doubt on lambda expression

[manustone]

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Hi All

I started playing with boost::lambda and I need some help or some clarification about it

or some hint to better understanding how does it work.

I have this piece of code

using namespace boost::lambda;

using boost::lambda::var;

int main(int argc, char** argv) {

   std::vector< int > myList;

   myList.push_back(100);

   myList.push_back(200);    

   int interval = 20;

   std::for_each( myList.begin(), myList.end(),

        (

        std::cout << "value: " << _1 << "\n",

        std::cout << "interval: " << var( interval ) << "\n",

        var( interval )++

        ));

}

I understood that writing a lambda expression would be a substitute

for writing a functor.

I expected the following result

value: 100

interval: 20

value: 200

interval: 21

Instead I had the following

intervalfirst value: 100

20

200

21

Why the strings "interval" and "first" are not written for each value and how can I achieve this?

Kind Regards

Mn

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[Roman Perepelitsa-3]

2009/11/11 Emanuele Rocci <rocciemanuele@...>

Hi All

I started playing with boost::lambda and I need some help or some clarification about it

or some hint to better understanding how does it work.

I have this piece of code

using namespace boost::lambda;

using boost::lambda::var;

int main(int argc, char** argv) {

   std::vector< int > myList;

   myList.push_back(100);

   myList.push_back(200);    

   int interval = 20;

   std::for_each( myList.begin(), myList.end(),

        (

        std::cout << "value: " << _1 << "\n",

        std::cout << "interval: " << var( interval ) << "\n",

        var( interval )++

        ));

}

Instead of

  std::cout << "value: " << _1 << "\n",

do this

  std::cout << constant("value: ") << _1 << "\n",

It's described here: http://www.boost.org/doc/libs/1_40_0/doc/html/lambda/using_library.html#lambda.introductory_examples.

By the way, if you are just starting with lambda, it's better to use Boost.Phoenix instead.

Roman Perepelitsa.

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[OvermindDL1]

On Wed, Nov 11, 2009 at 5:20 AM, Roman Perepelitsa
<roman.perepelitsa@...> wrote:

> 2009/11/11 Emanuele Rocci <rocciemanuele@...>
>>
>> Hi All
>> I started playing with boost::lambda and I need some help or some
>> clarification about it
>> or some hint to better understanding how does it work.
>> I have this piece of code
>> using namespace boost::lambda;
>> using boost::lambda::var;
>> int main(int argc, char** argv) {
>>    std::vector< int > myList;
>>    myList.push_back(100);
>>    myList.push_back(200);
>>    int interval = 20;
>>    std::for_each( myList.begin(), myList.end(),
>>         (
>>         std::cout << "value: " << _1 << "\n",
>>         std::cout << "interval: " << var( interval ) << "\n",
>>         var( interval )++
>>         ));
>> }
>
> Instead of
>   std::cout << "value: " << _1 << "\n",
> do this
>   std::cout << constant("value: ") << _1 << "\n",
> It's described here:
> http://www.boost.org/doc/libs/1_40_0/doc/html/lambda/using_library.html#lambda.introductory_examples.
> By the way, if you are just starting with lambda, it's better to use
> Boost.Phoenix instead.

This is another +1 for Boost.Phoenix2, it does everything lambda and
bind does, and a great deal more.  Boost.Phoenix3 is a proto'fied
Phoenix that is still being made and will be all the more powerful and
much more extendable and combinable with other things as well, so it
would be good to learn Boost.Phoenix2 now.
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[Ryan McConnehey]

OvermindDL1 wrote:
> Phoenix that is still being made
Since it's not in the standard documentation area it would mean it's in
the sandbox or vault?

Ryan
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[OvermindDL1]

On Wed, Nov 11, 2009 at 8:20 AM, Ryan McConnehey <mccorywork@...> wrote:
> OvermindDL1 wrote:
>>
>> Phoenix that is still being made
>
> Since it's not in the standard documentation area it would mean it's in the
> sandbox or vault?

Phoenix3 (note the *3*) is still being made.

Phoenix 1 is in boost now, but it is in the Spirit library, look in it.

Phoenix 2 is in Boost 1.39 (maybe earlier, unsure), also in the Spirit
library (phoenix2 includes).

Phoenix 3 will be completely stand-alone, but is on hold until someone
has time to donate to it.

So yes, it is in Boost right now and has been for a very long time,
but it is in the Spirit library, so look in its docs and search for
Phoenix until you find a link to the Phoenix sub-docs.
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[manustone]

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Hi Roman Thanks! 

It works and thanks for Phoenix advice!

Mn

---

Date: Wed, 11 Nov 2009 13:20:26 +0100
From: roman.perepelitsa@...
To: boost-users@...
Subject: Re: [Boost-users] doubt on lambda expression

2009/11/11 Emanuele Rocci <rocciemanuele@...>

Hi All

I started playing with boost::lambda and I need some help or some clarification about it

or some hint to better understanding how does it work.

I have this piece of code

using namespace boost::lambda;

using boost::lambda::var;

int main(int argc, char** argv) {

   std::vector< int > myList;

   myList.push_back(100);

   myList.push_back(200);    

   int interval = 20;

   std::for_each( myList.begin(), myList.end(),

        (

        std::cout << "value: " << _1 << "\n",

        std::cout << "interval: " << var( interval ) << "\n",

        var( interval )++

        ));

}

Instead of

  std::cout << "value: " << _1 << "\n",

do this

  std::cout << constant("value: ") << _1 << "\n",

It's described here: http://www.boost.org/doc/libs/1_40_0/doc/html/lambda/using_library.html#lambda.introductory_examples.

By the way, if you are just starting with lambda, it's better to use Boost.Phoenix instead.

Roman Perepelitsa.

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[Joel Falcou-3]

> Phoenix 3 will be completely stand-alone, but is on hold until someone
> has time to donate to it.
>  
I am actually doing this with the blessing of Joel & harmut ;)

--
___________________________________________
Joel Falcou - Assistant Professor
PARALL Team - LRI - Universite Paris Sud XI
Tel : [phone]


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[OvermindDL1]

On Wed, Nov 11, 2009 at 9:09 AM, joel <joel.falcou@...> wrote:
>
>> Phoenix 3 will be completely stand-alone, but is on hold until someone
>> has time to donate to it.
>>
>
> I am actually doing this with the blessing of Joel & harmut ;)

Ah, well I certainly eagerly await it!  :)
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