invalid operands to binary expression - using for and iterators

Question

/* strtok example */
#include <iostream>
#include <string>
#include<list>

using namespace std;

int main ()
{
  list<string> read;
  char str[] ="g 6 7 v 7 e 0 4 e 1 4 e 2 5 e 3 5 e 4 6 e 5 6";
  char * pch;
  pch = strtok (str," ");
  while (pch != NULL)
  {
    read.push_back(pch);
    pch = strtok (NULL, " ");
  }
  list<string>::iterator i;
  i = read.begin();
  i++;
  int grid_x = stoi(*i++,nullptr,10);
  int grid_y = stoi(*i++,nullptr,10);
  i++;
  int vertices = stoi(*i++,nullptr,10);
  i++;
  for(;i<read.end();)
  {
    cout<<*i++;
    cout<<*i++;
    i++;
  }
  return 0;
}

When my code gets into the for loop, it throws up an error saying that invalid operands to binary expression. I am doing a simple comparison to check if my iterator hasn't reached the end of the list. So, I am not sure as to why it is considering this comparison as invalid. Help on the same would be appreciated as I am new to STL and iterators.

Answer 1

std::list does not support random access iterators. That is, i < read.end() is not valid. You can test for equality: i != read.end() , but you have to be a bit careful, because those increments could push the iterator past the end, and the == test would be meaningless.

Or you could use std::vector instead of std::list ; that would make the < test okay.

You'd be better off creating a stream object to parse that input, as I suggested in a comment to your earlier question.

Answer 2

You need to use != instead of <

for(; i != read.end();)

FYI, a while loop would make more sense in this example:

while(i != read.end())