lists-set.rst

List, sets and other groups

Get unique elements from a list

python -c "print(set([1,1,4,5,6,2,2,4,3,5]))"

set() denoted by {} is a list of unique elements. When a list is converted into a set all the repeated values are discarded.

Find most common elements in a list with their count

python -c "import collections;print(collections.Counter([1,2,2,3,3,4,4,4,4,4,6,6,6,6,6,6,6,6,6,7]))"

collections.Counter will take the list and creates a list of dictionary as [{<item>: <frequency of item in the list>}, ...]

Get only elements which are duplicated from a list

python -c "import collections;x=dict(collections.Counter([1,2,3,4,5,5,6,6,6,7]));print([k for k,v in x.items() if v>1])"

Return only those items in the list that are repeated.

collections.Counter will take the list and creates a list of dictionary as [{<item>: <frequency of item in the list>}, ...] [k for k,v in x.items() if v>1] creates a new list with the items that are occurred more than ones in the list.

Reverse a list

python -c "x=[1,2,3,4,5,6,7,8,9];print(x[::-1])"

x[::-1] is used to slicing a list. Negative value creates a copy of the list in reverse order.

Generate all permutations from a list

python -c "import itertools;print(list(itertools.permutations(range(1,4))))"

itertools.permutations() generates all the possible permutations of the given list.

Generate power set of a set

python -c "import itertools;print([list(itertools.combinations([1,2,3], x)) for x in range(len([1,2,3])+1)])"

itertools.combinations() generates all the possible combinations of a given list. It take another optional argument, r represents number of items per combination.

Sort a dictionary by keys

python -c "y={1: 6, 2: 5, 3: 4, 4: 3, 5: 2, 6: 1};print(dict(sorted(y.items())))"

Sorts the dict by key. sorted() is python's in-built method to sorting iterables. By default, dictionaries are sorted by key.

Sort a dictionary by value

python -c "y={1: 6, 2: 5, 3: 4, 4: 3, 5: 2, 6: 1};print(dict(sorted(y.items(), key=lambda kv: kv[1])))"

Sorts the dict by value. sorted() takes an optional keyword argument key which take a callable to specify search key. In this case the search key is dict value instead of dict key.

Cartesian product of two lists

python -c "import itertools;print(list(itertools.product([['x','y','z'],[1,2,3]])))"

Generates cartesian product of two lists.