<algorithm> transform modification

[Jerry Coffin]

Re: &lt;algorithm&g t; transform modification

In article <f9b47917.03092 10613.16373a9b@ posting.google. com>,
s.z.s@web.de says...[color=blue]
> Jerry Coffin <jcoffin@taeus. com> wrote in message news:<MPG.19d6d cde10e03cd0989b 0a@news.clspco. adelphia.net>.. .[color=green]
> >
> > See $ 17.4.3.1/1 for the exact requirements.[/color]
>
> Where do I find that?[/color]

That's section 17.4.3.1, paragraph 1, of the C++ standard. If you don't
have a copy, you can get one as a PDF file for $18US from
webstore.ansi.o rg (this requires a credit or debit card they'll accept).

The DIN only seems to sell the paper version, at a much less attractive
price (242,70 Euros). You can also get it direct from the ISO (in
either PDF or paper format) for 364 CHF, which is roughly typical for a
paper copy, but clearly pretty high for a PDF.

Various other national bodies sell it as well, but I don't know of any
that sell the PDF for less than ANSI does. If you decided to get the
paper version, shipping costs would probably favor buying locally.

--
Later,
Jerry.

The universe is a figment of its own imagination.

[Howard Hinnant]

Re: &lt;algorithm&g t; transform modification

In article <3f6de554@news. greennet.net>,
"Fraser Ross" <fraserATmember s.v21.co.united kingdom> wrote:
[color=blue][color=green]
> > You might try out std::bind2nd in <functional> and stick with the
> > std::transform:
> >
> > std::transform( v.begin(), v.end(), v.begin(),
> > std::bind2nd(st d::multiplies<i nt>(), 3));[/color]
>
> Is there a reason for not using for_each? I have been wondering if for_each
> is faster.[/color]

I think it would be a matter of convenience rather than performance.
for_each could certainly be used here. But the use of for_each looks
more complicated to me than does transform.

On the inside transform will do something like:

*target = op(*source);

whereas for_each will do something like:

op(*source);

In our example, we're wanting to do:

*target = *target * 3;

or maybe:

*target *= 3;

So to use for_each you have to cook up a functor that takes a single
argument and performs the desired operation (both times and assign, or
the combined *=). There is no std::multiply_a ssign, but you could roll
your own:

template <class T, class U = T>
struct multiply_assign
: public std::binary_fun ction<T, U, T>
{
T& operator()(T& t, const U& u) const
{return t *= u;}
};

Then you could use this with for_each and bind2nd like:

std::for_each(v .begin(), v.end(),
std::bind2nd(mu ltiply_assign<i nt>(), 3));

I would expect this example to have the same performance as the
transform version, but I have not tested that expectation.

-Howard

[Fraser Ross]

Re: &lt;algorithm&g t; transform modification

transform might be as fast if return value optimisation is used. I would
prefer another algorithm for when the output iterator is the same as an
input iterator and two ranges are used. That isn't the situation the OP
has. e.g.

template <class ForwardIterator , class InputIterator, class BinaryOperation >
BinaryOperation transform_each (ForwardIterato r first1, InputIterator last1,
InputIterator first2, BinaryOperation binary_op) {
while (first1 != last1) {
binary_op(*firs t1, *first2);
++first1, ++first2;
}
return binary_op;
}


Fraser.


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[Steffen Brinkmann]

Re: &lt;algorithm&g t; transform modification

Sorry, pressed the wrong key.[color=blue]
>
> Is there a reason for not using for_each?[/color]

As I said, I'd have to write sth like

void f(double val){ val+=2.;}

and then call

for_each(v.begi n(),v.end(),f() );

What, if it is noct 2. but a value that is calculated at runtime?

Steffen

[red floyd]

Re: &lt;algorithm&g t; transform modification

Steffen Brinkmann wrote:
[color=blue]
> Sorry, pressed the wrong key.
>[color=green]
>>Is there a reason for not using for_each?[/color]
>
>
> As I said, I'd have to write sth like
>
> void f(double val){ val+=2.;}
>
> and then call
>
> for_each(v.begi n(),v.end(),f() );
>
> What, if it is noct 2. but a value that is calculated at runtime?
>
> Steffen[/color]

use a functor... There's probably a standard one that I can't remember at this time, but here's one,
and I realize I may have the syntax/semantics of for_each() slightly messed up. Please have pity, gurus!

struct add_value {
double val;
add_value (val_) : val(val_) { }
double operator()(doub le& d) { return d += val; }
}

then you can call:

for_each(v.begi n(), v.end(), add_value(some_ calculated_valu e_here));

[Steffen Brinkmann]

Re: &lt;algorithm&g t; transform modification

Hi!

Thank you all for your postings and learned a lot.
My solution is to use bind2nd(..) and only the stl without specialisation.

Thanks again

I'm out of here, Steffen