On 9/26/05, jarvi <jarvi@cs.tamu.edu> wrote:

the var function makes a lambda functor out of a variable, so you can write the expression as:

var(n) = _1

Thanks, I should have seen that one in the manual. Posting at the end of a work day is obviously not a good idea :-) I've got another lambda related question though. My example was a useless simplification, what I'm really trying to do is to read data from an STL vector into a legacy matrix class which doesn't have an iterator. for_each( raster.begin(), raster.end(), ( var( out2( h - 1 - c / w, c % w ) ) = _1, ++var( c ) ) ); -- Alex Borghgraef

On 9/27/05, Alexander Borghgraef <alexander.borghgraef.rma@gmail.com> wrote:

On 9/26/05, jarvi <jarvi@cs.tamu.edu> wrote:

...

the var function makes a lambda functor out of a variable, so you can write the expression as:

var(n) = _1

Thanks, I should have seen that one in the manual. Posting at the end of a work day is obviously not a good idea :-) I've got another lambda related question though. My example was a useless simplification, what I'm really trying to do is to read data from a container v into a legacy matrix class which doesn't have an iterator. I can do this with a for loop:

std::vector<char> v; MyMatrix<char> m; // Has h and w members for height and width, and element access using operator()( int i, int j ) std::vector<char>::iterator v_it = v.begin(); for ( long c = 0; c < m.h * m.w; ++c ) { m( c / m.w , c % m.w ) = *v_it; ++v_it; }

This works perfectly. I figured it should be possible to do the same thing using for_each and boost::lambda, so I tried this approach:

long c = 0; for_each( v.begin(), v.end(), ( var( m( c / m.w, c % m.w ) ) = _1, ++var( c ) ) );

This also fills MyMatrix, but with different values than the for loop. Which seems surprising to me. As far as I can tell, these two approaches should be equivalent, but apparently they're not. Anyone care to explain to me why?

P.S. Sorry for the unfinished post which got through. Please ignore that one.

-- Alex Borghgraef

That's not going to work because the operator() call in the loop (i.e. m(...)) is evaluated when the lambda expression is created, not each time the lambda expression is evaluated. Try replacing var( m( c / m.w, c % m.w ) ) with bind(&MyMatrix::operator(), var(m), var(c) / m->*&MyMatrix::w, var(c) % m->*&MyMatrix::w) This makes all accesses to m & c lazy (I think), so should update m as you want. Stuart Dootson

"Alexander Borghgraef" <alexander.borghgraef.rma@gmail.com> wrote in message news:9e8c52a20509270717488cef04@mail.gmail.com... On 9/27/05, Stuart Dootson <stuart.dootson@gmail.com> wrote: That's not going to work because the operator() call in the loop (i.e. m(...)) is evaluated when the lambda expression is created, not each time the lambda expression is evaluated. Try replacing var( m( c / m.w, c % m.w ) ) with bind(&MyMatrix::operator(), var(m), var(c) / m->*&MyMatrix::w, var(c) % m->*&MyMatrix::w) This makes all accesses to m & c lazy (I think), so should update m as you want. Ok, that fixes it. Now, hardcore algorithmism aside, I don't see any advantage in this over the for loop: it's slower, and can hardly be called an improvement in readability. So I think I'll stick to the old-fashioned syntax in this case. Still, interesting to know that boost::lambda actually can do this. Using std::copy and a function_output_iterator from the iterator adaptor library would IMHO, be more readable and resuable. #include <boost/function_output_iterator.hpp> struct matrix_filler { matrix_filler( matrix& m ): m(m), counter( 0 ) {} void operator()( int val )const { m( counter / m.w , counter % m.w ) = val; ++counter } matrix& m; long counter; }; std::copy( v.begin(), v.end(), boost::make_function_output_iterator(matrix_filler(m)) ); Or provide a "linear" iterator to your matrix class. Jeff Flinn

On 9/29/05, Joel de Guzman <joel@boost-consulting.com> wrote:

YMMV. In this case, I'd use BOOST_FOREACH.

Where do I find this? I've grepped my /usr/include/boost dir for BOOST_FOREACH, without success, and documentation seems to be hard to find as well, there's a foreach lib on the boost-consulting site, but the html file is lost, and on the boost.org <http://boost.org> site, there's no reference to it anymore. -- Alex Borghgraef

Alexander Borghgraef wrote:

On 9/29/05, *Joel de Guzman* <joel@boost-consulting.com <mailto:joel@boost-consulting.com>> wrote:

YMMV. In this case, I'd use BOOST_FOREACH.

Where do I find this? I've grepped my /usr/include/boost dir for BOOST_FOREACH, without success, and documentation seems to be hard to find as well, there's a foreach lib on the boost-consulting site,

That's it.

but the html file is lost,

What do you mean?

and on the boost.org <http://boost.org> site, there's no reference to it anymore.

The foreach.zip file in the File Vault on boost-consulting.com is what you want: http://www.boost-consulting.com/vault/index.php?directory=Algorithms The documentation is online at: http://boost-sandbox.sf.net/libs/foreach -- Eric Niebler Boost Consulting www.boost-consulting.com