lambda recursion

[Chris Forone]

hello group,

can a lambda function call itself for recursion purposes? thanks for
answering!

cheers, chris

 

[Chris Forone]

Am 28.04.2013 18:00, schrieb Chris Forone:

hello group,

can a lambda function call itself for recursion purposes? thanks for
answering!

cheers, chris

ok, it can because one can name it...

cheers, chris

 

[Stefan Ram]

Am 28.04.2013 18:00, schrieb Chris Forone:
ok, it can because one can name it...

When the Y combinator is used, it does not have to be named IIRC.

 

[Chris Forone]

Am 28.04.2013 18:02, schrieb Chris Forone:

Am 28.04.2013 18:00, schrieb Chris Forone:

ok, it can because one can name it...

cheers, chris

or it cant

"error: 'split' is not captured"

cheers, chris

 

[Chris Forone]

Am 28.04.2013 23:50, schrieb Andy Champ:

Are you using MS Visual Studio? Some versions have problems in capturing
variables from outer scopes.

Andy

i use mingw gcc 4.8.0 from page http://sourceforge.net/projects/mingwbuilds.

chris

 

[Chris Forone]

Am 29.04.2013 12:29, schrieb Andy Champ:

In that case we'll need a code sample.

Hack away at your code until you have the smallest _compilable_ program
that shows the problem. You never know on the way you may spot the
problem; if you don't, post the code up here and you'll get an answer.

Andy

#include <array>
#include <functional>
#include <iostream>
#include <numeric>

int main()
{
// five 3d vectors (xyz)
std::array<float, 5 * 3> store =
{
1.0f, 0.0f, 0.0f,
0.0f, 0.0f, 0.0f,
0.0f, 0.0f, 0.0f,
0.0f, 0.0f, 0.0f,
0.0f, 1.0f, 0.0f
};

std::function<void (std::size_t, std::size_t)> split =
[&](std::size_t first, std::size_t second)
{
if (first < second)
{
std::size_t middle((second - first) / 2);

// vector addidion
std::transform(&store[first], &store[first + 3], &store[second],
&store[middle], std:lus<float>());

// normalize new vector
std::transform(&store[middle], &store[middle + 3], &store[middle],
std::bind(std::divides<float>(), std:laceholders::_1,
std::sqrt(std::inner_product(&store[middle], &store[middle + 3],
&store[middle], 0.0f))));

// if you uncomment, gcc gives error, vc 2012 express compiles
// and crashes after start
//split(first, middle); split(middle, second);
}
};

split(0, 12);

std::copy(store.begin(), store.end(),
std:stream_iterator<float>(std::cout, " "));
}

 

[Chris Forone]

Am 29.04.2013 12:40, schrieb Chris Forone:

Am 29.04.2013 12:29, schrieb Andy Champ:

In that case we'll need a code sample.

Hack away at your code until you have the smallest _compilable_ program
that shows the problem. You never know on the way you may spot the
problem; if you don't, post the code up here and you'll get an answer.

Andy

#include <array>
#include <functional>
#include <iostream>
#include <numeric>

int main()
{
// five 3d vectors (xyz)
std::array<float, 5 * 3> store =
{
1.0f, 0.0f, 0.0f,
0.0f, 0.0f, 0.0f,
0.0f, 0.0f, 0.0f,
0.0f, 0.0f, 0.0f,
0.0f, 1.0f, 0.0f
};

std::function<void (std::size_t, std::size_t)> split =
[&](std::size_t first, std::size_t second)
{
if (first < second)
{
std::size_t middle((second - first) / 2);

// vector addidion
std::transform(&store[first], &store[first + 3], &store[second],
&store[middle], std:lus<float>());

// normalize new vector
std::transform(&store[middle], &store[middle + 3], &store[middle],
std::bind(std::divides<float>(), std:laceholders::_1,
std::sqrt(std::inner_product(&store[middle], &store[middle + 3],
&store[middle], 0.0f))));

// if you uncomment, gcc gives error, vc 2012 express compiles
// and crashes after start
//split(first, middle); split(middle, second);
}
};

split(0, 12);

std::copy(store.begin(), store.end(),
std:stream_iterator<float>(std::cout, " "));
}

pleas change line

std::size_t middle((second - first) / 2);

to

std::size_t middle((second + first) / 2);

 

[Chris Forone]

Am 29.04.2013 12:40, schrieb Chris Forone:

Am 29.04.2013 12:29, schrieb Andy Champ:

In that case we'll need a code sample.

Hack away at your code until you have the smallest _compilable_ program
that shows the problem. You never know on the way you may spot the
problem; if you don't, post the code up here and you'll get an answer.

Andy

#include <array>
#include <functional>
#include <iostream>
#include <numeric>

int main()
{
// five 3d vectors (xyz)
std::array<float, 5 * 3> store =
{
1.0f, 0.0f, 0.0f,
0.0f, 0.0f, 0.0f,
0.0f, 0.0f, 0.0f,
0.0f, 0.0f, 0.0f,
0.0f, 1.0f, 0.0f
};

std::function<void (std::size_t, std::size_t)> split =
[&](std::size_t first, std::size_t second)
{
if (first < second)
{
std::size_t middle((second - first) / 2);

// vector addidion
std::transform(&store[first], &store[first + 3], &store[second],
&store[middle], std:lus<float>());

// normalize new vector
std::transform(&store[middle], &store[middle + 3], &store[middle],
std::bind(std::divides<float>(), std:laceholders::_1,
std::sqrt(std::inner_product(&store[middle], &store[middle + 3],
&store[middle], 0.0f))));

// if you uncomment, gcc gives error, vc 2012 express compiles
// and crashes after start
//split(first, middle); split(middle, second);
}
};

split(0, 12);

std::copy(store.begin(), store.end(),
std:stream_iterator<float>(std::cout, " "));
}

sorry group for the chaos...

now i see the error in the line if (first < second)

i will fix it and then try again

cheers, chris

 

[Chris Forone]

Am 29.04.2013 12:40, schrieb Chris Forone:

Am 29.04.2013 12:29, schrieb Andy Champ:

In that case we'll need a code sample.

Hack away at your code until you have the smallest _compilable_ program
that shows the problem. You never know on the way you may spot the
problem; if you don't, post the code up here and you'll get an answer.

Andy

#include <array>
#include <functional>
#include <iostream>
#include <numeric>

int main()
{
// five 3d vectors (xyz)
std::array<float, 5 * 3> store =
{
1.0f, 0.0f, 0.0f,
0.0f, 0.0f, 0.0f,
0.0f, 0.0f, 0.0f,
0.0f, 0.0f, 0.0f,
0.0f, 1.0f, 0.0f
};

std::function<void (std::size_t, std::size_t)> split =
[&](std::size_t first, std::size_t second)
{
if (first < second)
{
std::size_t middle((second - first) / 2);

// vector addidion
std::transform(&store[first], &store[first + 3], &store[second],
&store[middle], std:lus<float>());

// normalize new vector
std::transform(&store[middle], &store[middle + 3], &store[middle],
std::bind(std::divides<float>(), std:laceholders::_1,
std::sqrt(std::inner_product(&store[middle], &store[middle + 3],
&store[middle], 0.0f))));

// if you uncomment, gcc gives error, vc 2012 express compiles
// and crashes after start
//split(first, middle); split(middle, second);
}
};

split(0, 12);

std::copy(store.begin(), store.end(),
std:stream_iterator<float>(std::cout, " "));
}

if (first + 3 < second) does it... sorry for that

cheers, chris

 

[Ike Naar]

Am 29.04.2013 12:40, schrieb Chris Forone:

[snip]
// five 3d vectors (xyz)
std::array<float, 5 * 3> store =
{
1.0f, 0.0f, 0.0f,
0.0f, 0.0f, 0.0f,
0.0f, 0.0f, 0.0f,
0.0f, 0.0f, 0.0f,
0.0f, 1.0f, 0.0f
};
[snip]
split(0, 12);
[snip]

if (first + 3 < second) does it... sorry for that

Are you sure?
Consider, for instance, what happens when the array has 4*3 elements
and split(0,9) is called (hint: at the next level of recursion,
split(0,4) and split(4,9) will be called).
It looks like you need to calculate middle in such a way that it
is a multiple of 3.

 

[Chris Forone]

Am 29.04.2013 13:41, schrieb Ike Naar:

Am 29.04.2013 12:40, schrieb Chris Forone:

[snip]
// five 3d vectors (xyz)
std::array<float, 5 * 3> store =
{
1.0f, 0.0f, 0.0f,
0.0f, 0.0f, 0.0f,
0.0f, 0.0f, 0.0f,
0.0f, 0.0f, 0.0f,
0.0f, 1.0f, 0.0f
};
[snip]
split(0, 12);
[snip]

if (first + 3 < second) does it... sorry for that

Are you sure?
Consider, for instance, what happens when the array has 4*3 elements
and split(0,9) is called (hint: at the next level of recursion,
split(0,4) and split(4,9) will be called).
It looks like you need to calculate middle in such a way that it
is a multiple of 3.

yes, i know this. i am only experimenting...

cheers, chris

 

[Chris Forone]

Am 29.04.2013 16:49, schrieb Andy Champ:

Not for me. You get a stack overflow from infinite recursion.

When called with (4,12) it passes the test. Middle is set to 4, so the
second recursive split call is with parameters (4, 12). After a while,
bang.

I can't say I know what your code is _supposed_ to do, but I'm sure it
isn't doing it!

Andy

yes, my post was to hasty. the call with (4, 12) will end with the calls
(4, 8) and (8, 12) because of post on 13:07. i will not post some kind
of experimental code in the future anymore.

cheers, chris