stoi() terminate after throwing and instance of 'std::invalid argument in c -- What am I doing-CodePudding

Fairly new to coding. Trying some of the easy projects at LeetCode, and failing... Ha! I am trying to take an integer and convert it to a string so I can reverse it, then re-convert the reversed string back into a integer.

This code is throwing the "terminate after throwing and instance of 'std::invalid argument' what(): stoi" error. I've spent an hour searching google and other questions here on SO, but can't figure out why it's not working.

bool isPalindrome(int x) {
  std::string backwards ="";
  std::string NumString = std::to_string(x);
     
  for (int i = NumString.size(); i >= 0 ; i--) {
    backwards  = NumString[i];
  }
       
  int check = std::stoi(backwards);
              
  if (check == x) {
    return true;
  }
  else {
    return false;
  }
}

EDIT: I think I figured it out. It was adding the null character to the end of the string upon first conversion, then adding it to the beginning of the string when I reversed it. Spaces can't be converted to integers.

So... I changed this line and it works:

for (int i = NumString.size() - 1; i >= 0 ; i--)

CodePudding user response：

you can also reverse number without using string.

     bool isPalindrome(int x) {
        long long  rev = 0;
        int cur = x;
        while( cur > 0) {
            rev *= 10;
            rev  = cur % 10;
            cur /=10;
        }
        return rev == x;

    }

CodePudding user response：

Its simpler than your answer that you edited in. YOu have

  for (int i = NumString.size(); i >= 0 ; i--) {
    backwards  = NumString[i];
  }

Imagine that Numstring has length 3 (no matter what spaces, digits,....)

So now you are efectively doing

  for (int i = 3; i >= 0 ; i--) {
    backwards  = NumString[i];
  }

So first loop goes

  backwards  = NumString[3];

well the indexes of things in an array of length 3 in c are 0,1,2. YOu are going one off the end

This is why you see loops doing

 for(int i = 0; i < len; i  ){}

Note the i < len not i <= len