RE: [Help-glpk] formulation of a problem

[Gottfried Lindner]

See below

Gottfried Lindner
 
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-----Original Message-----
From: address@hidden
[mailto:address@hidden On Behalf Of
Nico Moser
Sent: Donnerstag, 22. Juni 2006 13:33
To: address@hidden
Subject: [Help-glpk] formulation of a problem

Hi,

I'm in trouble with set in a problem in the glpk datastructure and I  
can't find answers to my questions (perhaps I looked at the wrong  
places).
The two main problems:
1. How can I set a variable to the right hand side?
If I have a constraint like this: x1+x2 <= y1 I can't set:

lpx_set_row_name(lp, 3, "constraint");
lpx_set_row_bnds(lp, 3, LPX_UP, 0, y1);

-> Yes, if y1 is a constant in your model and a variable in the program
containing this constant value.

2. How can I set 'holes' in the matrix (if a variable is not needed  
in one constraint)?

For instance:
x1+x2 = 3
x1 <= 1

then I would think the array assignments looks like that:

ia[1] = 1, ja[1] = 1, ar[1] = 1.0;
ia[2] = 1, ja[2] = 2, ar[2] = 1.0;
ia[3] = 2, ja[3] = 1, ar[3] = 1.0;
ia[4] = 2, ja[4] = 2, ar[4] = 0; // <- it doesn't work

(a similar problem: if I don't need a basic variable in the minimize  
or maximize funktion, can I set the the coefficient to zero? like:  
lpx_set_obj_coef(lp,1,0.0);  )

-> Only coefficients with value <> 0 may be explicitly assigned. The
others are assigned to 0 implicitly (sparse matrix storage).


Thanks a lot,
Nico


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