Counting the frequencies in a list using dictionary in Python
Given an unsorted list of some elements(may or may not be integers), Find the frequency of each distinct element in the list using a dictionary.
Example:
Recommended: Please try your approach on {IDE} first, before moving on to the solution.
The problem can be solved in many ways. A simple approach would be to iterate over the list and use each distinct element of the list as a key of the dictionary and store the corresponding count of that key as values. Below is the Python code for this approach:
Python
|
# Python program to count the frequency of
# elements in a list using a dictionary
def CountFrequency(my_list):
# Creating an empty dictionary
freq = {}
for item in my_list:
if (item in freq):
freq[item] += 1
else:
freq[item] = 1
for key, value in freq.items():
print ("% d : % d"%(key, value))
# Driver function
if __name__ == "__main__":
my_list =[1, 1, 1, 5, 5, 3, 1, 3, 3, 1, 4, 4, 4, 2, 2, 2, 2]
CountFrequency(my_list)
|
Output:
1 : 5
2 : 4
3 : 3
4 : 3
5 : 2
Time Complexity:O(N), where N is the length of the list.
Alternative way: An alternative approach can be to use the list.count() method.
Python
|
# Python program to count the frequency of
# elements in a list using a dictionary
def CountFrequency(my_list):
# Creating an empty dictionary
freq = {}
for items in my_list:
freq[items] = my_list.count(items)
for key, value in freq.items():
print ("% d : % d"%(key, value))
# Driver function
if __name__ == "__main__":
my_list =[1, 1, 1, 5, 5, 3, 1, 3, 3, 1, 4, 4, 4, 2, 2, 2, 2]
CountFrequency(my_list)
|
Output:
1 : 5
2 : 4
3 : 3
4 : 3
5 : 2
Time Complexity:O(N2), where N is the length of the list. The time complexity list.count() is O(N) alone, and when used inside loop it will become O(N2).
Alternative way:An alternative approach can be to use the dict.get() method. This makes the program much more shorter and makes understand how get method is useful instead of if…else.
Python
|
# Python program to count the frequency of
# elements in a list using a dictionary
def CountFrequency(my_list):
# Creating an empty dictionary
count = {}
for i in [1, 1, 1, 5, 5, 3, 1, 3, 3, 1 ,4, 4, 4, 2, 2, 2, 2]:
count[i] = count.get(i, 0) + 1
return count
# Driver function
if __name__ == "__main__":
my_list =[1, 1, 1, 5, 5, 3, 1, 3, 3, 1, 4, 4, 4, 2, 2, 2, 2]
print(CountFrequency(my_list))
|
Output:
{1: 5, 5: 2, 3: 3, 4: 3, 2: 4}
Related Article :
Count frequencies of all elements in array in Python using collections module
Article Tags :
Python
frequency-counting
Python dictionary-programs
python-dict
python-list
Practice Tags :
python-dict
python-list
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Counting the frequencies in a list using dictionary in Python
In this tutorial, you will learn to write a program in Python that will count the frequency of elements in a list using a dictionary. For a given list with multiple elements, we have to count the occurrence of each element in the list and display it as output. This can be done by creating a dictionary and using some built-in methods which will be discussed below.
Look at the input-output format to understand better.
Input: ['a', 'a', 'a', 'b','b']
Output: {'a': 3, 'b': 2}
To solve this problem we can follow these approaches-
- by iterating over the list and counting frequency
- using list.count() method
- using dict.get() method
Counting the frequencies in a list using dictionary in Python
PythonServer Side ProgrammingProgramming
In this article we develop a program to calculate the frequency of each element present in a list.
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Python: Count the frequency in a given dictionary
Last update on April 27 2021 12:48:12 (UTC/GMT +8 hours)What will we cover?
We will learn about dictionaries and how to use them for frequency count.
See the full tutorial on Python Dictionaries as well.