count and cycle

Learn infinite iterators with count and cycle.

count and cycle

count(start, step) produces infinitely increasing numbers; cycle(iterable) repeats an iterable forever. Both are infinite iterators; careless use leads to infinite loops.

Example: count(10, 2) produces 10, 12, 14, 16, .... cycle("AB") produces A, B, A, B, .... You must limit them with islice or zip; otherwise iteration never ends.

Using with a limit

If you write for c in count(1): on an infinite iterator, the program never stops. So you set a limit with islice or zip. zip stops when the shortest iterable ends; thanks to this, you can safely use infinite iterators.

Example: list(zip(count(1), ["a", "b"])) returns [(1, 'a'), (2, 'b')]. Since the second list has two items, count also produces two items and stops. This is a practical way for enumeration-like tasks.

Exercise Use cycle and islice

Start with from itertools import cycle, islice. Create a cycle with c = cycle([1, 2]). Take the first five items with result = list(islice(c, 5)). Then print with print(result).

Show example solution

# Limited iteration with cycle and islice
from itertools import cycle, islice

c = cycle([1, 2])
result = list(islice(c, 5))
print(result)

Test yourself

count import line

from itertools import count, ____

for i in zip(count(1), ["a", "b"]):
    print(i)

Show answer

from itertools import count, cycle

for i in zip(count(1), ["a", "b"]):
    print(i)

When does count(1) stop?
- Never
- At 100
- When some data ends
- With a limiter
Show answer

With a limiter — count stops only with a limiter like islice or zip.
Match the infinite iterator parts

count

Infinite numbers
cycle

Infinite repeat
islice

Limiter
zip

Stops at shortest

Next lesson: islice

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