GAMSPy for GAMS Users

This document is for users who are already GAMS users and interested in translating their existing GAMS models to GAMSPy.

Translating Symbols

The way to create symbols such as Set, Alias, Parameter, Variable, and Equation is explained in their respective documentation pages and you can see an example for the creation of each symbol below:

GAMSPy:

import gamspy as gp

m = gp.Container()
i = gp.Set(m, "i", records=['i1','i2'])
a = gp.Alias(m, "a", alias_with=i)
p = gp.Parameter(m, 'p', domain=[i], records=[['i1','1'], ['i2','2']])
v = gp.Variable(m, "v", domain=[i])
z = gp.Variable(m, "z")
e = gp.Equation(m, "e", domain=[i])
e[i] = v[i] + p[i] <= z
model = gp.Model(m, "my_model", equations=[e], problem="lp", sense="min", objective=z)
model.solve()

This GAMSPy code snippet above is equivalent to the following GAMS code:

GAMS:

Set i / i1, i2 /;
Alias (i, a);
Parameter p / i1 1, i2 2 /;
Variable v(i);
Equation e(i);
e(i) .. v(i) + p(i) =l= z
Model my_model / e /;
solve my_model using LP min z;

Translating Operations: Sum/Product/Smin/Smax

Frequently used GAMS operations which accept an index list and an expression can be translated as follows:

GAMSPy:

from gamspy import Sum, Product, Smin, Smax

m = gp.Container()
i = gp.Set(m, "i", records=['i1','i2'])
a = gp.Parameter(m, 'a', domain=[i], records=[['i1','1'], ['i2','2']])
z = gp.Variable(m, 'z')

eq = gp.Equation(m, name="eq")
eq[...] = Sum(i, a[i]) <= z

GAMS:

Set i / i1, i2 /;
Parameter a / i1 1, i2 2 /;
Variable z;
Equation eq;
eq .. sum(i, a(i)) =l= z;

Card/Ord

Card and Ord operations can be translated as follows:

GAMSPy:

import gamspy as gp
import math

m = gp.Container()
i = Set(m, name="i", records=[str(idx) for idx in range(0, 181)])
step = Parameter(m, name="step", records=math.pi / 180)
omega = Parameter(m, name="omega", domain=[i])
omega[i] = (Ord(i) - 1) * step

GAMS:

Set i / i0..i180 /;
Parameter step;
step = pi / 180;
Parameter omega(i);
omega(i) = (Ord(i) - 1) * step;

Domain

This class is exclusively for conditioning on a domain with more than one set.

GAMSPy:

import gamspy as gp

m = gp.Container()

bus = gp.Set(m, "bus", records=["i" + str(buses) for buses in range(1, 7)])
node = Alias(m, name="node", alias_with=bus)
conex = Set(m,"conex",domain=[bus, bus])

branch = Parameter(m,"branch",[bus, node, "*"],records=records)

p = Parameter(m, name="M")

conex[bus, node].where[branch[bus, node, "x"]] = True
conex[bus, node].where[conex[node, bus]] = True

p[...] = Smax(
    Domain(bus, node).where[conex[bus, node]],
    branch[bus, node, "bij"] * 3.14 * 2,
)

GAMS:

Set bus / i1..i6 /;
Alias (bus, node);
Set conex(bus, bus);

Parameter branch(bus, node, "*") / ...... /;
Parameter p;

conex(bus, node)$(branch(bus, node, "x")) = yes;
conex(bus, node)$(conex(node, bus)) = yes;

p = smax((bus, node) $ (conex(bus, node)), branch(bus, node, "bij" * 3.14 * 2))

Number

This is for conditions on numbers or yes/no statements.

GAMSPy:

import gamspy as gp

m = gp.Container()
i = gp.Set(m, "i", records=[str(i) for i in range(1,5)])
ie = gp.Set(m, "ie", domain=[i])
x = gp.Variable(m, "x", domain=[i])
ie[i] = gp.Number(1).where[x.lo[i] == x.up[i]]

GAMS:

Set i / 1..4 /;
Set ie(i);
Variable x(i);
ie(i) = yes$(x.lo(i) = x.up(i));

math package

This package is for the mathematical operations of GAMS.

GAMSPy:

import gamspy.math as gams_math
import math

sigma = Variable(m, name="sigma", domain=[i, k], type="Positive")
sigma.l[i, k] = uniform(0.1, 1) # Generates a different value from uniform distribution for each element of the domain.
sigma.l[i, k] = math.uniform(0.1, 1) # This is not equivalent to the statement above. This generates only one value for the whole domain.

Logical Operations

Since it is not possible in Python to overload keywords such as and, or, and not, you need to use bitwise operatiors &, |, and ~.

Mapping:

• and -> &

• or -> |

• not -> ~

GAMSPy:

error01[s1,s2] = rt[s1,s2] & (~lfr[s1,s2]) | ((~rt[s1,s2]) & lfr[s1,s2])

GAMS:

error01(s1,s2) = rt(s1,s2) and not lfr(s1,s2) or not rt(s1,s2) and lfr(s1,s2);