Warning for adding constraints: zero or small (< 1e-13) coefficients, ignored

[Jennifer Gossels]

Hello,

I'm trying to solve an LP, and with certain inputs I get the message, "Warning for adding constraints: zero or small (< 1e-13) coefficients, ignored."  Here's the code where I'm adding the problematic constraints.  This is a snippet from inside a larger function.  self.m is the model, var is a Gurobi variable, and then I'm printing weight and on_virt_link.

logger.info(DASHES*3)
self.m.addConstr(var == weight * on_virt_link, name=cname)
self.m.update()
logger.info("just added constraint %s with weight %s and on_virt_link = %s"
                                    % (cname, weight, on_virt_link))
logger.info("max coefficient:  %s" % self.m.maxcoeff)
logger.info("min coefficient:  %s" % self.m.mincoeff)
logger.info("max bound:  %s" % self.m.maxbound)
logger.info("min bound:  %s" % self.m.minbound)
logger.info(DASHES*3)

Here's the log file for the first constraint that produces a warning:

---------------------------------------------------------------------
Warning for adding constraints: zero or small (< 1e-13) coefficients, ignored
just added constraint 818_path_weight_521(5,16,1,3,21) with weight 1.0 and

    on_virt_link = gurobi.LinExpr: Z_818521(5,16,1,3,21) + Z_818521(5,16,10,3,21) + Z_818521(5,16,1,7,21) + Z_818521(5,16,10,17,7,21)
max coefficient: 5000.0

min coefficient: 0.0262601362977
max bound: 0.0
min bound: 0.0
---------------------------------------------------------------------

I don't see how this constraint has zero or small coefficients?  The only coefficient is weight, and its value is 1.0.  I'd greatly appreciate any help.

Jennifer Gossels

[Kostja Siefen]

Hi Jennifer,

You could try to debug this by printing the constraint itself, e.g.

logger.info("just added constraint %s" % (var == weight * on_virt_link))

Kostja

[Jennifer Gossels]

Hi Kostja,

Thanks for the suggestion.  But, here's what I get:

-------------------------------------------------------------------------------------------

Warning for adding constraints: zero or small (< 1e-13) coefficients, ignored 

just added constraint gurobi.TempConstr: gurobi.LinExpr: Z_818521(5,16,1,3,21) == gurobi.LinExpr: 1.0 Z_818521(5,16,1,3,21) + 1.0 Z_818521(5,16,10,3,21) + 1.0 Z_818521(5,16,1,7,21) + 1.0 Z_818521(5,16,10,17,7,21)

-------------------------------------------------------------------------------------------

This is consistent with what I was seeing before -- the only coefficients I see are 1.0.  Do you have any idea what's happening?

Thanks,

Jennifer

[Michael Winkler]

Hi Jennifer,

internally constraints are changed such that sum ai*xi <=/= constant. Also, if a variable appears multiple times, all coefficients are summed up. In your example you have Z_818521(5,16,1,3,21) twice in your constraint, and if you transform it into the standard form both coefficients cancel each other resulting in a zero coefficient. This leads to the warning you are seeing.

Cheers,
Michael

[Jennifer Gossels]

Ah, thank you!