how to count the frequency of letters in text excluding whitespace and numbers?

Question

Use a dictionary to count the frequency of letters in the input string. Only letters should be counted, not blank spaces, numbers, or punctuation. Upper case should be considered the same as lower case. For example, count_letters("This is a sentence.") should return {'t': 2, 'h': 1, 'i': 2, 's': 3, 'a': 1, 'e': 3, 'n': 2, 'c': 1}

def count_letters(text):
      result = {}
      # Go through each letter in the text
      for letter in text:
        # Check if the letter needs to be counted or not
        if letter not in result:
          result[letter.lower()] = 1
        # Add or increment the value in the dictionary
        else:
          result[letter.lower()] += 1
      return result

    print(count_letters("AaBbCc"))
    # Should be {'a': 2, 'b': 2, 'c': 2}

    print(count_letters("Math is fun! 2+2=4"))
    # Should be {'m': 1, 'a': 1, 't': 1, 'h': 1, 'i': 1, 's': 1, 'f': 1, 'u': 1, 'n': 1}

    print(count_letters("This is a sentence."))
    # Should be {'t': 2, 'h': 1, 'i': 2, 's': 3, 'a': 1, 'e': 3, 'n': 2, 'c': 1}

Answer 1

def count_letters(text):
  result = {}
  text = text.lower()
  # Go through each letter in the text
  for letter in text:
   
    # Check if the letter needs to be counted or not
    if letter.isalpha() :
      # Add or increment the value in the dictionary
      count = text.count(letter)
      result[letter] = count
  return result

print(count_letters("AaBbCc"))
# Should be {'a': 2, 'b': 2, 'c': 2}

print(count_letters("Math is fun! 2+2=4"))
# Should be {'m': 1, 'a': 1, 't': 1, 'h': 1, 'i': 1, 's': 1, 'f': 1, 'u': 1, 'n': 1}

print(count_letters("This is a sentence."))
# Should be {'t': 2, 'h': 1, 'i': 2, 's': 3, 'a': 1, 'e': 3, 'n': 2, 'c': 1}

Answer 2

This should work:

>>> from collections import Counter
>>> from string import ascii_letters
>>> def count_letters(s) :
...     filtered = [c for c in s.lower() if c in ascii_letters]
...     return Counter(filtered)
... 
>>> count_letters('Math is fun! 2+2=4')
Counter({'a': 1, 'f': 1, 'i': 1, 'h': 1, 'm': 1, 'n': 1, 's': 1, 'u': 1, 't': 1})
>>> 

Answer 3

So I got your question,

def count_letters(text):
  result = {}
  text = text.lower()
  # Go through each letter in the text
  for letter in text:
    # Check if the letter needs to be counted or not
    if letter in "abcdefghijklmnopqrstuvwxyz":
      #
      if letter not in result:
        result[letter] = 1
      # Add or increment the value in the dictionary
      else:
        result[letter] += 1
  return result

  print(count_letters("AaBbCc"))

  # Should be {'a': 2, 'b': 2, 'c': 2}

  print(count_letters("Math is fun! 2+2=4"))
  # Should be {'m': 1, 'a': 1, 't': 1, 'h': 1, 'i': 1, 's': 1, 'f': 1, 'u': 
  1, 'n': 1}

  print(count_letters("This is a sentence."))
  # Should be {'t': 2, 'h': 1, 'i': 2, 's': 3, 'a': 1, 'e': 3, 'n': 2, 'c': 
  1}