multiplication of CyLPExpr by constant not commutative

[sdementen]

When multiplying by a constant before or after a variable.sum() I get a wrong / good answer

from cylp.cy.CyClpSimplex import CyClpSimplex

s = CyClpSimplex()
x = s.addVariable("x", 2)
y = s.addVariable("y", 2)
s += y >= 0
s += y.sum() - x.sum() * 3 <= -1  # ==> gives wrong solution x=[0,1]
# s += y.sum() - 3 * x.sum() <= -1  # ==> gives correct solution x=[0,0.3333]
s.objective = x.sum()
s.primal()

The two expressions y.sum() - x.sum() * 3 <= -1 and y.sum() - 3 * x.sum() <= -1 should represent the same constraint and give the same results but only the second gives the correct model.

[rsemenoff]

I believe this is a documented limitation.  On Friday, January 28, 2022, 05:08:03 AM PST, sdementen @.***> wrote:

When multiplying by a constant before or after a variable.sum() I get a wrong / good answer from cylp.cy.CyClpSimplex import CyClpSimplex

s = CyClpSimplex() x = s.addVariable("x", 2) y = s.addVariable("y", 2) s += y >= 0 s += y.sum() - x.sum() * 3 <= -1 # ==> gives wrong solution x=[0,1]

s += y.sum() - 3 * x.sum() <= -1 # ==> gives correct solution x=[0,0.3333]

s.objective = x.sum() s.primal() The two expressions y.sum() - x.sum() 3 <= -1 and y.sum() - 3 x.sum() <= -1 should represent the same constraint and give the same results but only the second gives the correct model.

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