The Next Palindrome .. Strange Character Output

Question

I made this code that is supposed to increment a number till get the next palindrome number of of this inputted number.

The program take the number as string " cause it may be a very big digits number ( 0 < digits < 1000000 ) " ....

The Code

int main ()
{
string number = "1243";
int position = number.length()-1;
do
{
    if (number[position] == '9')
    {
        //cout << "hereee";
        number[position] = '0';
        int n1 = (int)number[position-1] - '0';
        n1++;
        number[position-1] = n1 + '0';
        nextPalindrome[position-1];
         cout << number <<"hereee2"<< endl; // only to determine if i get in "if"
    }
    else
    {
        int n1 = (int)number[position] - '0';
        n1++;
        number[position] = n1 + '0';
        cout << number <<"hereee1" << endl; // only to determine if i get in "else"
    }
} while (isPalindrome(number) == false);
}

It's start to take the digit in the current position and increment it and return it as character again

The Problem

cout << number <<"hereee1" << endl;

this line show the number status while runing and it is like that :

12"6 hereee1

12"7 hereee1

12"8 hereee1

12"9 hereee1

12#0 hereee2

12#1 hereee1

while it must to be

1236 hereee1

1237 hereee1

1238 hereee1

1239 hereee1

1240 hereee2

1241 hereee1

i don't know where is the error .. can any one help

NOTE : "isPalindrome" is a function take a string as parameter and if the original string equal to its reverse, it returns true .. else return false

Answer 1

You don't handle carryover well... This part is invalid:

    int n1 = (int)number[position-1] - '0';
    n1++;
    number[position-1] = n1 + '0';

This just increases the digit on the previous place. And if it happens to be '9' (same as what Mats Petersson tried to suggest), it just overflows... It should however be carried over to the next digit too... This is a recursive solution to this (beware, there might be syntax errors, I haven't coded in C++ since ages...):

/*
* This function adds one to the specified digit of a 
* string containing a decimal integer.
*
* Contains no checks whatsoever. Behavior is undefined when 
* not supplied a valid input string.
*/
int addOneToDigit (string number, int digit)
{
    if (number[digit] == '9')
    {
        number[digit] = '0';
        //we need to handle getting a longer string too...
        if(digit>0) 
        {
            return addOneToDigit(number, digit-1);
        }
        else
        {
            return "1" + number;
        }
    }
    else
    {
        int n1 = (int)number[digit] - '0';
        n1++;
        number[digit] = n1 + '0';
    }
    return number;
}

and the main() would look like something like this:

int main ()
{
    string number = "1243";
    do
    {
      number = addOneToDigit(number,number.length()-1)
    } 
    while (isPalindrome(number) == false);
}