Having trouble encrypting and decrypting messages

Here's the homework problem I have, and here is the code I put in for it. I keep getting this question wrong. What could I be doing wrong?

[Walter Roberson]

Unfortunately MATLAB does not have any facility for executing pictures of code, so we cannot test your code.

[Christian]

Okay, for sure. Here is the code:

% Testing the encryption function
key = 7;
message = 'My Bday is Nov 28';
encrypted_message = encrypt(message, key);
disp(['Encrypted Message: ', encrypted_message]);

% Decrypting the given message
encrypted_msg = 'VJCUJK Ena 1912j';
for possible_key = 4:9
    decrypted_message = encrypt(encrypted_msg, -possible_key);
    disp(['Key: ', num2str(possible_key), ', Decrypted Message: ', decrypted_message]);
end


function encrypted_message = encrypt(message, key)
    % Function to encrypt or decrypt a message based on a key.
    % Spaces are not encrypted and the encryption wraps around for letters and numbers.

    encrypted_message = "";

    for i = 1:length(message)
        charr = message(i);

        % Check if character is a space
        if charr == ' '
            encrypted_message = strcat(encrypted_message, char);
            continue;
        end

        % Determine the ASCII value of the character
        ascii_value = double(charr);

        % Check if character is an uppercase letter
        if 'A' <= charr && charr <= 'Z'
            % Wrap around within uppercase letters
            encrypted_message = strcat(encrypted_message, char(mod(ascii_value - 65 + key, 26) + 65));
        % Check if character is a lowercase letter
        elseif 'a' <= charr && charr <= 'z'
            % Wrap around within lowercase letters
            encrypted_message = strcat(encrypted_message, char(mod(ascii_value - 97 + key, 26) + 97));
        % Check if character is a digit
        elseif '0' <= charr && charr <= '9'
            % Wrap around within digits
            encrypted_message = strcat(encrypted_message, char(mod(ascii_value - 48 + key, 10) + 48));
        else
            % If it's not a letter or digit, just add the character as it is
            encrypted_message = strcat(encrypted_message, char);
        end
    end
end

Answer 1

[Voss]

Notice that a space character should be encrypted as a space character. The input message you are given has spaces, but the encrypted message you generate does not. This indicates a problem with how spaces are encrypted in your code.

The reason is explained in the strcat documentation:

"For character array inputs, strcat removes trailing ASCII whitespace characters: space, tab, vertical tab, newline, carriage return, and form feed. For cell array and string array inputs, strcat does not remove trailing white space."

This means that when you try to strcat a space onto your encrypted string, it doesn't work:

strcat("message",' ')

ans = "message"

There's no space at the end because strcat removed the trailing space from ' ' and concatenated '' (i.e., an empty character array) onto "message".

But if you convert the character ' ' into a string before doing strcat, then you get the space concatenated properly:

strcat("message",string(' '))

ans = "message "

[Walter Roberson]

Nice catch!

[Voss]

Thanks!

[Christian]

I understand, but I'm not 100% sure where to make the changes. Could you tell me which specific lines need adjustments in my code, and how to adjust them? Thank you for your help.

[Voss]

        % Check if character is a space
        if charr == ' '
            encrypted_message = strcat(encrypted_message, string(char));
            continue;
        end

[Christian]

Okay, I ran the code, and this is what it gave me:

Given this, what should I put in for my homework answers? I will attach the homework page in another message since it will only let me attach one photo per message.

[Christian]

This is what I got for my latest attempt on my homework. My apologies for the excessive zoom on these images.

[Walter Roberson]

The first character of msg is 'M'. The 7'th letter after 'M' is 'T' but your output is 'O' -- which is only 2 letters after 'M' not 7.

[Voss]

Sorry, it should've been

encrypted_message = strcat(encrypted_message, string(charr));

instead of

encrypted_message = strcat(encrypted_message, string(char));

Running the whole thing:

% Testing the encryption function
key = 7;
message = 'My Bday is Nov 28';
encrypted_message = encrypt(message, key);
disp(sprintf('Encrypted Message: %s', encrypted_message));

Encrypted Message: Tf Ikhf pz Uvc 95

% Decrypting the given message
encrypted_msg = 'VJCUJK Ena 1912j';
for possible_key = 4:9
    decrypted_message = encrypt(encrypted_msg, -possible_key);
    disp(sprintf('Key: %d, Decrypted Message: %s', possible_key, decrypted_message));
end

Key: 4, Decrypted Message: RFYQFG Ajw 7578f
Key: 5, Decrypted Message: QEXPEF Ziv 6467e
Key: 6, Decrypted Message: PDWODE Yhu 5356d
Key: 7, Decrypted Message: OCVNCD Xgt 4245c
Key: 8, Decrypted Message: NBUMBC Wfs 3134b
Key: 9, Decrypted Message: MATLAB Ver 2023a

function encrypted_message = encrypt(message, key)
    % Function to encrypt or decrypt a message based on a key.
    % Spaces are not encrypted and the encryption wraps around for letters and numbers.

    encrypted_message = "";

    for i = 1:length(message)
        charr = message(i);

        % Check if character is a space
        if charr == ' '
            encrypted_message = strcat(encrypted_message, string(charr));
            continue;
        end

        % Determine the ASCII value of the character
        ascii_value = double(charr);

        % Check if character is an uppercase letter
        if 'A' <= charr && charr <= 'Z'
            % Wrap around within uppercase letters
            encrypted_message = strcat(encrypted_message, char(mod(ascii_value - 65 + key, 26) + 65));
        % Check if character is a lowercase letter
        elseif 'a' <= charr && charr <= 'z'
            % Wrap around within lowercase letters
            encrypted_message = strcat(encrypted_message, char(mod(ascii_value - 97 + key, 26) + 97));
        % Check if character is a digit
        elseif '0' <= charr && charr <= '9'
            % Wrap around within digits
            encrypted_message = strcat(encrypted_message, char(mod(ascii_value - 48 + key, 10) + 48));
        else
            % If it's not a letter or digit, just add the character as it is
            encrypted_message = strcat(encrypted_message, char);
        end
    end
end