Question

I wrote a function to check whether a word is palindrome or not but “unexpectedly”, that function failed quite badly, here it is:

bool isPalindrome (const string& s){
 string reverse = "";
 string original = s;
 for (string_sz i = 0; i != original.size(); ++i){
  reverse += original.back();
  original.pop_back();
 }

 if (reverse == original)
  return true;
 else
  return false;
}

It gives me “string iterator offset out of range error” when you pass in a string with only one character and returns true even if we pass in an empty string (although I know its because of the intialisation of the reverse variable) and also when you pass in an unassigned string for example:

string input;
isPalindrome(input);

Later, I found a better function which works as you would expect:

bool found(const string& s)
{
 bool found = true;
 for (string::const_iterator i = s.begin(), j = s.end() - 1; i < j; ++i, --j) {
  if (*i != *j)
   found = false;
 }
 return found;
}

Unlike the first function, this function correctly fails when you give it an unassigned string variable or an empty string and works for single characters and such…

So, good people of stackoverflow please point out to me why the first function is so bad…

Thank You.

Answer 1

for (string_sz i = 0; i != original.size(); ++i) {
    reverse += original.back();
    original.pop_back();
}

original.size() changes as you pop elements off the back. Effectively, you keep incrementing i and decrementing original.size(); they may never be equal.

if (reverse == original)

This will never be true since you’ve just removed all of the elements from original and added them in reverse order to reverse. original will always be empty at this point.