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Re: [Help-glpk] Problems with TSPSOL

From: Andrew Makhorin
Subject: Re: [Help-glpk] Problems with TSPSOL
Date: Fri, 9 Mar 2007 15:28:05 +0300

> I'm having some issues with tspsol. When I regenerate the cost function
> using the optimal tour I do not get the same result. Hence, I'm wondering 
> if I'm possibly doing something stupid extracting the solution.
> (I'm using a symmetric TSP defined with a lower diagonal matrix)
> grep '*' example.out | cut -b 8-18 | cut -f 1 -d '*' | awk -F-
> '{printf("edge(%d) = %d <-> %d\n",NR,$1,$2);}'

See the routine tsp_distance (file src/glptsp.c) which computes the
distance between two nodes, or the report by G.Reinelt:
Probably you are using wrong formulae.

To check the tspsol's result you can solve your tsp with Concorde by
submitting it to the NEOS server:

> Another aspect: Can it possibly be overflow issues? I have fractional 
> values < 50 that I multiply with 1e6 and round to get integers. Is there 
> a limit on the number of bits for the edge weights?

Edge lengths are limited to 32-bit int's; however, big values (> 1e6)
can cause overflow or inexactness on solving the netflow problem to
generate subtour elimination constraints.

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