Solve fails on problem with only 1 variable

[snorfalorpagus]

The following code runs correctly for count = 2, but fails with count = 1.

#!/usr/bin/env python

from cylp.cy import CyClpSimplex
from cylp.py.modeling.CyLPModel import CyLPArray

count = 2

s = CyClpSimplex()
x = s.addVariable('x', count)

s += 0.0 <= x <= 10.0

s.objective = CyLPArray([20]*count) * x
s.optimizationDirection = 'max'

status = s.primal()
result = s.primalVariableSolution['x']

print(status)
print(result)

The error raised:

meh.py:9: FutureWarning: comparison to `None` will result in an elementwise object comparison in the future.
  x = s.addVariable('x', count)
meh.py:12: FutureWarning: comparison to `None` will result in an elementwise object comparison in the future.
  s += 0.0 <= x <= 10.0
Traceback (most recent call last):
  File "meh.py", line 14, in <module>
    s.objective = CyLPArray([20]*count) * x
  File "CyClpSimplex.pyx", line 141, in cylp.cy.CyClpSimplex.CyClpSimplex.objective.__set__ (cylp/cy/CyClpSimplex.cpp:4559)
  File "CyClpSimplex.pyx", line 1192, in cylp.cy.CyClpSimplex.CyClpSimplex.setObjectiveArray (cylp/cy/CyClpSimplex.cpp:17461)
ValueError: Buffer has wrong number of dimensions (expected 1, got 0)

This can be worked around by adding another variable, which can be completely unconstrained and not included in the objective function definition. However, this shouldn't be required.

[dafinguzman]

I had a similar issue and worked around it by deleting an "np.squeeze" operation in CyLPModel.py, which transforms an np.array([[0]]) into np.array(0).

I guess deleting that operation in the function def objective(...): could make your case work.

[sdementen]

I found another example that leads to a very similar error message but removing the 2 np.squeeze at the beginning of the CyLPConstraint.perform() did not solve it:

from cylp.cy.CyClpSimplex import CyClpSimplex

s = CyClpSimplex()
x = s.addVariable("x", 1)

s += x >= 1
s.objective = x.sum()
s.primal()

Traceback (most recent call last):
  File "...", line 7, in <module>
    s.objective = x.sum()
  File "cylp/cy/CyClpSimplex.pyx", line 147, in cylp.cy.CyClpSimplex.CyClpSimplex.objective.__set__
  File "cylp/cy/CyClpSimplex.pyx", line 1207, in cylp.cy.CyClpSimplex.CyClpSimplex.setObjectiveArray
ValueError: Buffer has wrong number of dimensions (expected 1, got 0)