Stl methods with cmath functions

Question

I was trying to write an STL method to take the log of a vector:

for_each(vec.begin(),vec.end(),log);

But I get

no matching function for call to ‘for_each(__gnu_cxx::__normal_iterator<double*, std::vector<double, std::allocator<double> > >, __gnu_cxx::__normal_iterator<double*, std::vector<double, std::allocator<double> > >, <unresolved overloaded function type>)’

Which I gather is due to log function's multiple versions. Obviously I can write a simple wrapper around the log function and call it with that. Is there a simpler way to specify which log function I want inline?

Answer 1

Yes. You can cast the function to the appropriate type:

for_each(vec.begin(),vec.end(),(double(*)(double))log);

Another possibility would be creating your functor that would accept any type:

struct log_f
{
  template <class T> T operator()(const T& t) const { return log(t); }
};

for_each(vec.begin(),vec.end(), log_f());

And, as Billy O'Neal pointed out, you want rather transform than for_each.

Answer 2

I believe std::for_each is looking for a function with a void return type. You're passing a function with a double return type. jpalecek's answer is the correct one, and +1 to him. However, you still have the semantic issue that doing for_each with log doesn't make any sense:

If you want all the members of the vector to be the log of the previous members, i.e.:

//pseudocode
foreach( var x in myvector )
   x = log(x);

Then you don't want for_each, you want transform.

std::transform(vec.begin(), vec.end(), vec.begin(), log);