Python Count Character Occurrences in String

Hey guys, Minh Vu here.

In this tutorial, I will show you how to count the occurrences or calculate the frequency of a character in a string in Python.

In short, we will use a dictionary to store the character as the key and the frequency as the value.

Table of Contents

To make sure everyone can understand, I will try to cover everything from basic to the solution.

• What is a Dictionary in Python?
• Count Character Occurrences in String in Python
• Sort the Occurrences Alphabetically
• Conclusion

What is a Dictionary in Python?

A dictionary is a data structure in Python that allows you store data in key-value pairs.

For example, you can store the information of a person using a dictionary like this:

person = {
	"name": "Minh Vu",
	"age": 21,
	"location": "Vietnam",
}

Here, we created a dictionary called person with 3 key-value pairs.

• Key: "name", Value: "Minh Vu"
• Key: "age", Value: 21
• Key: "location", Value: "Vietnam"

Similarly, we can use this to store the frequency of a character in a string.

For example, a string "wisecode" has the following frequency of characters:

count = {
	"w": 1,
	"i": 1,
	"s": 1,
	"e": 2,
	"c": 1,
	"o": 1,
	"d": 1,
}

Let's see how to do this in Python.

Count Character Occurrences in String in Python

To count the occurrences of a character in a string, we will loop through the characters in the string and count them all.

Here are the steps:

• Create a dictionary called count.
• Loop through the characters in the string.
  • If the character is not in the dictionary, add it to the dictionary with the value of 1.
  • If the character is already in the dictionary, increase its value by 1.

Here is the code:

def count_char_occurrences(text):
	count = {}

	for char in text:
		if char not in count: # we can optimize this by using count.get(char, 0)
			count[char] = 1
		else:
			count[char] += 1

	return count

text = "wisecode"
print(count_char_occurrences(text))

The output will be:

{
  "w": 1,
  "i": 1,
  "s": 1,
  "e": 2,
  "c": 1,
  "o": 1,
  "d": 1
}

The optimized version of the code will be:

def count_char_occurrences(text):
	count = {}

	for char in text:
		count[char] = count.get(char, 0) + 1

	return count

text = "wisecode"
print(count_char_occurrences(text))

Sort the Occurrences Alphabetically

To output the dictionary in an alphabetical order, we can use the sorted() function.

def count_char_occurrences(text):
	count = {}

	for char in text:
		count[char] = count.get(char, 0) + 1

	return count

text = "wisecode"
count = count_char_occurrences(text)

count_alphabetically_sorted = sorted(count.items(), key=lambda x: x[0])
count_alphabetically_sorted = dict(count_alphabetically_sorted)
print(count_sorted)

To help you understand better, here is the output of count.items():

dict_items([('w', 1), ('i', 1), ('s', 1), ('e', 2), ('c', 1), ('o', 1), ('d', 1)])

The sorted() function will sort the items in the dictionary by the key.

In this case, we use a lambda function to sort the items by the key, which is x[0], as one item in the dictionary is a tuple with 2 values: x[0] is the key and x[1] is the value.

So the output will be:

{
  "c": 1,
  "d": 1,
  "e": 2,
  "i": 1,
  "o": 1,
  "s": 1,
  "w": 1
}

As you can see, the characters are sorted alphabetically.

Conclusion

In general, to count the occurrences of a character in a string, we can use a dictionary to store the character as the key and the frequency as the value.

I will add more use cases as I face them so you can have a wider view of this problem.