Programming Fundamentals › Programming Basics
Iteration
Stepping through the items of a collection one at a time.
Also known as: iterating, looping over a collection, for each
Iteration means stepping through the items of a collection one at a time and doing something with each.
names = ["Ana", "Bo", "Cy"]
for name in names:
print(name)
for i, name in enumerate(names): # when you need the position too
print(i, name)
for key, value in {"a": 1}.items(): # dictionaries
...for (const name of names) console.log(name);
names.forEach((name, i) => console.log(i, name));Choosing a style
| Need | Use |
|---|---|
| Do something with each item | a for loop |
| Build a new list from an old one | a comprehension or map (map, filter, reduce) |
| One result from all items | sum, reduce, or a loop with an accumulator |
| Stop at the first match | a loop with break, or any/find |
Mistakes to avoid
- Changing a collection while looping over it. Removing items from a list you’re iterating skips elements. Loop over a copy, or build a new list.
- Index loops with the wrong bounds (off-by-one errors). Prefer looping over items directly.
- Doing slow work inside the loop, like a database query per item (N+1 queries).
- Iterating over
None/undefined. Check or default to an empty list.
Under the hood, many languages let you iterate over anything that follows an iterator protocol, including files and generators that produce items lazily (iterators).