Storing words from an input stream into a vector

Everything said by @Jamal

In addition I think you are counting the words in a hard way.

If you were using a std::map<> counting becomes real easy. But we will leave that solution to next month (when you read the next chapter).

But you should still become aware of the functions available in the standard library.

This function finds the range of iterators that match a specific value:

template <class ForwardIterator, class T>
pair<ForwardIterator,ForwardIterator>
equal_range (ForwardIterator first, ForwardIterator last, const T& val);

This function calculates the distance between two iterators:

template< class InputIt >
typename std::iterator_traits<InputIt>::difference_type
distance( InputIt first, InputIt last );

// Not always efficient. But for std::vector iterators it is very
// efficient being a simple subtraction.

If we plug this into your code:

sort(words.begin(), words.end());
for(auto rangeStart = words.begin(); rangeStart != words.end(); )
{
     auto wordGroup = equal_range(rangeStart, words.end(), *rangeStart);
     std::cout << *rangeStart << ": " << std::distance(wordGroup.first, wordGroup.last) << "\n";

     rangeStart = wordGroup.second;
}

This has the characteristics of \$O(nlog(n))\$ which is better than the \$O(n^2)\$ suggested by counting each of the words (as suggested by @liv902).

Since you used std::sort(), I assume you can use other functions from the standard library. Using the standard library functions, you entire program can be simplified to this:

#include <algorithm>
#include <iostream>
#include <iterator>
#include <string>
#include <vector>

int main (void)
{
    std::vector <std::string> words ;

    std::copy (
        std::istream_iterator <std::string> (std::cin),
        std::istream_iterator <std::string> (),
        std::back_inserter (words)
    ) ;

    std::vector <std::string> unique_words ;
    std::unique_copy (
        std::begin (words), 
        std::end (words), 
        std::back_inserter (unique_words)
    ) ;

    std::cout << "Total number of words: " << words.size () << "." "\n" ;
    std::cout << "Total number of unique words: " << unique_words.size () << "." "\n" ;

    for (const std::string &s : unique_words) {
        std::cout << s << ": " << std::count (std::begin (words), std::end (words), s) << "\n" ;
    }

    return 0 ;
}

Please note that I used a few C++11 features that may or may not be available to your compiler. std::begin() and std::end() can be replaced with words.begin() and words.end(). The range-based for-loop could be replaced with a normal for-loop.

One downside of doing it this way is that the std::count part of this code would be inefficient. If performance matters, you could sort the std::vector and create a function that would count adjacent words and return an iterator to the next set of words.

Some other things I noticed:

1. Give your run() function a more meaningful name.
2. Consider whether you really need to throw an exception. If this function performed something critical that needed to happen, then I could see the justification of throwing an exception. But in this case, I think outputting nothing or outputting something like "vector is empty" would be fine.

3. Avoid multiple statements on a single line.

   if (v.size() == 0) throw domain_error("Nothing to count");
   

   If you are writing an inline function within a class definition, then I guess that would be okay. I prefer using braces everywhere, but at least do this:

   if (v.size() == 0) 
       throw domain_error("Nothing to count");
   

4. I know that you are constrained to using only std::vector for this problem, but in the real world, always try to think about the best data structure for the job.