Loops: Repeating Without Retyping

Python for Absolute Beginners · Lesson 10

What you’ll learn

• What a loop is, and why programmers love them
• How a for loop repeats a set number of times
• How range() counts for you
• How a while loop repeats until a condition changes
• How to avoid the one loop mistake every beginner makes

Why loops exist

• A loop repeats a block of code for you, so you write the action once and say how many times to do it
• Printing 1 to 100 without a loop would take a hundred lines — and changing the plan means rewriting all of them

Without a loop: five lines for 1 to 5

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

Output:
1
2
3
4
5

That works, but it does not scale.

For loops and range()

• A for loop repeats its block once for each item in a sequence
• The simplest sequence comes from range(), which counts
• range(5) produces 0, 1, 2, 3, 4 — five numbers, starting at 0
• range(5) means “five times”, and the counting variable takes each value in turn
• range(start, stop) counts from start up to, but not including, stop

A for loop with range()

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

Output:
0
1
2
3
4

That zero start surprises everyone once, then becomes habit.

Range with a start and stop

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

Output:
1
2
3
4
5

The stop value is always one past the last number you get.

A loop through a string

for letter in "cat":
    print(letter)

Output:
c
a
t

Same shape: “for each thing in this sequence, do the indented block” — here the sequence is characters.

While loops: repeat until something changes

• A while loop repeats its block as long as its condition stays true, and stops the moment it becomes false
• The block must change something the condition tests — otherwise nothing ever changes

A while loop in code

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

Output:
1
2
3
4
5

When count reaches 6, the condition is false and the loop stops.

The one loop mistake every beginner makes

• A while loop whose condition never becomes false runs forever — an infinite loop
• It usually comes from forgetting the update line, so the tested value never changes
• If a program prints the same thing endlessly, stop it — in IDLE, use Shell → Restart Shell, or close the shell window
• A for loop cannot run forever this way, because range() always runs out

An infinite loop (do not run this as-is)

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

This prints 1 forever, because count never changes.

Hands-on exercise

• Use a for loop with range() to print the numbers 1 to 10
• Use a while loop to count down from 5 to 1, then print “Blast off!” after the loop
• Use a for loop to print each letter of your name on its own line
• Predict what range(3) produces before running it, then check

Key takeaways

• A loop repeats an indented block so you write an action once
• for repeats once per item in a sequence; range(n) counts 0 to n−1
• range(start, stop) counts from start up to, but not including, stop
• while repeats while its condition is true; the block must change something the condition tests
• A while loop that never changes its condition runs forever

Take the quiz

Take the Lesson 10 quiz — 5 quick questions to check what you learned.

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