Count character frequency in Python

A string is a sequence of characters, so you can loop over it and count each character in a dictionary.

The program

word = "mississippi"

counts = {}
for ch in word:
    counts[ch] = counts.get(ch, 0) + 1

print(counts)

{'m': 1, 'i': 4, 's': 4, 'p': 2}

How it works

• for ch in word gives one character at a time: m, i, s, s, and so on.
• counts.get(ch, 0) is the count so far, or 0 the first time a character is seen. Adding 1 and storing it back updates the count.
• The keys appear in the order each character was first seen in the word.

Another way: Counter

collections.Counter counts any sequence in one step. most_common(n) returns the n largest counts as (character, count) pairs:

from collections import Counter

sentence = "Hello, World"
counts = Counter(ch for ch in sentence.lower() if ch.isalpha())

print(counts.most_common(3))
print("Times 'z' appears:", counts["z"])

[('l', 3), ('o', 2), ('h', 1)]
Times 'z' appears: 0

• ch.isalpha() keeps only letters, so the comma and the space are not counted. lower() makes H and h the same.
• Asking a Counter for a missing key gives 0, not a KeyError. A normal dictionary would raise the error.
• When counts are equal, most_common() keeps the order in which the characters were first seen. That is why h comes before e, w, r and d.

Related programs: Find the most frequent character in a string.