count and cycle
- Python course
- Intermediate
- Lesson 95/116
- 5 min
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.
Practice 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).
Starter code
# Limited iteration with cycle and isliceSample solution
# Limited iteration with cycle and islice
from itertools import cycle, islice
c = cycle([1, 2])
result = list(islice(c, 5))
print(result)Expected output
[1, 2, 1, 2, 1]
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 count import line
from itertools import count, ____
for i in zip(count(1), ["a", "b"]):
print(i)from itertools import count, cycle
for i in zip(count(1), ["a", "b"]):
print(i)Explanation: cycle is also in the itertools module.
2 When does count(1) stop?
- Never
- At 100
- When some data ends
- With a limiter Correct answer
Explanation: count stops only with a limiter like islice or zip.
3 Match the infinite iterator parts
| Term | Meaning |
|---|---|
count |
Infinite numbers |
cycle |
Infinite repeat |
islice |
Limiter |
zip |
Stops at shortest |