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

Subject: Re: [Boost-users] apply std::pow to vector or matrix
From: Robert Jones (robertgbjones_at_[hidden])
Date: 2010-07-07 03:49:06

On Wed, Jul 7, 2010 at 8:06 AM, Robert Jones <robertgbjones_at_[hidden]>wrote:

> On Wed, Jul 7, 2010 at 3:30 AM, Max S. Kaznady <max.kaznady_at_[hidden]>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 <math.h>
>> #include "boost/bind.hpp"
>> #include <boost/numeric/ublas/vector.hpp>
>> #include <boost/numeric/ublas/io.hpp>
>>
>> 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 <math.h>
#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.