> For the complete documentation index, see [llms.txt](https://lei-d.gitbook.io/leetcode/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://lei-d.gitbook.io/leetcode/python-tips/list-comprehension.md).

# List Comprehension

*List comprehensions are used to **derive a new list from an existing list**.* It's a short way to write one or multiple for loops.

A list comprehension consists of

1. an input sequence
2. an expression that yields the elements of the derived list
3. an iterator over the input sequence
4. a logical condition over the iterator (optional)

```python
# Suppose you have a list of numbers and you want to get a corresponding list 
# with all the numbers multiplied by 2 only when the number itself is greater than 2. 
# List comprehensions are ideal for such situations.
listone = [2, 3, 4]
listtwo = [2*i for i in listone if i > 2]
print listtwo
```

**List comprehension can support multiple levels of looping.** But it's best practice to avoid more than two nested comprehensions and use conventional nested for loops.

```python
# create product of two sets
A = [1, 2, 3]
B = ['a', 'b']
[(x, y) for x in A for y in B]
# [(1, 'a'), (1, 'b'), (2, 'a'), (2, 'b'), (3, 'a'), (3, 'b')]

# convert 2D list to 1D list
A = [[1, 2, 3], ['a', 'b', 'c']]
[x for y in A for x in y]   # first write outer for loop, then inside for loop
# [1, 2, 3, 'a', 'b', 'c']

# iterate over each entry in a 2D list
A = [[1, 2, 3], [4, 5, 6]]
[[x**2 for x in y] for y in A]
# [[1, 4, 9], [16, 25, 36]]
```

List comprehension also works on set.