Specific Characters Frequency in String List-Python

Last Updated : 01 Mar, 2025

The task of counting the frequency of specific characters in a string list in Python involves determining how often certain characters appear across all strings in a given list.

For example, given a string list ["geeksforgeeks"] and a target list ['e', 'g'], the output would count how many times 'e' and 'g' appear across all the strings in the list. The output in this case would be {'g': 2, 'e': 4}, indicating that 'g' appears twice and 'e' appears four times across all the strings in the list.

Using collections.Counter

Counter from the collections module is a convenient tool for counting occurrences of elements in an iterable. By converting a string list into a single string and applying Counter, we can quickly count the frequency of all characters. To focus on specific characters, a dictionary comprehension is used to filter only those characters present in the target list.

Python

from collections import Counter

# test list
a = ["geeksforgeeks is best for geeks"]

# char list
b = ['e', 'b', 'g']

# join the list into a single string and use Counter
res = {key: val for key, val in Counter("".join(a)).items() if key in b}
print(res)

Output

{'g': 3, 'e': 7, 'b': 1}

Explanation: This code combines the list a into a string, counts the frequency of each character using Counter() and filters the result to include only the characters found in list b, storing them in a dictionary.

Using dictionary

Manually iterating through each character in the list and counting its occurrences gives us more control over the process. A dictionary is used to track the frequency of characters in the string. By checking if each character is in the target list, we update the count accordingly.

Python

# test list
a = ["geeksforgeeks is best for geeks"]

# char list
b = ['e', 'b', 'g']

# manually count the frequency of characters in the list
char_count = {}
for s in a:
    for c in s:
        if c in b:
            char_count[c] = char_count.get(c, 0) + 1
print(char_count)

Output

{'g': 3, 'e': 7, 'b': 1}

Explanation: This code iterates through each string in list a and each character in the string. For each character, it checks if it is in list b. If it is, the character's count is updated in the char_count dictionary using get() to handle missing keys.

Using filter()

filter() function is used to remove characters from the string that are not part of the target list. After filtering, Counter is applied to count the occurrences of the remaining characters. This method combines functional programming with the ease of counting using Counter.

Python

from collections import Counter

# test list
a = ["geeksforgeeks is best for geeks"]

# char list
b = ['e', 'b', 'g']

# filter the string and count using Counter
filtered_str = "".join(filter(lambda x: x in b, "".join(a)))
res = dict(Counter(filtered_str))
print(res)

Output

{'g': 3, 'e': 7, 'b': 1}

Explanation: "".join(a) combines the list a into a string, then filter() with a lambda retains characters from b. The filtered characters are rejoined and Counter(filtered_str) counts their occurrences, converting the result into a dictionary with dict().

Using dictionary comprehension

List comprehension offers a more Pythonic way to count the frequency of specific characters in a string. By first creating a list of only the characters that are in the target list, Counter is then used to count the occurrences of each character. This approach is compact and efficient for solving the problem in a single line.

Python

from collections import Counter

# test list
a = ["geeksforgeeks is best for geeks"]

# char list
b = ['e', 'b', 'g']

# list comprehension to count the characters
res = {key: val for key, val in Counter([char for char in "".join(a) if char in b]).items()}
print(res)

Output

{'g': 3, 'e': 7, 'b': 1}

Explanation: This code combines list a into a string, filters characters from b using a dictionary comprehension, counts their frequencies with Counter(), and converts the result into a dictionary.