down, move to the next envelope, press it down again. Same action. Repeated. Automated.

Loops are your rubber stamp in Python.

Instead of writing the same code fifty times, you write it once and tell Python: repeat this for every item in this collection or keep repeating this until a condition changes.

This is not a small convenience. It is a fundamental superpower of programming. Loops are what allow a single developer to process a million records, generate a thousand images, or check ten thousand passwords — without writing ten thousand lines of code.

Why Loops Exist: The Problem Without Them

Let's say you want to print the numbers 1 through 5. Without loops, you'd write:

print(1)
print(2)
print(3)
print(4)
print(5)

Fine for 5. Now imagine 1 through 1,000. You'd need 1,000 print statements. What if the number changed at runtime — what if the user typed in how many they wanted? Without loops, that's impossible.

With a loop:

for i in range(1, 6):
    print(i)

Output:

1
2
3
4
5

Five lines of output from two lines of code. That ratio gets more dramatic as the task grows.

The for Loop: Iterating Over Sequences

A for loop says: "for each item in this collection, do this thing."

Syntax:

for variable in sequence:
    # code to repeat

The variable takes the value of each item in the sequence, one at a time, each time through the loop.

The range() Function

range() generates a sequence of numbers without you having to list them all.

range(5) — five numbers starting from 0:

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

0
1
2
3
4

range(1, 11) — numbers from 1 up to (but not including) 11:

for i in range(1, 11):
    print(i)

1
2
3
4
5
6
7
8
9
10

range(0, 10, 2) — even numbers, stepping by 2:

for i in range(0, 10, 2):
    print(i)

0
2
4
6
8

The three arguments are: range(start, stop, step). Stop is always excluded.

Looping Over a List

range() isn't the only sequence you can loop over. Any list works:

fruits = ["apple", "banana", "mango", "grape"]

for fruit in fruits:
    print(f"I love {fruit}s!")

Output:

I love apples!
I love bananas!
I love mangos!
I love grapes!

The loop variable fruit automatically becomes each item in fruits, one at a time.

The while Loop: Repeat While a Condition Is True

A for loop is great when you know how many times to repeat. A while loop is for when you want to keep going as long as a condition holds.

Syntax:

while condition:
    # code to repeat

Python checks the condition before each iteration. When it becomes False, the loop stops.

Simple counter example:

count = 1
while count <= 5:
    print(f"Count: {count}")
    count += 1
print("Done!")

Output:

Count: 1
Count: 2
Count: 3
Count: 4
Count: 5
Done!

Notice count += 1 inside the loop — this is critical. Without it, count stays at 1 forever and the loop never ends.

The Infinite Loop Danger

# DANGER — do not run this
while True:
    print("This will print forever")

This is an infinite loop. It never stops because True is always... true. Your program freezes. If this happens, press Ctrl+C to force-stop it.

The safe pattern: always make sure something inside the loop will eventually make the condition False, or use break to exit.

For vs While: When to Use Each

A useful mental model: for = "do this N times" or "do this for each item"; while = "keep doing this until something changes."

Real Example 1 (for): Multiplication Table

A teacher wants to print the multiplication table for any number a student requests. Without a loop, this means 10 print statements per number — tedious. With a loop, it's elegant.

number = int(input("Enter a number: "))
print(f"\nMultiplication table for {number}:")

for i in range(1, 11):
    result = number * i
    print(f"{number} × {i} = {result}")

What each part does:

• int(input(...)) — reads the user's input and converts to integer
• range(1, 11) — generates numbers 1 through 10
• result = number * i — calculates the product for this row
• The f-string formats each line with consistent spacing

Sample run (user enters 7):

Enter a number: 7

Multiplication table for 7:
7 × 1 = 7
7 × 2 = 14
7 × 3 = 21
7 × 4 = 28
7 × 5 = 35
7 × 6 = 42
7 × 7 = 49
7 × 8 = 56
7 × 9 = 63
7 × 10 = 70

Real Example 2 (while): Number Guessing Game

Now something more interactive. The user guesses a secret number. We don't know how many guesses they'll need — it could be 1, it could be 20. A while loop is perfect here.

secret = 42
attempts = 0

while True:
    guess = int(input("Guess the number: "))
    attempts += 1

    if guess == secret:
        print(f"Correct! You got it in {attempts} attempts!")
        break
    elif guess < secret:
        print("Too low! Try higher.")
    else:
        print("Too high! Try lower.")

What each part does:

• while True: — loops indefinitely until we explicitly break out
• attempts += 1 — counts each guess (equivalent to attempts = attempts + 1)
• if guess == secret: — the winning condition; break exits the loop immediately
• elif guess < secret: — gives a helpful hint without revealing the answer
• else: — covers the case where the guess is too high

Sample run:

Guess the number: 70
Too high! Try lower.
Guess the number: 30
Too low! Try higher.
Guess the number: 50
Too high! Try lower.
Guess the number: 42
Correct! You got it in 4 attempts!

The player gets immediate, useful feedback every round. The loop does the heavy lifting — we never had to predict how many rounds to allow.

Key Takeaways

• Loops eliminate repetition — write the logic once, execute it many times
• Use a for loop when you know the sequence or count in advance
• Use a while loop when you repeat based on a changing condition
• range(start, stop, step) generates number sequences for for loops
• Always ensure while loops have an exit condition to avoid infinite loops
• break exits a loop immediately from anywhere inside it

Next lesson: Nested loops and loop control — how to manage loops inside loops, and how break and continue give you fine-grained control.