Use for-loop on characters in a textfile

I have a plain text file that is going to be encrypted - where I'll use double as part of the process. I read the text file

text = fileread('textfile.txt')

Then I need to get the letters into numbers

lettertoNumber = double(text)

Now comes the part I'm struggling with. I need to have a for-loop that goes through lettertoNumber, which also needs to take lower/captial letters into consideration.

for ii = 1:length(lettertoNumber)
    ?? = ???????
    ?? = ???????
end

Greatly appreciate any input.

[Walter Roberson]

What needs to be done inside the for loop? And is it mandatory to use for if it could be done by vectorization ?

[Josh Samnja]

There is no restriction on what can be used. The script or the program is going to be a simplified enigma machine. Where the input (encrypt/decrypt) is given from a text file. So the implementation I've thought of is to change letters into numbers.

lettertoNumber = double(text)

But this is only for a single letter right? Or am I wrong and literally the entire text is already converted to numbers? There is also the case of handling case sensitivity when it goes from letters/numbers vice versa. The enigma machine contains 3 rotors and 1 reflector that is already given. After the letters are changed into numbers I will use something like

cipher = rotor1(rotor2(rotor3(reflector(rotor3(rotor2(rotor1(letter)))))));

Where cipher is again a value between 1 and 26, and that needs to be converted to a letter in the same way again, and of course make sure that capitals are turned into capital and lower case into lower case..

[Walter Roberson]

double(text) would apply double() to the entire vector of characters individually.

Answer 1

[Walter Roberson]

Use lookup tables. Like

MAP_INVALIDS_TO = -1;
mapping = zeros(1,255) + MAP_INVALIDS_TO;
mapping('a':'z') = 1:26;
mapping('A':'Z') = 1:26;

mapped = mapping(text);      %valid to use characters as indices
mapped(mapped == MAP_INVALIDS_TO) = [];      %delete characters that had no mapping

Modified versions of this can be used for situations such as encoding the other characters as pairs of values, or for using "shift-in" / "shift-out" to switch between encodings.