Frequency of Letters | Dataford Interview Questions

Systems Planning and Analysis tools may need simple text statistics when processing mission or project notes. Given a string, count how often each English letter appears.

Treat uppercase and lowercase versions of a letter as the same character. Ignore spaces, digits, punctuation, and other nonalphabetic characters. Return a dictionary containing only letters that appear, with lowercase letters as keys. The order of dictionary entries does not matter.

Formal Specification

Implement count_letter_frequency(text), where text is a string. Return a dictionary mapping each lowercase English letter to its integer frequency.

Constraints

0 <= len(text) <= 10^5
text contains standard ASCII characters
Only the 26 English letters are counted
Return an empty dictionary when no letters appear

Function Signature

def count_letter_frequency(text):

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