Re: [Boost-users] [lambda] Variable assignment from lambda expression

From: Alexander Borghgraef (alexander.borghgraef.rma_at_[hidden])
Date: 2005-09-27 08:12:00

On 9/26/05, jarvi <jarvi_at_[hidden]> 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