In this cell, I have a linear programming problem: http://sagecell.sagemath.org/?q=oatgsd
However, the answer is wrong. The optimal objective has to be 1 whereas it is 0. The optimal values for x and y lie on the line between (-2,1) and (1,-2). I have shown the values of the constraints for the point (-1,0)
Helloooooooooooooooooooooo !!
That's because all variables are assumed to be >= 0 by default unless specified otherwise. Sorry for that, it's something that all LP solvers that I know assume too (and that's why we do, too), and it's pretty hard to advertise in the doc and make sure that everybody sees it O_o
So if you want to make this work with negative values, you have to add lines like :
p.set_min(p['x'],None)
p.set_min(p['y'],None)in your code. And it will work, at least it does on my computer.
Perhaps I should add warnings everywhere in the doc O_o
Nathann
By the way, you can show it with your code by adding a p.show() at the end. It will tell you that your variables are nonnegative things.
I thought we handled this in ticket 12332
Well, the doc mentions it since 12332. But printing stuff in the doc is not always enough, especially when the LP above does not even call new_variable :-/ I created http://trac.sagemath.org/ticket/15482 to make it more visible.
That's ... quite a convention. See my comments on that ticket.
Thanks for the answer. Maybe it's better to leave the constraints to the user.
Asked: 11 years ago
Seen: 444 times
Last updated: Dec 04 '13
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I'm baffled here. The only suggestion I have is that somehow it doesn't like maximizing functions that have many negative values - I get similar problems with anything negative for coefficients.