function Freq = LetterFrequencies(Str)
% Freq = LetterFrequencies(Str)
%Input:  Str = a string of upper case ciphertext
%Output: Freq = a vector of percentages of occurences of each of the
%letters A -> Z in Str.  A summary table will also be produced in the
%command window.  This program calls on UCText2Int

Vec = UCText2Int(Str);
totChars = length(Str); %total number of characters in Str.

fprintf('   Letter        Frequency            \r')
fprintf('===============================\r')
for k=0:25
   Freq(k+1) = sum(Str == (65+k)*ones(size(Str)))/totChars;
   fprintf('      %s          %f          \r', char(65+k), Freq(k+1));
end;

function Vec = UCText2Int(str)
% Vec = UCText2Int(str)
% Input: str = a string of Upper Case text (alphabet: A, B, ..., Z)
% Output: Vec = the vector of corresponding nonnegative integers:
% A -> 0,  B->1, ..., Z -> 25

if  max((str < 'A') + (str > 'Z')) > 0
   error('Inputted text must only contain capital letters for UCText2Int');
end

Vec = str - 'A';   %MATLAB converts arithmetic operations on strings to
                   %their integer equivalents.