Re: [boost] [BGL] problem w/ push_relabel_max_flow and floating point capacities

Subject: Re: [boost] [BGL] problem w/ push_relabel_max_flow and floating point capacities
From: Marc Schafer (Marc.Schafer_at_[hidden])
Date: 2011-01-07 09:48:17

Jeremiah,
For my test case, is_flow fails on a comparison x==y where x and y are both approximately 1. and x-y is 1.e-14.

However, the graphs I want to process could have capacities that are any valid floating point numbers.  This leads to several questions:

is_flow is a consistency check after the algorithm is complete, does the algorithm need modifications to work correctly with floating point numbers?

How do I determine the correct tolerances to use for comparisons?

How do I deal with the relational operators (<, >)?

-Marc

"Jeremiah Willcock" <jewillco_at_[hidden]> wrote in message news:<alpine.LRH.2.00.1101051355460.26649_at_[hidden]>...
> On Wed, 22 Dec 2010, Marc Schafer wrote:
> 
> > Using the push_relabel_max_flow feature on graphs that have edge 
> > capacities in double or single precision often results in a failed 
> > assertion (line 706 push_relabel_max_flow.hpp) on debug builds although 
> > the answer provided is correct for builds with assertions disabled.
> >
> > The is_flow() method (line 564, push_relabel_max_flow.hpp) performs some 
> > consistency checking after the algorithm completes and there is an 
> > assertion that it returns true.  The comparisons inside is_flow are done 
> > using ==, !=, >, and <.  Using a locally modified version, I was able to 
> > see that all the cases where is_flow returns false are caused by errors 
> > on the order of machine precision.
> >
> > I believe the error is a faulty comparison mechanism inside is_flow 
> > rather than a problem with my graphs or the push_relabel_max_flow 
> > algorithm.
> 
> I agree with what you are saying.  If you just replace the == and != 
> operations in is_flow (and not < and >), does the code work correctly? 
> Boost already has floating point equality comparisons with a tolerance, 
> but does not appear to have inequalities.
> 
> -- Jeremiah Willcock
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