Perform vector operation

Question

Hi,

I'm using the vector container to store an array of doubles. Is there any quick way of multiplying each element in my vector by some scalar without using a loop.

For example:

  vector<double> Array(10,1);

will initialise an array of 10 doubles with initial value 1. To multiply this array by 0.5 I would write:

  for(unsigned int i=0; i<Array.size(); i++) 
     Array[i] = 0.5*Array[i]; 

Is there there another way? I have used valarray which overloads the '*' operator so that:

     Array = 0.5 * Array; 

is valid but I'd rather not use valarray as it seems the vector container is a more standard approach for manipulating arrays.

Thanks!

Answer 1

as i know there is not.

if there is one, probably it encapsulates this loop for you. so i dont think the performance would change.

Answer 2

You could do this:

std::transform(Array.begin(), Array.end(), Array.begin(),
                std::bind2nd(std::multiplies<double>(), 0.5));

In response to getting the sum of elements:

double sum = std::accumulate(Array.begin(), Array.end(), 0.0);

And in response to getting sqrt'ing each element:

std::transform(Array.begin(), Array.end(), Array.begin(),
                static_cast<double (*)(double)>(std::sqrt));

That cast is to select the correct overload.

Answer 3

You can use std::transform:

 std::transform(Array.begin(), Array.end(), Array.begin(), std::bind1st(std::multiplies<double>(), 0.5)));

Answer 4

The STL vector itself does not allow elementwise scaling in a single operation.

You could wrap your vector with a decorator which applys a scaling factor. The application of a new factor would be O(1) regardless of the size of the vector. This is comes not for free as the drawbacks are increased complexity and a somewhat larger access per element.

Answer 5

Consider using an std::valarray.