Description

This illustrates the MIP formulations of some difficult functions.

Small Model of Type : MIP

Category : GAMS Model library

Main file : absmip.gms

$title Discontinous Functions abs, min, max, and sign as MIPs (ABSMIP,SEQ=208)

$onText
This illustrates the MIP formulations of some difficult functions.


GAMS Development Corporation, Formulation and Language Example.

Keywords: mixed integer linear programming, discontinous functions,
          GAMS language example, mathematics, transformation techniques
$offText

Variable x 'argument to the various functions';

Positive Variable
   xp 'positive part of x'
   xn 'negative part of x';

Binary Variable b 'decision if we have positive or negative x';

Equation
   e1 'split x in positive and negative part'
   e2 'restrict positive part of x'
   e3 'restrict negative part of x';

e1.. x  =e= xp - xn;

e2.. xp =l= abs(x.up)*b;

e3.. xn =l= abs(x.lo)*(1 - b);

Variable y 'result of function evaluation';

Equation
   defabs  'definition of y = abs(x)'
   defzmax 'definition of y = zmax(x) = max(x,0)'
   defzmin 'definition of y = zmin(x) = min(x,0)';

defabs..  y =e= xp + xn;

defzmax.. y =e= xp;

defzmin.. y =e=    - xn;

Model
   absM  / e1, e2, e3, defabs  /
   zmaxM / e1, e2, e3, defzmax /
   zminM / e1, e2, e3, defzmin /;

Scalar tol 'tolerance for being zero' / 1e-04 /;

Binary Variable
   b1      'decides if xp is positive'
   b2      'decides if xn is positive';

Equation
   e2up    'upper restriction for xp'
   e2lo    'lower restriction for xp'
   e3up    'upper restriction for xn'
   e3lo    'lower restriction for xn'
   one     'either b1 or b2'
   defsign 'definition of y = sign(x)';

e2up..    xp =l=  abs(x.up)*b1;

e2lo..    xp =g=  tol*b1;

e3up..    xn =l=  abs(x.lo)*b2;

e3lo..    xn =g=  tol*b2;

one..     b1 + b2 =l= 1;

defsign.. y =e= b1 - b2;

Model signM / e1, e2up, e2lo, e3up, e3lo, one, defsign /;

option limCol = 0, limRow = 0, optCr = 1e-6;

Set
   runs 'number of test runs' / 1*5 /
   nos                        / "v.lo", "v.up", "w.lo", "w.up" /
   mm   'min and max'         / max, min /;

Parameter bnds_unary(runs,*)  / 1.lo  -5, 1.up  5
                                2.lo  -5, 2.up -2
                                3.lo   2, 3.up  5
                                4.lo   0, 4.up  5
                                5.lo  -5, 5.up  0 /;

Parameter rep_unary;

loop(runs,
   x.lo = bnds_unary(runs,"lo"); rep_unary(mm,runs,"x.lo") = x.lo;
   x.up = bnds_unary(runs,"up"); rep_unary(mm,runs,"x.up") = x.up;

   solve absM  max y using mip;  rep_unary("max",runs,"abs" ) = y.l;
   solve zmaxM max y using mip;  rep_unary("max",runs,"zmax") = y.l;
   solve zminM max y using mip;  rep_unary("max",runs,"zmin") = y.l;
   solve signM max y using mip;  rep_unary("max",runs,"sign") = y.l;
   solve absM  min y using mip;  rep_unary("min",runs,"abs" ) = y.l;
   solve zmaxM min y using mip;  rep_unary("min",runs,"zmax") = y.l;
   solve zminM min y using mip;  rep_unary("min",runs,"zmin") = y.l;
   solve signM min y using mip;  rep_unary("min",runs,"sign") = y.l;
);

Variable
   z 'result of function evaluation'
   v 'argument to max/min'
   w 'argument to max/min';

Equation
   defvwx 'substitution x = v - w'
   defmXX 'definition of z = max/min(v,w) = max/min(x,0) + w';

defvwx.. x =e= v - w;

defmXX.. z =e= y + w;

Model
   maxM / e1, e2, e3, defzmax, defvwx, defmXX /
   minM / e1, e2, e3, defzmin, defvwx, defmXX /;

Parameter bnds_binary(runs,*) / 1.vlo  -5, 1.vup  5, 1.wlo  -2, 1.wup  2
                                2.vlo  -2, 2.vup  2, 2.wlo  -5, 2.wup  5
                                3.vlo  -5, 3.vup -2, 3.wlo   2, 3.wup  5
                                4.vlo   2, 4.vup  5, 4.wlo  -5, 4.wup -2
                                5.vlo  -5, 5.vup  2, 5.wlo  -2, 5.wup  5 /;

Parameter rep_binary;

loop(runs,
   v.lo = bnds_binary(runs,"vlo"); rep_binary(mm,runs,"v.lo") =  v.lo;
   v.up = bnds_binary(runs,"vup"); rep_binary(mm,runs,"v.up") =  v.up;
   w.lo = bnds_binary(runs,"wlo"); rep_binary(mm,runs,"w.lo") =  w.lo;
   w.up = bnds_binary(runs,"wup"); rep_binary(mm,runs,"w.up") =  w.up;
   x.lo = -max(abs(w.up-v.lo), abs(v.up-w.lo));         x.up  = -x.lo;

   solve maxM max z using mip; rep_binary("max",runs,"max") = z.l;
   solve minM max z using mip; rep_binary("max",runs,"min") = z.l;
   solve maxM min z using mip; rep_binary("min",runs,"max") = z.l;
   solve minM min z using mip; rep_binary("min",runs,"min") = z.l;
);

display rep_unary, rep_binary;