[gecode-users] int vs. unsigned int for Matrix dimensions

Tue Jan 25 11:01:23 CET 2011

Hi,

While I buy into the iterator stuff I have to admit that I am not (yet)
excited over the wrap stuff: that looks very very specific. There are only
few cases where you really can rely on the fact that variables are assigned.

For example, a common misconception is that the print() function of a script
can only be called on a solution and hence all variables are assigned. This
is wrong, printing must also be able to deal with not yet assigned variables
as the print function can be called on any script and not only solutions.
This for example happens when you click a node in Gist.

So I will include the iterators but not the wrap stuff. Is that okay?

Thanks a lot for your effort!

Christian

--

Christian Schulte, www.ict.kth.se/~cschulte/

From: users-bounces at gecode.org [mailto:users-bounces at gecode.org] On Behalf
Of Gregory Crosswhite
Sent: Monday, January 24, 2011 10:57 PM
To: users at gecode.org
Subject: Re: [gecode-users] int vs. unsigned int for Matrix dimensions

Oops, I just realized that I forgot to include in the patch an extra line in
Makefile.in to install the new header.  The amended patch is attached to
this e-mail.

Also, I forgot in my previous to mention that also included in the patch is
a test for the WrapVal iterator in test/iter.hpp;  that test program
includes checks on some of the examples that I described in the e-mail to
make sure that they have the expected result.

Cheers,
Greg

On 01/24/2011 12:12 PM, Gregory Crosswhite wrote: 

Well then, I am glad that I got the patch ready over the weekend.  :-) 

The patch has been attached to this e-mail as the file "patch";  it is the
result of running "svn diff" against revision 11556. 

I have modified "gecode/kernel/array.hpp" along the lines that we discussed,
and have added a new test to your suite under "test/array.cpp" to verify
that it works as expected. 

While I was add it, I added another feature.  The patch includes a file
"gecode/iter/wrap-val.hpp" (and a patch to "gecode/iter.hh" to include this
file) which contains an iterator class "WrapVal" that wraps another iterator
and upon dereferencing returns the the result of calling ".val()" on the
dereferenced wrapped iterator, as well as a function "wrap_val" that is a
convenience function for constructing a "WrapVal" (since this causes the
template parameters to automatically be inferred). 

That might sound complicated, but the point of it is to allow someone to do
the following: 

    #include <algorithm> 
    #include <gecode/iter.hh> 
    #include <iterator> 
    #include <vector> 
    ... 
    using namespace Gecode; 
    using namespace Gecode::Iter; 
    using namespace std; 
    ... 
    IntVarArray a; 
    ... 
    vector<int> v; 
    copy(wrap_val(a.begin()),wrap_val(a.end()),back_inserter(x)); 

which has the effect of copying all of the values from the array a into the
vector v, since a.begin() returns an iterator that dereferences to an IntVar
and wrap_val(a.begin()) returns a wrapped iterator that returns the result
of calling .val() on the IntVar. 

The basic use case of the iterators is as follows: 

    IntVarArgs a; 
    ... 
    for(IntVarArgs::iterator i = a.begin(); i != a.end(); ++i) {
rel(space,*i,IRT_EQ,0); } 

Of course, you could more easily do this with rel(space,a,IRT_EQ,0).  A more
interesting example is: 

    IntVarArgs a; 
    ... 
    for(IntVarArgs::iterator i = a.begin(); i != a.end()-1; ++i) {
rel(space,*i > *(i+1)+2); } 

More powerfully, iterators give you the ability to work with STL algorithms:

    #include <algorithm> 
    ... 
    void f(IntVar& v) { ... } 
    ... 
    IntVarArgs a; 
    ... 
    std::for_each(a.begin(),a.end(),f); 

The iterators returned by a.begin() and a.end() dereference to a reference
to an IntVar/BoolVar (or a view in the case of ViewArray).  If you are
interested in iterating over the *values* of the array --- that is, the
value currently assigned to to each variable then you can use the function
wrap_val to wrap these iterators so that dereferencing them returns the
value assigned to the variable.  For example, the last two lines in the
following have exactly the same result: 

    #include <algorithm> 
    #include <iostream> 
    ... 
    void f(IntVar& v) { std::cout << v.val() << " "; } 
    void g(int i) { std::cout << i << " "; } 
    ... 
    IntVarArgs a; 
    ... 
    std::for_each(a.begin(),a.end(),f); std::cout << std::endl; 
    std::for_each(wrap_val(a.begin()),wrap_val(a.end()),g); std::cout <<
std::endl; 

Or if you want to copy the values assigned to the variables into a
std::vector: 

    #include <algorithm> 
    #include <iterator> 
    #include <vector> 
    ... 
    IntVarArray a; 
    ... 
    std::vector<int> v; 
    std::copy(wrap_val(a.begin()),wrap_val(a.end()),std::back_inserter(x)); 

Or if you are interested in computing the sum of the values in the
variables: 

    #include <numeric> 
    ... 
    IntVarArray a; 
    ... 
    int sum = std::accumulate(wrap_val(a.begin()),wrap_val(a.end()),0); 

Or alternatively if you are interested in counting the number of zeros in
the variables: 

    #include <algorithm> 
    ... 
    IntVarArray a; 
    ... 
    unsigned int number_of_zeros =
std::count(wrap_val(a.begin()),wrap_val(a.end()),0); 

=== 

Anyway, I hope that this patch can make it into Gecode this week; you have
my official permission to release it under whatever license Gecode uses. 

Let me know if you have any questions!  :-) 

Cheers, 
Gregory Crosswhite 

On 01/24/2011 10:54 AM, Christian Schulte wrote: 

Christian Schulte <mailto:cschulte at ...> <cschulte at ...>  writes: 

Hi again, 

There is one thing I forgot to say: we intend to release end of this week 
(hopefully). So if your patch arrives until then, it'll be included in the 
next version. 

Cheers 
Christian 

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