Count Letters in a String | Dataford Interview Questions

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Problem

ZoomInfo SalesOS processes text fields that may contain names, labels, punctuation, and numeric identifiers. Write a function that counts each English alphabet character in a given string, treating uppercase and lowercase forms as the same character.

Return a dictionary containing only the letters that appear, with keys in alphabetical order. Ignore whitespace, digits, punctuation, and any character outside the English alphabet. The result must be produced with a single left-to-right scan of the input.

Formal Specification

Implement count_alphabets(text).

Input: text, a string containing ASCII letters, digits, whitespace, and punctuation.
Output: A dictionary mapping lowercase letters to their integer frequencies. Return {} when no English letters appear.

Constraints

0 <= len(text) <= 10^6
Input contains ASCII characters
Only A-Z and a-z are counted
Matching is case-insensitive

Function Signature

def count_alphabets(text):