How to detect first or last element iterating over a container?

Question

How to do the following in more stylish/short way?

for(i=container.begin(); i!=container.end(); ++i) {
    if (i!=container.begin()) {
        cout << ", ";
    }
    cout << *i;
    j=i;
    if (++j==container.end()) {
        cout << "!" << endl; 
    }
}

Solutions like foreach are acceptable (actions on first and last elements need to be configurable, though).

P.S. There are many answers that are handling first element, but not last. Here is what I mean by handling last element:

for(i=container.begin(); i!=container.end(); ++i) {
    j=i;
    if (i==container.begin()) {
        cout << "[" << *i << "]" << endl;
    } else if (++j==container.end()) {
        cout << ", (" << *i << ")" << "!" << endl; 
    } else {
         cout << ", " << *i;
    }
}

Don't you think it's very easy to handle first element outside the cycle body? The real problem is the last one! I'm sorry for not being able to clarify the important point asking the question. I think I'll just accept the top ranked answer eventually.

Answer 1

In your code,

if (i==container.end()) {
    cout << "!" << endl; 
}

will never happen.

My own approach would be to use the container size (I think size() is now constant time for all Standard Library containers). Maintain a count in the loop and you are at the end when count == size() - 1, and at the beginning when count == 0, obviously.

Answer 2

Shift the ++i a bit:

i = container.begin();
while(i != container.end()) {
    if (i != container.begin()) {
        cout << ", ";
    }
    cout << *i;
    if (++i == container.end()) {
        cout << "!" << endl; 
    }
}

Answer 3

My advice here would be: there is no point in detecting anything within this loop !

Since your special cases are at the beginning and the end of your container, it is easy to remove their processing from within the loop.

The following function will print the contents of any container class whose elements can be <<'ed to an std::ostream:

template < class Container >
void print(Container const & container)
{
    typename Container::const_iterator current = container.begin();
    typename Container::const_iterator const end = container.end();
    if (current != end)
    {
        std::cout << *current;
        for (++current; current != end; ++current)
        { 
            std::cout << ", " << *current;
        }
        std::cout << "!" << std::endl;
    }
}

Answer 4

template < class TContainerType>
void print(TContainerType const & i_container)
  {
  typename TContainerTypeconst ::const_iterator current = i_container.begin();
  typename TContainerTypeconst ::const_iterator const end = i_container.end();
  if(current != end)
    {
    std::cout << *current++;
    while(current != end)
      std::cout << ", " << *current++;
     }
  std::cout << "!" << std::endl;
  }

Answer 5

As container is not defined by you, I used the simplest - vector

template <class T>
string vector_join( const vector<T>& v, const string& token ){
  ostringstream result;
  for (typename vector<T>::const_iterator i = v.begin(); i != v.end(); i++){
    if (i != v.begin()) result << token;
    result << *i;
  }
  return result.str();
}

//usage
cout << vector_join( container, ", " ) << "!";

Answer 6

Boost has next / prior which can sometimes help in such situations.

for(i=container.begin(); i!=container.end(); ++i) {
    if (boost::next(i) == container.end()) {
         std::cout << "!" << std::endl;
    }
}

Although for this specific case, I'd simply output the first element, loop from second till last while always outputting the ',' and then output the '!' after the loop has ended. (as others have suggested already)

I don't see the point in moving the special cases inside the loop, and then checking inside the loop for them....

Answer 7

Take the second part out of the loop.

for(i=container.begin(); i!=container.end(); ++i) {
    if (i != container.begin()) {
        cout << ", ";
    }
    cout << *i;
}
cout << "!" << endl;