Dictionary to List by Repeating keys corresponding value times - Python
Last Updated :
28 Jan, 2025
Given a dictionary where each key is a character and its corresponding value is a number, the task is to generate a list by repeating each keys character according to its value and if a character is repeated as a key in the dictionary then the value from its last occurrence will be considered. For example, If we have a dictionary d = {'g': 2, 'f': 3, 'g': 1, 'b': 4, 'e': 1, 's': 4, 't': 3} then output will be ['g', 'f', 'f', 'f', 'b', 'b', 'b', 'b', 'e', 's', 's', 's', 's', 't', 't', 't'].
Using loop and * operator
In this method we use a loop to iterate through dictionary items and repeats the key value times using the * operator.
Python
d = {'g': 2, 'f': 3, 'g': 1, 'b': 4, 'e': 1, 's': 4, 't': 3}
res = []
for k, v in d.items():
res += [k] * v
print(res)
Output['g', 'f', 'f', 'f', 'b', 'b', 'b', 'b', 'e', 's', 's', 's', 's', 't', 't', 't']
Using List Comprehension
This method uses list comprehension to repeat each dictionary key according to its value, creating a flat list of repeated elements.
Python
d = {'g': 2, 'f': 3, 'g': 1, 'b': 4, 'e': 1, 's': 4, 't': 3}
# list comprehension repeat key `v` times
res = [k for k, v in d.items() for _ in range(v)]
print(res)
Output['g', 'f', 'f', 'f', 'b', 'b', 'b', 'b', 'e', 's', 's', 's', 's', 't', 't', 't']
Explanation:
- for k, v in d.items() retrieves the dictionary's keys and their values.
- for _ in range(v) ensures each key is repeated exactly v times in the output list.
Using the collections module
This method leverages collections.Counter to handle the dictionary and repeat the elements and return them as a list using list().
Python
import collections
d = {'g': 2, 'f': 3, 'g': 1, 'b': 4, 'e': 1, 's': 4, 't': 3}
# using collections.Counter to flatten dictionary into a list
res = list(collections.Counter(d).elements())
print(res)
Output['g', 'f', 'f', 'f', 'b', 'b', 'b', 'b', 'e', 's', 's', 's', 's', 't', 't', 't']
Explanation:
- collections.Counter(d) creates a Counter object from the dictionary d.
- .elements() returns an iterator that yields the elements repeated by their counts, which is converted to a list using list().
This method uses itertools.repeat to repeat keys based on their values and itertools.chain to flatten the results into a single list.
Python
import itertools
d = {'g': 2, 'f': 3, 'g': 1, 'b': 4, 'e': 1, 's': 4, 't': 3}
# using itertools to repeat keys according to their values
res = list(itertools.chain.from_iterable(itertools.repeat(k, v) for k, v in d.items()))
print(res)
Output['g', 'f', 'f', 'f', 'b', 'b', 'b', 'b', 'e', 's', 's', 's', 's', 't', 't', 't']
Explanation:
- itertools.repeat(k, v) repeats each key k v times.
- itertools.chain.from_iterable() flattens the iterator of repeated keys into a single list.
In this method we use reduce() function to accumulate values by repeating the dictionary's key values.
Python
from functools import reduce
d = {'g': 2, 'f': 3, 'g': 1, 'b': 4, 'e': 1, 's': 4, 't': 3}
# using reduce to construct the list
res = reduce(lambda lst, val: lst + [val[0]] * val[1], d.items(), [])
print(res)
Output['g', 'f', 'f', 'f', 'b', 'b', 'b', 'b', 'e', 's', 's', 's', 's', 't', 't', 't']
Explanation: reduce() function accumulates results by applying the lambda function, which repeats the key val[0] for val[1] times.
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