Caesar's cypher working incorrectly

Hello,

I'm writing a Matlab code that reads text from a .txt file and encodes it using Caesar's cypher. The shift value is inputted from the keyboard.

As long as I only use uppercase letters, the code works pefectly. The probles is that lowercase letters are encoded wrongly. I understand that Matlab converts it to ASCII values and applies the shift on that value, then reconverts it, but i can't seem to identify the problem. I've used the same formula for Python also and there it works fine.

This is my code:

clc; close all; clear;
% Read input text file
input_file = 'input.txt';
fid = fopen(input_file, 'r');
input_text = fscanf(fid, '%c');
fclose(fid);

% Get shift from user input
shift = input('Enter shift value: ');

% Apply Caesar cipher to each character in input text
output_text = '';
for i = 1:length(input_text)
    % Shift uppercase letters
    if isletter(input_text(i)) && upper(input_text(i))
        output_text = [output_text, char(mod(double(input_text(i)) - 65 + shift, 26) + 65)];
    % Shift lowercase letters
    elseif isletter(input_text(i)) && islower(input_text(i))
        output_text = [output_text, char(mod(double(input_text(i)) - 97 + shift, 26) + 97)];
    % Leave non-letter characters unchanged
    else
        output_text = [output_text, input_text(i)];
    end
end

% Write output text file
output_file = 'output1.txt';
fid = fopen(output_file, 'w');
fprintf(fid, '%s', output_text);
fclose(fid);

And these are the input and outputs that i get, using a shift of 1:

I'm open to any suggestions.

Thank you.

Answer 1

[Voss]

I can't find the function islower in MATLAB. Maybe that's a function you wrote yourself, and if so maybe it works correctly, but it's never going to be called in this code. The reason is that the if condition

if isletter(input_text(i)) && upper(input_text(i))
%                             ^^^^^^^^^^^^^^^^^^^^ this is a character

is always true when isletter(input_text(i)) is true, because upper(input_text(i)) returns a character whose ASCII value is something other than 0 when input_text(i) is a letter, so upper(input_text(i)) evaluates to true, so the entire if condition is true when isletter(input_text(i)) is true.

Therefore the elseif condition

elseif isletter(input_text(i)) && islower(input_text(i))
%                                 ^^^^^^^^^^^^^^^^^^^^^^ this is never evaluated

is only checked when isletter(input_text(i)) is false, and in that case the first part of the elseif condition is false, so the second part is never checked because && short-circuits.

Maybe you have a function called isupper that works that you can use in place of upper, or maybe you had isupper there originally and you got an error because that function doesn't exist, so you changed it to upper, which created this situation where the corresponding error trying to use islower will never be seen because islower is never called for the reason explained above. I don't know, but in any case, one way to solve this without using the unknown functions isupper/islower is to use isstrprop, as shown below.

% Read input text file
input_file = 'input.txt';
fid = fopen(input_file, 'r');
input_text = fscanf(fid, '%c');
fclose(fid);

% Get shift from user input
% shift = input('Enter shift value: ');
shift = 1;

% Apply Caesar cipher to each character in input text
output_text = '';
for i = 1:length(input_text)
    % Shift uppercase letters
    if isstrprop(input_text(i),'upper')
        output_text = [output_text, char(mod(double(input_text(i)) - 65 + shift, 26) + 65)];
    % Shift lowercase letters
    elseif isstrprop(input_text(i),'lower')
        output_text = [output_text, char(mod(double(input_text(i)) - 97 + shift, 26) + 97)];
    % Leave non-letter characters unchanged
    else
        output_text = [output_text, input_text(i)];
    end
end

% Write output text file
output_file = 'output1.txt';
fid = fopen(output_file, 'w');
fprintf(fid, '%s', output_text);
fclose(fid);

type output1.txt % show the output file's contents

B
C
D
E
F
G
H

BOB BSF NFSF
bob bsf nfsf

Bob bsf nfsf

Note that you can process your entire file at once instead of one character at a time:

% Read input text file
input_file = 'input.txt';
fid = fopen(input_file, 'r');
input_text = fscanf(fid, '%c');
fclose(fid);

% Get shift from user input
% shift = input('Enter shift value: ');
shift = 1;

% Apply Caesar cipher to each character in input text
output_text = input_text;
isU = isstrprop(input_text,'upper');
isL = isstrprop(input_text,'lower');
output_text(isU) = char(mod(double(input_text(isU)) - 65 + shift, 26) + 65);
output_text(isL) = char(mod(double(input_text(isL)) - 97 + shift, 26) + 97);

% Write output text file
output_file = 'output1.txt';
fid = fopen(output_file, 'w');
fprintf(fid, '%s', output_text);
fclose(fid);

type output1.txt % show the output file's contents

B
C
D
E
F
G
H

BOB BSF NFSF
bob bsf nfsf

Bob bsf nfsf

[Carmen Chira]

Hi!

yes, i forgot edit this post after i saw some errors in my code. Now it looks like this:

clc; close all; clear;
% Read input text file
input_file = 'input.txt';
fid = fopen(input_file, 'r');
input_text = fscanf(fid, '%c');
fclose(fid);

% Get shift from user input
shift = input('Enter shift value: ');

% Apply Caesar cipher to each character in input text
output_text = '';
for i = 1:length(input_text)
    
    % Shift uppercase letters
    if (isletter(input_text(i)) && upper(input_text(i)))
        output_text = [output_text, char(mod(double(input_text(i)) - 65 + shift, 26) + 65)];
   
    % Shift lowercase letters
    elseif (isletter(input_text(i)) && lower(input_text(i)))
        output_text(i) = [output_text, char(mod((double(input_text(i))+ shift - 97),26) + 97)];

   % Leave non-letter characters unchanged
    else
        output_text = [output_text, input_text(i)];
    end
end

% Write output text file
output_file = 'output2.txt';
fid = fopen(output_file, 'w');
fprintf(fid, '%s', output_text);
fclose(fid);

But thank you so much for your help! This definetly simplifies everything.

[Voss]

You're welcome!

Note that your updated code still suffers from the same problems as the original. Specifically, lower will never be called for the reason described in my answer, and you should use isstrprop to test for upper- or lower-case.

Answer 2

[John D'Errico]

Question: What do the functions upper, and islower do in MATLAB?

help upper

 UPPER Convert to uppercase
    NEWSTR = UPPER(STR) converts any lowercase characters in STR to the
    corresponding uppercase character and leaves all other characters
    unchanged. STR can be a string, character vector, or a cell array of
    character vectors. NEWSTR is the same type and shape as STR.
 
    Example:
        STR = 'DATA.tar.gz';
        upper(STR)          
 
        returns  
 
        DATA.TAR.GZ
 
    Example:
        STR = ["FUNDS.xlsx";"PAPER.docx"];
        upper(STR)
 
        returns
 
            "FUNDS.XLSX"
            "PAPER.DOCX"
 
    See also LOWER, STRING, ISSTRPROP, REVERSE

    Documentation for upper
       doc upper

    Other uses of upper

       codistributed/upper    tokenizedDocument/upper

So upper converts a string to entirely uppercase. It is NOT a test.

upper('The quick brown Fox')

ans = 'THE QUICK BROWN FOX'

As well, there is no islower function in MATLAB, but it does seem to exist in python.

which islower

'islower' not found.