Finding the frequency of letters in a string

Question

Given the following string:

sentence = "Never in the delirious dream of a disordered brain could 
anything more savage, more appalling, more hellish, be conceived than 
that dark form and savage face which broke upon us out of the wall of 
fog";

I am trying to find the frequency of occurrence of each letter. Working towards this, I have find the occurrence of each letter by:

CountLettersSorted[y_] := 
  Reverse[SortBy[
  Tally[Characters[
  StringReplace[
   ToLowerCase[y], {"," -> "", " " -> "", "." -> "", 
    "-" -> ""}]]], Last]];

Giving:

{{"e", 39}, {"a", 35}, {"s", 26}, {"n", 25}, {"o", 24}, {"l", 
  24}, {"t", 23}, {"d", 21}, {"u", 17}, {"i", 17}, {"h", 17}, {"r", 
  14}, {"m", 13}, {"w", 11}, {"c", 11}, {"y", 9}, {"b", 9}, {"p", 
  7}, {"k", 7}, {"v", 6}, {"g", 6}, {"f", 3}}

Now I'm trying to define a function f that can be mapped to this list ie:

f:= #/Length[string[s]]
findingfrequency=Map[f,#]   

But this is not working, any ideas?

Answer 1

Why not use LetterCounts

LetterCounts[sentence, IgnoreCase -> True] // KeySort

N[#]/Total[#] &[%]

<|"a" -> 16, "b" -> 3, "c" -> 5, "d" -> 9, "e" -> 19, "f" -> 6, 
  "g" -> 5, "h" -> 9, "i" -> 10, "k" -> 2, "l" -> 8, "m" -> 5, 
  "n" -> 10, "o" -> 15, "p" -> 3, "r" -> 12, "s" -> 6, "t" -> 7, 
  "u" -> 5, "v" -> 4, "w" -> 2, "y" -> 1|>

<|"a" -> 0.0987654, "b" -> 0.0185185, "c" -> 0.0308642, 
  "d" -> 0.0555556, "e" -> 0.117284, "f" -> 0.037037, "g" -> 0.0308642,
  "h" -> 0.0555556, "i" -> 0.0617284, "k" -> 0.0123457, 
  "l" -> 0.0493827, "m" -> 0.0308642, "n" -> 0.0617284, 
  "o" -> 0.0925926, "p" -> 0.0185185, "r" -> 0.0740741, 
  "s" -> 0.037037, "t" -> 0.0432099, "u" -> 0.0308642, 
  "v" -> 0.0246914, "w" -> 0.0123457, "y" -> 0.00617284|>

BTW, your sentence does not give 39 "e"s.

Answer 2

Not totally sure if this is what you want:

    tallyChars[sentence_String] := Module[{work},
  work = StringCases[Characters[sentence], 
     head___ ~~ LetterCharacter ~~ tail___] /. {} -> Nothing;
  work = Flatten@(ToLowerCase /@ work);
  SortBy[Tally[work], Last] // Reverse
  ]

This can be done shorter, but so it is easy to read:

Step1: get the letter characters and delete the {} coming form punctuation marks etc. Step2: Convert to Lowercase and flatten the list Step3: Sorting

You can then use the function as `tallyChars /@ listOfStrings

remark: your example sentence has only 19 times the letter "e".

Answer 3

Does this not accomplish what you're after?

Reverse[SortBy[
  Tally[ToLowerCase@
    StringCases[sentence, LetterCharacter]], #[[2]] &]]