CODEWITHSUNDEEP
c++stl
Frank
Frank

Reputation: 66214

Best practices: How to improve this code that sums container values?

I am writing a simple function that iterates over the numbers in a container and computes some summarizing value (the entropy in this case).

I want to write it as generic as possible, so that it works with different containers (vector, set, ...) over different value types (double, float, ...), so I'm doing this:

template<class Iterator>
typename Iterator::value_type Entropy(Iterator begin, Iterator end) {
  typedef typename Iterator::value_type Value;
  Value result = Value(); // assignment needed?
  for (Iterator it = begin; it != end; ++it) {
    result -= *it * log(*it);
  }
  return result;
}

I'm interested in what this code would look like according to all best practices. In other words, what exactly would an STL implementation of this little function look like?

Upvotes: 2

Views: 196

Answers (4)

Steve Townsend
Steve Townsend

Reputation: 54178

In C++0x the following lambda version can be used:

template<class Iterator>
typename Iterator::value_type Entropy(Iterator begin, Iterator end) {
  typedef typename Iterator::value_type Value;
  return accumulate(begin, end, Value(), 
      [] (Value i, Value j)-> Value { return i - j * log(j); });  
}

Upvotes: 4

Edward Strange
Edward Strange

Reputation: 40877

template < typename T1, typename T2 >
T1 entropy_sum(T1 sum_so_far, T2 number)
{
  return sum_so_far - static_cast<T1>(number) * log(static_cast<T1>(number));
}
double entropy = std::accumulate(set.begin(), set.end(), 0.0, &entropy_sum<double, set_type::value_type>);

You method is OK except for the fact that Value() might not be a legal statement.

Edited to reflect comment.

Upvotes: 4

UncleBens
UncleBens

Reputation: 41351

It would be better to use std::iterator_traits for this:

typedef typename std::iterator_traits<Iterator>::value_type Value;

This allows Iterator to be a pointer.

Upvotes: 2

David
David

Reputation: 3442

look into std::transform and std::accumulate

Upvotes: 4

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