Python · Chapter 41 of 45
map() & filter()
`map(func, seq)` applies func to every item. `filter(func, seq)` keeps items where func returns truthy.
Both return iterators — wrap in `list(...)` to see results.
Prefer comprehensions
`[f(x) for x in seq]` and `[x for x in seq if cond]` are usually clearer than map/filter.
With lambda
Map/filter commonly appear with a small lambda expression as the function.
Example 1 (python)
nums = [1, 2, 3, 4]
doubled = list(map(lambda x: x*2, nums))
print(doubled)Output
[2, 4, 6, 8]Double every number.
Example 2 (python)
nums = [1, 2, 3, 4, 5]
evens = list(filter(lambda x: x % 2 == 0, nums))
print(evens)Output
[2, 4]Keep only even numbers.
Key points
- map transforms; filter selects.
- Both return iterators (lazy).
- Comprehensions are usually clearer.
- Use with `list()` to materialise.
💡 Note: `list(map(int, ["1","2","3"]))` is a classic idiom to convert strings to ints.