Question

I have a simple function, that checks if the strings given match a certain condition, then generate a 3rd string based on the 2 ones received as arguments. The 3rd string is fine but when I return it it suddenly turn into “\n“.

string sReturn = "";
if (sText.size() != sPassword.size()) {
     //Checks to see if the texts match a condition
     return sReturn;
}
for (int foo = 0; foo < sText.size(); foo++) {
    sReturn = "";
    sReturn += (char)sText[foo] ^ (char)sPassword[foo];
}
return sReturn;

In the for sReturn is fine and has the right contents, but as soon as it exists the loop, the debugger suddenly tells me it’s contents are “\n“. What am I doing wrong ?

Answer 1

1. You don’t have to initialize string
   with empty character array like:

   std::string sReturn = "";
   

   Default constructor meant to do it
   for you and is much more efficient.
   Correct code:

   std::string sReturn;
   

2. Assigning an empty string to sReturn
   on each iteration in your loop is
   incorrect. Not to mention that to
   clear a string you have to call
   std::string::clear ():

   sReturn = "";
   

   Correct code:

   sReturn.clear (); 
   

   But this should be removed from the
   loop at all in your case.

3. There is not need to explicitly
   convert the result of operator []
   (size_t) into a character because it
   is a character:

   sReturn += (char)sText[foo] ^ (char)sPassword[foo];
   

   Correct code:

   sReturn += sText[foo] ^ sPassword[foo];
   

4. Using post-increment in for
   loop is not necessary. It makes an
   extra copy of the “foo” on every
   increment:

   for (int foo = 0; foo < sText.size(); foo++)
   

   This probably will be optimized by
   compiler but you have to get rid of
   this bad habit. Use
   pre-increment instead. Correct
   code:

   for (int foo = 0; foo < sText.size(); ++foo)
   

5. Calling std::string::size () on
   every iteration when string size
   does not change is not efficient:

   for (size_t foo = 0; foo < sText.size(); ++foo)
   

   Better code:

   for (size_t foo = 0, end_foo = sText.size(); foo < end_foo; ++foo)
   

   Note size_t type. You cannot store
   string size in 32-bit signed integer
   as it has not enough capacity to
   store large number. Correct type is
   size_t, which is returned by
   std::string::size () method.

Taking into account all above, the correct function should look something like this:

std::string
getMixedString (const std::string & text, const std::string & password)
{
    std::string result;
    if (text.length () != password.length ())
        return result;
    for (size_t pos = 0, npos = text.length (); pos < npos; ++pos)
        result += text[pos] ^ password[pos];
    return result;
}

But there is a problem if you want the final string to be human-readable. Using eXclusive OR (XOR) operator on two ASCII characters might or might not give you human readable character or even an ASCII character. So you may end up having resulting string that contains newlines, unreadable characters, some garbage whatsoever.

To solve this, you have to come up with some better algorithm to generate a string basing on two other strings. For example, you may use MD5 hash of both strings or encode them in base64.

Good luck!