Array attribution with all elements

Hi,

one way to set values to all elements of an array is:

Do i=1, N

    A(i)=2*B(i)
    C(1,i)=2*B(i)

End do

Another possibility is to use this:

A(:)=2*B(:)

C(1,:)=2*B(:)

Which is the optimum way to set values of an entire array? And with parallel computing?

Thank you.