Loops — for, while, comprehensions

for iterates over any iterable. while runs until a condition is false. Comprehensions build lists in one line.

script.py v3.1

# for loop over a range
for i in range(5):
    print('range:', i)

# for loop over a list
fruits = ['apple', 'banana', 'cherry']
for f in fruits:
    print('fruit:', f)

# for loop with enumerate (index + value)
for i, f in enumerate(fruits):
    print(f'{i}: {f}')

# while loop
n = 10
while n > 1:
    n //= 2
    print('n =', n)

# List comprehension (Pythonic)
squares = [x*x for x in range(5)]
print('squares:', squares)

# Comprehension with filter
evens = [x for x in range(10) if x % 2 == 0]
print('evens:', evens)

Python output click Run to execute

Click "Run" to execute the code on the left.
The output will appear here.

🧠 How it works

for variable in iterable: runs the loop body once per item. range(5) produces 0,1,2,3,4. range(2, 5) produces 2,3,4. range(0, 10, 2) produces 0,2,4,6,8. enumerate(iterable) yields (index, value) pairs — use it instead of range(len(iterable)). while condition: runs while condition is True. Risk: infinite loop if condition never becomes False. List comprehension: [expr for var in iter if cond] — faster than a for+append loop. Dict comprehension: {k: v for k, v in pairs}. Set comprehension: {x for x in iter}.

✏️ Try these changes

Change range(5) to range(2, 10)
→ loop runs from 2 to 9
Change range(5) to range(0, 10, 2)
→ steps by 2: 0,2,4,6,8
Change 'while n > 1' to 'while n > 0'
→ extra iteration when n becomes 1
Change squares to [x*x for x in range(5) if x % 2 == 0]
→ only squares of evens: [0, 4, 16]
Add 'break' inside the for loop after the first print
→ loop exits after first iteration

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