template functio need help

[Rex_chaos]

Hi all,
Here is a std::vector containing some std::complex<do uble>. I would
like to take the amplitude of the complex array.
i.e. v = [c1, c2, c3, c4 ...]
amp of v = [abs(c1)^2, abs(c2)^2, abs(c3)^2, abs(c4)^2,
abs(c5)^2]
I try a template function but didn't work

std::vector< complex<double> > v(5);
std::vector< double > av(5);

// initialize v here
....

transform( v.begin(), v.end(), av.begin(), ptr_fun(abs) );

Here abs is a template function defined in std. The error message is

" no matching function for call to `ptr_fun(<unkno wn type>)' "

It seems that the system don't know the function abs, so I try this

template <typename T>
T myabs(complex<T > c)
{
return abs(c);
}

transform( v.begin(), v.end(), av.begin(), ptr_fun(myabs) );

Now, it works fine. I wonder why it fails in the first transform
???????

Anyway, what I get now is only the abs of the complex element (|u|)
but not the amplitude (|u|^2). The problem can obviously be solved by
taking the transform again like

transform( av.begin(), av.end(), av.begin(), bind2nd(ptr_fun (pow),
2.0) );

However, it really take times when the above code is running. If it's
possible to obtain the last result with only one trasnform? And how?

Thanks in advance.

[Julián Albo]

Rex_chaos escribió:

// initialize v here
...

transform( v.begin(), v.end(), av.begin(), ptr_fun(abs) );

Here abs is a template function defined in std. The error message is

" no matching function for call to `ptr_fun(<unkno wn type>)' "
Try this:

std::transform( v.begin(), v.end(), av.begin(), std::ptr_fun (&
std::abs<double > ) );
Anyway, what I get now is only the abs of the complex element (|u|)
but not the amplitude (|u|^2). The problem can obviously be solved by
taking the transform again like

transform( av.begin(), av.end(), av.begin(), bind2nd(ptr_fun (pow),
2.0) );

However, it really take times when the above code is running. If it's
possible to obtain the last result with only one trasnform? And how?

Do the same you make with myabs, write a myamplitude functor.

Regards.

[Rex_chaos]

> std::transform( v.begin(), v.end(), av.begin(), std::ptr fun (&

std::abs<double > ) );

Are u sure it's ok? Actually, I have already add 'using namespace
std;' at the beginning of the source, still no help.

[Julián Albo]

Rex_chaos escribió:

std::transform( v.begin(), v.end(), av.begin(), std::ptr fun (&
std::abs<double > ) );

Are u sure it's ok? Actually, I have already add 'using namespace
std;' at the beginning of the source, still no help.

I put the std:: for clarity (and it seems I obscured things instead),
but the key is to use abs<double> instead of abs alone.

Regards.

[Rex_chaos]

Julián Albo <JU********@ter ra.es> wrote in message news:<3F******* ********@terra. es>...

Rex chaos escribi :

std::transform( v.begin(), v.end(), av.begin(), std::ptr fun (&
std::abs<double > ) );

Are u sure it's ok? Actually, I have already add 'using namespace
std;' at the beginning of the source, still no help.

I put the std:: for clarity (and it seems I obscured things instead),
but the key is to use abs<double> instead of abs alone.

It's confusing. Take a look at this class

template <typename A>
class Tester
{
...
A myfun(A a) {...}
template <typename FunObj>
void mytransform(Fun Obj fun)
{
transform( v.begin(), v.end(), v.begin(), fun);
}

void testing(void)
{
mytransform( myfun );
}
}

when calling testing(), error occurs. it's all right unless I replace
myfun with an external function ??????

[Julián Albo]

Rex_chaos escribió:

template <typename A>
class Tester
{
...
A myfun(A a) {...}


template <typename FunObj>
void mytransform(Fun Obj fun)
{
transform( v.begin(), v.end(), v.begin(), fun);
}

void testing(void)
{
mytransform( myfun );
}
}

when calling testing(), error occurs. it's all right unless I replace
myfun with an external function ??????

You can't use a member function directly with a standard algorithm, they
expect something that can be called directly.

Regards.

[Rex_chaos]

Julián Albo <JU********@ter ra.es> wrote in message news:<3F******* ********@terra. es>...

Rex chaos escribi :

template <typename A>
class Tester
{
...
A myfun(A a) {...}

template <typename FunObj>
void mytransform(Fun Obj fun)
{
transform( v.begin(), v.end(), v.begin(), fun);
}

void testing(void)
{
mytransform( myfun );
}
}

when calling testing(), error occurs. it's all right unless I replace
myfun with an external function ??????

You can't use a member function directly with a standard algorithm, they
expect something that can be called directly.

It's annoything. I am trying to have an inner-class casting a function
object. However, no help.

[tom_usenet]

On 7 Oct 2003 04:25:17 -0700, re*******@21cn. com (Rex_chaos) wrote:

Julián Albo <JU********@ter ra.es> wrote in message news:<3F******* ********@terra. es>...

Rex chaos escribi :

> template <typename A>
> class Tester
> {
> ...
> A myfun(A a) {...}
>

>

> template <typename FunObj>
> void mytransform(Fun Obj fun)
> {
> transform( v.begin(), v.end(), v.begin(), fun);
> }
>

> void testing(void)
> {
> mytransform( myfun );
> }
> }
>

> when calling testing(), error occurs. it's all right unless I replace
> myfun with an external function ??????

You can't use a member function directly with a standard algorithm, they
expect something that can be called directly.

It's annoything. I am trying to have an inner-class casting a function
object. However, no help.

You can use a binder, from the <functional> header:

void testing()
{
mytransform(std ::bind1st(std:: mem_fun(&Tester <A>::myfun), this));
}

Tom