|  | Initialize one vector using elements in second vector... | | |
| | | Peter Seidler |  |
|  Posted: Fri Aug 22, 2008 1:01 pm Post subject: Initialize one vector using elements in second vector... |  |
Hi,
I have the following problem:
Given a vector: vec[10, 20, 30, 40, 50, 60]
and another one: idx[ 2, 3, 5]
I wish to generate a new vector based on the
elements in vec given by the indices in idx,
i.e. the result should be res[30, 40, 60].
Is there any way to accomplish this using the
STL vectors / algorithms.
I would like to do something like:
transform(idx.begin(), idx.end(),
back_inserter(res),
vec.at());
However this doesn't work.
Any suggestions?
--
Peter
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| |
| | | Guest | |
| | Posted: Sat Aug 23, 2008 11:58 pm Post subject: Re: Initialize one vector using elements in second vector... | |
| |  | |
On Aug 22, 3:01 pm, Peter Seidler
<seidler@chem._INGEN_au_SPAM_.dk.sunsite.dk> wrote:
| Quote: | Hi,
I have the following problem:
Given a vector: vec[10, 20, 30, 40, 50, 60]
and another one: idx[ 2, 3, 5]
I wish to generate a new vector based on the
elements in vec given by the indices in idx,
i.e. the result should be res[30, 40, 60].
Is there any way to accomplish this using the
STL vectors / algorithms.
I would like to do something like:
transform(idx.begin(), idx.end(),
back_inserter(res),
vec.at());
However this doesn't work.
Any suggestions?
|
Your idea is basically sound, but you can't pass member functions this
way. You need a binder, e.g. Boost.Bind or Std/TR1 bind.
#include <tr1/functional>
std::transform(idx.begin(), idx.end(), std::back_inserter(res),
std::tr1::bind(&std::vector<int>::at, vec, _1));
This might not work with this exact syntax, because at() is overloaded
on constness. It may have to look something like this:
static_cast<const int& (std::vector<int>::*)(typename
std::vector<int>::size_type) const>(&std::vector<int>::at)
I don't know the syntax for "pointer to const member function",
though.
Sebastian
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| |
| | | Guest | |
| | Posted: Sat Aug 23, 2008 11:58 pm Post subject: Re: Initialize one vector using elements in second vector... | |
| | | |
| Quote: | Hi,
I have the following problem:
Given a vector: vec[10, 20, 30, 40, 50, 60]
and another one: idx[ 2, 3, 5]
I wish to generate a new vector based on the
elements in vec given by the indices in idx,
i.e. the result should be res[30, 40, 60].
Is there any way to accomplish this using the
STL vectors / algorithms.
I would like to do something like:
transform(idx.begin(), idx.end(),
back_inserter(res),
vec.at());
However this doesn't work.
Any suggestions?
|
std::transform(idx.begin(), idx.end(), std::back_inserter(res),
elem_by_index(vec));
Where Im using a unaryfunction called elem_by_index that saves a
reference to the backend vector, and simply returns the element
specified by the index
struct elem_by_index {
elem_by_index(std::vector<int> &v) : backend(v) {}
int operator()(int index) const{
return backend[index];
}
private:
std::vector<int> &backend;
};
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| |
| | | Christian Hackl | |
| | Posted: Sat Aug 23, 2008 11:58 pm Post subject: Re: Initialize one vector using elements in second vector... | |
Peter Seidler wrote:
| Quote: | Given a vector: vec[10, 20, 30, 40, 50, 60]
and another one: idx[ 2, 3, 5]
I wish to generate a new vector based on the
elements in vec given by the indices in idx,
i.e. the result should be res[30, 40, 60].
Is there any way to accomplish this using the
STL vectors / algorithms.
I would like to do something like:
transform(idx.begin(), idx.end(),
back_inserter(res),
vec.at());
t |
You could accomplish it with the Boost Lambda Library. Here's an example:
typedef std::vector<int> Vector;
Vector::value_type const &(Vector::*at)(Vector::size_type) cons =
&Vector::at;
std::transform(idx.begin(), idx.end(), std::back_inserter(res),
bind(at, vec, _1));
The temporary variable "at" is used because the at() member function is
overloaded for const and non-const. See
LINK
--
Christian Hackl
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| |
| | | JSoft | |
| | Posted: Sat Aug 23, 2008 11:58 pm Post subject: Re: Initialize one vector using elements in second vector... | |
On Fri, 22 Aug 2008 07:01:51 -0600, Peter Seidler wrote:
| Quote: | Hi,
I have the following problem:
Given a vector: vec[10, 20, 30, 40, 50, 60] and another one: idx[ 2,
3, 5]
I wish to generate a new vector based on the elements in vec given by
the indices in idx, i.e. the result should be res[30, 40, 60].
Is there any way to accomplish this using the STL vectors / algorithms.
I would like to do something like:
transform(idx.begin(), idx.end(),
back_inserter(res),
vec.at());
However this doesn't work.
Any suggestions?
|
What's wrong with this?
typeof idx::iterator it;
typeof vec new_vec;
for (it = vec.begin(); it < vec.end(); ++it)
new_vec.push_back(vec[*it]);
Jake
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| |
| | | red floyd | |
| | Posted: Sat Aug 23, 2008 11:58 pm Post subject: Re: Initialize one vector using elements in second vector... | |
| | | |
On Aug 22, 6:01 am, Peter Seidler
<seidler@chem._INGEN_au_SPAM_.dk.sunsite.dk> wrote:
| Quote: | Hi,
I have the following problem:
Given a vector: vec[10, 20, 30, 40, 50, 60]
and another one: idx[ 2, 3, 5]
I wish to generate a new vector based on the
elements in vec given by the indices in idx,
i.e. the result should be res[30, 40, 60].
Is there any way to accomplish this using the
STL vectors / algorithms.
I would like to do something like:
transform(idx.begin(), idx.end(),
back_inserter(res),
vec.at());
However this doesn't work.
Any suggestions?
|
How about something like:
struct converter {
const std::vector<int>& v;
converter(const std::vector<int>& v_) : v(v_) { }
converter(const converter& c) : v(c.v) { }
int operator()(int idx) const { return v.at(idx); }
};
std::transform(idx.begin(), idx.end(),
std::back_inserter(res),
converter(vec));
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| |
| | | uuilly | |
| | Posted: Sat Aug 23, 2008 11:58 pm Post subject: Re: Initialize one vector using elements in second vector... | |
| | | |
On Aug 22, 6:01 am, Peter Seidler
<seidler@chem._INGEN_au_SPAM_.dk.sunsite.dk> wrote:
| Quote: | Hi,
I have the following problem:
Given a vector: vec[10, 20, 30, 40, 50, 60]
and another one: idx[ 2, 3, 5]
I wish to generate a new vector based on the
elements in vec given by the indices in idx,
i.e. the result should be res[30, 40, 60].
Is there any way to accomplish this using the
STL vectors / algorithms.
I would like to do something like:
transform(idx.begin(), idx.end(),
back_inserter(res),
vec.at());
However this doesn't work.
Any suggestions?
--
Peter
|
I always find that when you have to store indices, it's easiest to
just use a traditional for loop. Esp w/ vector.
Otherwise, the only way I can think to do it would be to use
boost::bind and the distance function acting on a reference from
idx.at. While it could probably be done I think it will look pretty
messy in the end. Though I would be keen on knowing an "STL way" of
doing this.
--
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| |
| | | Guest | |
| | Posted: Sat Aug 23, 2008 11:58 pm Post subject: Re: Initialize one vector using elements in second vector... | |
| | | |
On Aug 22, 9:01 am, Peter Seidler
<seidler@chem._INGEN_au_SPAM_.dk.sunsite.dk> wrote:
| Quote: | Hi,
I have the following problem:
Given a vector: vec[10, 20, 30, 40, 50, 60]
and another one: idx[ 2, 3, 5]
I wish to generate a new vector based on the
elements in vec given by the indices in idx,
i.e. the result should be res[30, 40, 60].
Is there any way to accomplish this using the
STL vectors / algorithms.
I would like to do something like:
transform(idx.begin(), idx.end(),
back_inserter(res),
vec.at());
However this doesn't work.
Any suggestions?
--
Peter
|
#include "boost/functional.hpp" // boost::mem_fun_ref
// tr1::...
transform(
idx.begin( ),
idx.end( ),
res.begin( ),
boost::bind1st(
boost::mem_fun_ref<
int&,
vector< int, allocator<int> >,
size_t
| Quote: | ( &vector< int, allocator<int> >::at ), vec )
); |
//...
no boost:
//...
using namespace std;
template<
typename From
| Quote: |
struct select_element_of |
: public unary_function<
const typename From::size_type&,
void> {
explicit select_element_of( const From & from )
: from_( from ) {
}
operator const From ( ) {
return result_;
}
void operator ()( const typename From::size_type & val ) {
result_.push_back( from_[ val ] );
}
private:
const From & from_;
From result_;
};
res = for_each( idx.begin( ), idx.end( ), select_element_of< vector<
int > >( vec ) );
//...
cheers,
gil
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| |
| | | Greg Herlihy | |
| | Posted: Sat Aug 23, 2008 11:58 pm Post subject: Re: Initialize one vector using elements in second vector... | |
| | | |
On Aug 22, 6:01 am, Peter Seidler
<seidler@chem._INGEN_au_SPAM_.dk.sunsite.dk> wrote:
| Quote: | I have the following problem:
Given a vector: vec[10, 20, 30, 40, 50, 60]
and another one: idx[ 2, 3, 5]
I wish to generate a new vector based on the
elements in vec given by the indices in idx,
i.e. the result should be res[30, 40, 60].
I would like to do something like:
transform(idx.begin(), idx.end(),
back_inserter(res),
vec.at());
However this doesn't work.
|
I think that you are on the right track. To complete the solution, I
would suggest something along these lines:
#include <iostream>
#include <iterator>
#include <algorithm>
#include <tr1/functional>
#include <vector>
#include <functional>
using std::tr1::placeholders::_1;
using std::tr1::bind;
using std::back_inserter;
using std::tr1::mem_fn;
using std::vector;
typedef vector<int> IntVector;
// declare an AtMemFunPtr member function pointer typedef
// for std::vector<int>::at() const
typedef IntVector::const_reference
(IntVector::*AtMemFunPtr)(IntVector::size_type) const;
const int va[] = { 10, 20, 30, 40, 50, 60 };
const int ia[] = { 2, 3, 5};
const int vaEnd = sizeof(va)/sizeof(va[0]);
const int iaEnd = sizeof(ia)/sizeof(ia[0]);
int main()
{
IntVector values( &va[0], &va[vaEnd] );
IntVector indexes( &ia[0], &ia[iaEnd] );
IntVector v; // holds output
std::transform( indexes.begin(), indexes.end(),
back_inserter(v),
bind( AtMemFunPtr(&IntVector::at), values,
_1));
std::cout << v[0] << "\n" << v[1] << "\n" << v[2] << "\n";
}
Program output:
30
40
60
Greg
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| |
| | | gast128@hotmail.com | |
| | Posted: Sat Aug 23, 2008 11:58 pm Post subject: Re: Initialize one vector using elements in second vector... | |
| | | |
| Quote: | Hi,
I have the following problem:
Given a vector: vec[10, 20, 30, 40, 50, 60]
and another one: idx[ 2, 3, 5]
I wish to generate a new vector based on the
elements in vec given by the indices in idx,
i.e. the result should be res[30, 40, 60].
Is there any way to accomplish this using the
STL vectors / algorithms.
I would like to do something like:
transform(idx.begin(), idx.end(),
back_inserter(res),
vec.at());
However this doesn't work.
Any suggestions?
|
It is quite simple if you are not afraid of boost (or tr1):
#include <algorithm>
#include <iterator>
#include <vector>
#include <boost/bind.hpp>
#include <boost/assign/list_of.hpp>
void f()
{
typedef std::vector<int> int_vector;
int_vector vec = boost::assign::list_of(10)(20)(30)(40)(50)(60);
int_vector idx = boost::assign::list_of(2)(3)(5);
int_vector res;
std::transform(idx.begin(), idx.end(),
std::back_inserter(res),
boost::bind(&int_vector::at, &vec, _1));
}
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