Re: using dot_product from c++ II
Gerry Ford wrote:
"Jim Langston" <tazmaster@rocketmail.com> wrote in message
news:Ppoxj.405$bc1.96@newsfe06.lga...
Gerry Ford wrote:
[snipped and re-ordered]
So, if a points to vec_a, and likewise with b, how do I rewrite Jim
Langston's function to calculate an inner product from scratch?
This may be, but the errors you posted have nothing to do with the
code as supplied. Are you missing linking some library. I don't
know what library it is however.
It sounds like you may be attempting to link the fortran object with
the C++ object but forgetting to also link one of the required c++
libraries. What library that is depends on your compiler and OS, etc...
What
are you compiling with? What compiler and what OS? Looks like
windows because of the path, but doesn't look like MSVC++
The compiler I was using is dev-cpp on the platform that dare not
speak its name. I think I have configuration troubles with too many
compilers on the block. I believe this compiles with the exception
of the pointer re-reference and the function call that has mismatched
types; it's a gemisch of what you and Jason wrote:
#include <cmath>
#include <vector>
#include <iostream>
#include <iterator>
// using pointers instead of references
double dot_product_sqrt (const std::vector<double> *a, const
std::vector<double> *b) {
// you can remove this check if you know you'll never pass NULLs.
if (a == NULL || b == NULL) {
std::cerr << "NULL pointer passed to dot_product.\n";
return 0.0;
}
// you can remove this check if you know a and b are always the
//same size.
if (a->size() != b->size()) {
std::cerr << "Vectors for dot product are not same size!\n";
return 0.0;
}
double sum = 0.0;
for (size_t i = 0; i < a->size(); ++ i)
sum += ((*a)[i] * (*b)[i]);
// just use sqrt(), no need for std::pow(sum,.5):
return sqrt(sum);
}
int main()
{
std::vector<double> vec_a;
std::vector<double> vec_b;
float r = 42.0;
float s = 43.56;
float *rp = &r;
float *sp = &s;
//first error is next line
rp = & vec_a;
for ( int i = 0; i < 4; ++i )
{
vec_a.push_back( std::sqrt( static_cast<double>( i )) );
vec_b.push_back( i * i );
}
std::cout.precision(16);
std::copy(vec_a.begin(), vec_a.end(),
std::ostream_iterator<double>(std::cout, "\n"));
std::copy(vec_b.begin(), vec_b.end(),
std::ostream_iterator<double>(std::cout, "\n"));
std::cout << "Dot Product: " << dot_product_sqrt(vec_a, vec_b) <<
"\n"; return 0;
}
// end source. The first error is:
44 C:\Dev-Cpp\c++_source\vector10.cpp cannot convert
`std::vector<double, std::allocator<double> >*' to `float*' in
assignment
The above version has main in c++, which is going to be different
from the ultimate program that calls the function from elsewhere. I
don't have the kinks worked out in herding the address of two vectors
into the function. It looks like I either need a radical cast, or I
need to revise either main's or the external function on what is
being passed.
Grateful for any tips.
Okay, after reading the thread, it seems that you want to call this function
from Fortran. The first obvious question is: why don't you just code the
function in Fortran?
Now, as you want to call this from another language a few things come in:
1. You don't want to use in the function prototype any C++ specific stl or
classes. Therefore this is a valid reason to use a double*
2. You don't want to have to link in any C++ libraries if you can help it.
Meaning no iostream, no vector, etc... This will become a problem, however,
with the sqrt function. You are going to need to link that in whatever
library it's in.
3. You need to use C external linkage.
Since you don't want to use any C++ specific code in this function, you may
just want to code it in C in the first place. It's going to wind up with
the same function in C or C++ with the restrictions imposed.
#include <cmath>
extern "C" {
// using pointers instead of references
double dot_product_sqrt (const double*a, const double*b) {
// you can remove this check if you know you'll never pass NULLs.
if (a == NULL || b == NULL) {
// Without any C++ stuff, best we can do to show error.
return 0.0;
}
// It is presumed that the pointers are pointing to valid arrays
// of 4 doubles. If not this function has undefined behavior.
double sum = 0.0;
for (int i i = 0; i < 4; ++i)
sum += (a[i] * b[i]);
return sqrt(sum);
}
}
Few things not good about this code is it is possible to do a buffer
overflow. We are presuming that the double pointers are pointing to 4
doubles. This may or may not be the case. Also, a library still needs to
be linked in for the sqrt function. You can search for some code to do a
squareroot inline which will probably be slow, or link in the library, or
code it in Fortran in the first place.
The const in the prototype may give linking problems with Fortran, I don't
know. They are just there for constant correctness. If you have a problem
linking with the const there, remove them, although I'd probably document
that they were removed on purpose:
double dot_product_sqrt (/*const*/ double*a, /*const*/ double*b) {
--
Jim Langston
tazmaster@rocketmail.com