PhilippeCouronne
commented
3 years ago Hello, You're not mentioning the original format of the model you want to convert to docplex, seems like OPL?
To explain how to map the constraint syntax you posted to Docplex, let me proceed in steps:
- First, the format of constraint is of the form
=> , in other terms, when some logical condition is true (i.e. equal to 1), the constraint has to be satisfied (otherwise, it is free, maybe satisfied, maybe not, we don't know). What's more in your case, the logical condition is built using constraints, so the general pattern here is:
if [ C1 and C2 are satisfied] then C3 must be satisfied.
This can be mapped to docplex easily. The essential point here is that DOcplex provides a binary variable mirroring the "truth value" of a constraint, that is, equal to 1 if and only if the constraint is satisfied. Of course the constraints should not be added to the model, otherwise they would always be satisfied. Let me illustrate with a small simple code:
from docplex.mp.model import Model
m = Model()
x = m.integer_var()
y = m.integer_var()
z = m.integer_var(lb=1, ub=10)
ctx3 = (x == 3)
cty5 = (y == 5)
ctz7 = (z == 7)
m.add(x == 3)
m.add(y == 5)
m.add((ctx3 & cty5) <= ctz7)
m.minimize(x + y)
s1 = m.solve()
s1.display()Here I create two constraints ctx3 and cty5; note they are NOT added to the model, so we don't know in advance what their status will be .
The syntax:
m.add((ctx3 & cty5) <= ctz7)
illustrates several points:
- a constraint can be used as an expression, it is converted into its binary status variable
- the '&' operator computes the logical AND of the two status variables
- this binary logical is in turn usable as a binary variable, equal to 1 when both statuses are 1 Then we state that the And is less than the status of the target constraint (y==7), which means when the And is true, the status of (y==7) becomes true, hence y==7
Applied to your (simplified) case , this would look like:
m.add( ( (X == 1) & (F >= 0)) <= (S2 -S1 + F >=0))
To summarize:
- a linear constraint can be used in expressions, converted to its binary truth variable
- operator
&computes logical AND of two logical expressions - a simple <= constraint can be used to express implication
-
Let me know if you need more help.
Julia000199
Hi everybody! Currently I am writing cplex to python docplex conversion model and I have cplex side binding as follows:
forall (i in point: i == 0, j in point: j != 0, k in truck) x[0][j][k] == 1 && f[j][k] >= 0 => sl[j][k] - sl[0][k] + f[j][k] > = 0;
I want to convert this binding to docplex, however I don't know how to write it. Can you help me? Thanks.