comp.lang.c++

Gerry Ford wrote:
> I've been pecking away at writing a program that will calculate the
> inner product of two double-width four-vectors.
>
> Larry thinks I'm well started with the following source to populate a
> vector:
> #include
> #include
> #include
> #include
> #include
>
> int main() {
> std::vector four_vector;
>
> for (double i=0.0; i<4.0; i++)
> four_vector.push_back(sqrt(i));
>
> std::cout.precision(16);
>
> std::copy(four_vector.begin(), four_vector.end(),
> std::ostream_iterator(std::cout, "\n"));
> return 0;
> }
>
> How do I imitate the following fortran source:
> program vector2
> implicit none
> integer index, i
> integer, parameter :: some_kind_number = selected_real_kind (p=16)
> real (kind = some_kind_number), dimension(4):: vec_a, vec_b
> real (kind = some_kind_number) :: res
>
>
> index = 4
>
> do i = 1, index
> vec_a(i)= i**.5
>
> vec_b(i)= (-1)*(i**2)
>
> end do
>
> res = dot_product(vec_a, vec_b)
>
>
> write (*,*) vec_a, vec_b
> write (*,*) res
>
>
> end program vector2
>
> ! gfortran2 -o vector2 vector2.f95
> ! vector2 >text55.txt 2>text56.txt
> //end source continue comment
> , except making the inner product calculated externally. I have zero
> chance of getting it correct, so I'll spare you the flailing attempt.
> Screenshot here: http://zaxfuuq.net/c++5.jpg
>
> To imitate it, I believe the appropriate c++ inner product would be
> around negative 25.

You are not actually showing the fortran function that calculates the dot
product. That fortran source code simply populates 2 arrays of 4 elements
then calls the function dot_product(). It is the function dot_product you
want to code, but you aren't showing the souce for that.

In fortran ** is the power symbol, in C and C++ you can use the pow
function. Although some number raised to the power of 2 it's just as simple
to multiply the number by itself. Raising a number to the power of .5 is
taking the square root of it. So basically all that fortran code is filling
one array of 4 elements with the square roots of 1 to 4, the second 4
element array with the squares of 1 to 4, then calling the function
dot_product on them which returns a single number.


--
Jim Langston
tazmaster@rocketmail.com