How to use the penultimate position of an iterator in c++

Question

I have the following piece of code which helps me to write a bunch of values into a comma separated file format. My problem is, that I do not want a comma after the last element written to normcsv . How can I use beg in an If clause of the kind:

if(beg == penultimate element)
then.... bla bla...

Everything I tried out ended up with the iterator being mad invalid

ReadLine.erase(0,17);
int offsets[] = {8,8,8,8,8,8};
boost::offset_separator f(offsets, offsets+6);
boost::tokenizer<boost::offset_separator> RVBEARline(ReadLine,f);
boost::tokenizer<boost::offset_separator>::iterator beg;

for( beg=RVBEARline.begin(); beg!=RVBEARline.end();++beg )
{                                   
    copy=*beg;
    boost::trim(copy);
    if(copy.compare(0,1,".")==0)
    {
        copy.insert(0,"0");
    }

    normcsv << copy <<",";
}

Answer 1

Instead of printing the comma after the element except during the last iteration, print it before the element except during the first iteration. For that, you can use if(beg != RVBEARline.begin()) .

Answer 2

An alternative to ruakh's "first plus rest" approach, you can do with one less local variable by using a loop-and-a-half construct:

{
  auto it = x.begin(), end = x.end();

  if (it != end)
  {
    for ( ; ; )
    {
      process(*it);

      if (++it == end) break;

      print_delimiter();
    }
  }
}

Here x.begin() and x.end() are only called once. There is one mandatory comparison per loop round, the minimum possible. The check for emptiness is hoisted outside.

Answer 3

您不能总是删除最后一个字符，因为您知道这将是多余的逗号吗？