Re: [Boost-users] apply std::pow to vector or matrix

7 Jul 2010

      Great example, thanks! Before your post, I wrote something like:

namespace my {
void my_vec_pow(boost::numeric::ublas::vector<double> &vec, double expon) {
  boost::numeric::ublas::vector<double>::iterator it;
  for (it = vec.begin(); it != vec.end(); ++it)
    *it = std::pow(*it, expon);
}
}

I'm just curious, but from performance perspective, how much more
efficient is it to use iterators and functors? I would assume that
most compilers can exploit this to improve memory management and speed
up the computation... I'll write some benchmarks later on this week,
efficiency is crucial for what I'm doing.

Max

On Wed, Jul 7, 2010 at 3:49 AM, Robert Jones  wrote:
...
On Wed, Jul 7, 2010 at 8:06 AM, Robert Jones 
wrote:
...
On Wed, Jul 7, 2010 at 3:30 AM, Max S. Kaznady 
wrote:
...
Hi Robert,
Yep, the following works great. Not sure why the same code doesn't
work when I have it in my larger project, but I guess that's for me to
figure out.
There is still the problem with the std::for_each, see below:
#include <iostream>
#include <vector>
#include <algorithm>
#include 
#include "boost/bind.hpp"
#include 
#include 
int main( )
{
 boost::numeric::ublas::vector<double> v(10);
 double exponent = 2.1;
 std::fill(v.begin(), v.end(), 8.0);
 // The following works just fine:
 std::transform( v.begin(), v.end(), v.begin(), boost::bind( pow, _1,
exponent ) );
 // But the for_each doesn't apply the power function, just leaves
the vector unaltered
 std::for_each( v.begin(), v.end(), boost::bind( pow, _1, exponent ) );
 // Print the result
 std::cout<< v << std::endl;
 return 0;
}
Compiled with: g++ -O2 -Wfatal-errors -Wall -g -ansi
-I/usr/local/boost/include -o main main.cpp
Hi Max
I haven't taken the time to try this, but does this do the trick?
std::for_each(v.begin(), v.end(), boost::bind(pow, boost::ref(_1),
exponent));
Cheers
- Rob.
Forget that, it was complete gibberish. Your example doesn't do as you
expect
because std::pow does not alter its argument, but rather returns a result.
You have
to do something with the result of std::pow, hence my original use of
transform.
To understand this look at this example.
#include <vector>
#include <algorithm>
#include 
#include "boost/bind.hpp"
void my_pow( double & val, double exponent )
{
    val = pow( val, exponent );
}
int main( )
{
    std::vector<double> v;
    double exponent = 2.1;
    std::for_each( v.begin(), v.end(), boost::bind( my_pow, _1, exponent )
);
}
HTH
- Rob.
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