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:
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:
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# 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)
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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.
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# 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)
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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.
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# 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))
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Related Article :
Count frequencies of all elements in array in Python using collections module
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Dictionaries
A dictionary is like a list, but more general. In a list, the index positions have to be integers; in a dictionary, the indices can be (almost) any type.
You can think of a dictionary as a mapping between a set of indices (which are called keys) and a set of values. Each key maps to a value. The association of a key and a value is called a key-value pair or sometimes an item.
As an example, we’ll build a dictionary that maps from English to Spanish words, so the keys and the values are all strings.
The function dict creates a new dictionary with no items. Because dict is the name of a built-in function, you should avoid using it as a variable name.
>>> eng2sp = dict() >>> print(eng2sp) {}The curly brackets, {}, represent an empty dictionary. To add items to the dictionary, you can use square brackets:
>>> eng2sp['one'] = 'uno'This line creates an item that maps from the key 'one' to the value “uno”. If we print the dictionary again, we see a key-value pair with a colon between the key and value:
>>> print(eng2sp) {'one': 'uno'}This output format is also an input format. For example, you can create a new dictionary with three items. But if you print eng2sp, you might be surprised:
>>> eng2sp = {'one': 'uno', 'two': 'dos', 'three': 'tres'} >>> print(eng2sp) {'one': 'uno', 'three': 'tres', 'two': 'dos'}The order of the key-value pairs is not the same. In fact, if you type the same example on your computer, you might get a different result. In general, the order of items in a dictionary is unpredictable.
But that’s not a problem because the elements of a dictionary are never indexed with integer indices. Instead, you use the keys to look up the corresponding values:
>>> print(eng2sp['two']) 'dos'The key 'two' always maps to the value “dos” so the order of the items doesn’t matter.
If the key isn’t in the dictionary, you get an exception:
>>> print(eng2sp['four']) KeyError: 'four'The len function works on dictionaries; it returns the number of key-value pairs:
>>> len(eng2sp) 3The in operator works on dictionaries; it tells you whether something appears as a key in the dictionary (appearing as a value is not good enough).
>>> 'one' in eng2sp True >>> 'uno' in eng2sp FalseTo see whether something appears as a value in a dictionary, you can use the method values, which returns the values as a type that can be converted to a list, and then use the in operator:
The in operator uses different algorithms for lists and dictionaries. For lists, it uses a linear search algorithm. As the list gets longer, the search time gets longer in direct proportion to the length of the list. For dictionaries, Python uses an algorithm called a hash table that has a remarkable property: the in operator takes about the same amount of time no matter how many items there are in a dictionary. I won’t explain why hash functions are so magical, but you can read more about it at wikipedia.org/wiki/Hash_table.
Exercise 1: Download a copy of the file www.py4e.com/code3/words.txt
Write a program that reads the words in words.txt and stores them as keys in a dictionary. It doesn’t matter what the values are. Then you can use the in operator as a fast way to check whether a string is in the dictionary.
That One Thing
Create a Dictionary From a List
Count the number of times an item appears in a Python list, with code examples
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