Loops — How They Work

A loop repeats a block of code multiple times. Python has two main types: for loops and while loops.

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The for Loop

Use for when you know how many times to repeat.

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

Output:

0
1
2
3
4

range(5) gives you the numbers 0, 1, 2, 3, 4. The variable i takes each value, one at a time.

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range() Explained

Code	Numbers produced
range(5)	0, 1, 2, 3, 4
range(1, 6)	1, 2, 3, 4, 5

Two forms:

• range(stop) — starts at 0, goes up to (but not including) stop
• range(start, stop) — starts at start, goes up to stop

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Looping Over a List

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

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

Output:

I like apple
I like banana
I like cherry

The variable fruit takes each item in the list, one at a time.

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The while Loop

Use while when you want to repeat until a condition becomes False.

count = 0
while count < 5:
    print(count)
    count += 1

Output:

0
1
2
3
4

Be careful: if the condition never becomes False, you get an infinite loop!

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Accumulator Pattern

A very common pattern — start with a variable and build it up inside a loop:

total = 0
for i in range(1, 11):
    total += i

print(total)  # 55

This adds 1 + 2 + 3 + ... + 10 = 55.

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break and continue

• break — exits the loop immediately
• continue — skips to the next iteration

for i in range(10):
    if i == 5:
        break
    print(i)
# Prints: 0 1 2 3 4

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

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for vs while — When to Use Which?

Use for when...	Use while when...
You know the number of iterations	You don't know when to stop
Looping over a list or range	Waiting for user input
Counting from A to B	Running until a condition is met

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Key Takeaways

• for loops repeat a known number of times
• while loops repeat until a condition is False
• range(n) gives you numbers 0 to n-1
• You can loop over lists directly with for item in list:
• break exits, continue skips
• The accumulator pattern builds up a value over many iterations