How to cipher an input file?

Hi,

I'm woring on a program that ciphers files (Caesar cipher). The program works when an user inputs a sentence, but protests when somebody is trying to do the same with a file, any idea what am I doing wrong?

clear all
fid = input('Podaj tekst:','s')

if exist(fid, 'file')%File exists

Alfabet = ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o','p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z', 'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J', 'K', 'L', 'M', 'N', 'O', 'P', 'Q', 'R', 'S', 'T', 'U', 'W','V', 'X', 'Y', 'Z' ]; 
i = 1;
key = input('Podaj skok:'); %Jump
plik = dlmread(fid)% Tried plik = fopen(fid) and plik = importdata(fid) as well, but those don't work either.

while key > 26
    key = mod(key,26); 
end

while i <= length(plik);
        
    if isletter(plik(i)) == 1 
        y = strfind(Alfabet,plik(i)); 
        z = y+key; 
        if z > 26 
        z = z - 26;
        end
        Szyfr(i) = Alfabet(z); 
    
    elseif isletter(plik(i)) == 0
        if plik(i) == ' ' 
            Szyfr(i)=plik(i);
        end
                        if plik(i) == '.' 
            Szyfr(i) = plik(i);
                end
                               if plik(i) == ',' 
            Szyfr(i) = plik(i);
                               end
                                               if plik(i) == ':' 
            Szyfr(i) = plik(i);
                end
    
    end
    
    i = i+1;

end

dlmwrite('Cezar.txt',Szyfr,'\n')
dlmwrite('Skok.txt',key,'\n')
else 

fprintf('Podany plik nie istnieje:\n%s', plik); %File doesn't exist

end

Answer 1

[Guillaume]

dlmread is not for reading text files.The easiest way to import a text file is with fileread.

plik = fileread(fid); %note that using fid as a variable name for a filename is very misleading

Similarly, dlmwrite is not for writing text files. There's no one-line equivalent to fileread unfortunately:

filename = input('name of file to write', 's');
fid = fopen(filename, 'wt');
fprintf(fid, 'Cezar: %s\n', Szyfr);
fprintf(fid, 'Skok %s\n', key);
fclose(fid);

WIth regards to your code:

while key > 26
    key = mod(key,26); 
end

is a pointless loop. If key is greater than 26, the mod guarantees that it will be less than 26 after that regardless of how big it is. If it is less than 26, then the mod wouldn't change it, so the whole lot is simply:

key = mod(key, 26); %no loop needed

I note that later on in your code, you seem to have forgotten about mod.

Finally, I'll leave you with this code:

plain = 'the quick brown fox jumps over the lazy fox!'  %demo text
key = mod(input('Podaj skok:'), 26);
cypher('a':'z') = circshift('a':'z', -key);
encoded = plain;
encoded(isletter(plain)) = cypher(plain(isletter(plain)))

[Karou]

Thank you kindly sir, worked :D