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Re: [Help-glpk] Understanding solution answer: infeas??
From: |
Andrew Makhorin |
Subject: |
Re: [Help-glpk] Understanding solution answer: infeas?? |
Date: |
Mon, 2 Nov 2009 21:16:59 +0300 |
> Hi guys, I am new in GLPK use and I would like to ask you about some
> solution that I get: Basically I define an LP problem where the
> variable x10, x11 and x12 take values bw. 0 and 1 and the sum of
> x10+x11+x12 = 1, according to that and for 18 structural variables I
> get the following answer:
> ./a.out
> 0: obj = 0.000000000e+00 infeas = 6.000e+00 (0)
> * 8: obj = 1.300000000e+03 infeas = 0.000e+00 (0)
> * 13: obj = 1.600000000e+03 infeas = 8.171e-16 (0)
> OPTIMAL SOLUTION FOUND
> z = 1600
> x10 = 0; x11 = 0; x12 = 1
> x20 = 0; x21 = 0; x22 = 1
> x30 = 0; x31 = 0; x32 = 1
> x40 = 0; x41 = 0; x42 = 1
> x50 = 1; x51 = 0; x52 = -8.17124e-16
> x60 = 1; x61 = 0; x62 = 0
> The value of z correspond to the right value, but how do I understand
> the value of x52??, is it a real "0" value and how is it related to
> the "infeas = 8.171e-16" value?, does it mean that my answer is wrong
> (since I should have only expect x50 = 1; x51 = 0; x52 = 0)?
The solution is correct. You should understand x52 = -8.17124e-16 as
zero within a working precision. To perform calculations the glpk
simplex solver uses floating-point arithmetic, so it reports a solution,
which is an approximation to the mathematically exact result.