Interesting – running on my computer, the results file doesn't mention any errors or infeasibility (but it’s also returning a different result for the objective).
Here’s what I see:
Rows: 11 Columns: 11 (11 integer, 0 binary) Nonzeros: 32 Status: INTEGER OPTIMAL Objective: out = 82892752 (MAXimum)
[...]
Integer feasibility conditions:
KKT.PE: max.abs.err = 0.00e+00 on row 0 max.rel.err = 0.00e+00 on row 0 High quality
KKT.PB: max.abs.err = 0.00e+00 on row 0 max.rel.err = 0.00e+00 on row 0 High quality On Dec 17, 2019, at 7:33 PM, Heinrich Schuchardt < address@hidden> wrote:
On 12/18/19 12:12 AM, Matthew Keeter wrote:Hi folks,
I used GLPK to solve a problem in this year’s Advent of Code [1], and noticed that it produces a very slightly wrong answer for one of the examples.
It’s an integer linear programming problem, pasted below the fold in CPLEX LP format.
The correct answer is 82892753, and I’m getting 82892752.
I’ve pasted my glpsol output at [2]
Strangely, other folks have gotten the correct answer with the same input; there’s a reddit thread discussing it at [1]. I'm on a Mac OS 10.13.6, GLPK 4.65, GMP 6.1.2, compiled with Apple LLVM version 10.0.0 (clang1000.11.45.5)
Any ideas?
There are several tolerances taken into account by GLPK includingtol_int defaulting to 1e5. If a problem is illconditioned, you may geterrors due to these tolerances.Looking at your ore_consumption constraint you are looking for asolution that is exact to a factor of 1 in 177 trillions. This isnothing GLPK can deliver.When running your problem with./glpsol lp ore.lp o resultI get in file result:Status: INTEGER OPTIMALObjective: out = 82892753 (MAXimum)KKT.PB: max.abs.err = 1.00e+00 on row 10 max.rel.err = 1.00e+00 on row 10 SOLUTION IS INFEASIBLESo equation PSHF is not fulfilled by the solution GLPK provides.Best regardsHeinrich
