Give an obvious wrong result for a very simple problem

[kmeng]

Details for the issue

What did you do?

Write a very simple program for learning:

def solve_by_pulp(): model = pulp.LpProblem('Sample3_1', LpMaximize)

# define decision variables
x1 = pulp.LpVariable('x1', lowBound=0)
x2 = pulp.LpVariable('x2', lowBound=0)
x3 = pulp.LpVariable('x3', lowBound=0)

# set objective
model += 2*x1 + 3*x2 - 5+x3

# set constraint
model += x1 + x2 + x3 == 7
# 下面的两个约束条件没有起作用
model += 2*x1 - 5*x2 + x3 >= 10
model += x1 + 3*x2 + x3 >= 12

model.solve()

print('x1 = {}'.format(x1.varValue))
print('x2 = {}'.format(x2.varValue))
print('x3 = {}'.format(x3.varValue))
print('max z= ', value(model.objective))

What did you expect to see?

It should be no result.

What did you see instead?

x1 = 6.4285714 x2 = 0.57142857 x3 = 0.0 max z= 9.57142851

Useful extra information

The info below often helps, please fill it out if you're able to. :)

What operating system are you using?

• [x] Windows: ( version: ___ )
• [ ] Linux: ( distro: ___ )
• [ ] Mac OS: ( version: ___ )
• [ ] Other: ___

I'm using python version:

• [ ] 2.7
• [ ] 3.4
• [ ] 3.5
• [ ] 3.6
• [x] Other: 3.11

I installed PuLP via:

• [ ] pypi (python -m pip install pulp)
• [ ] github (python -m pip install -U git+https://github.com/coin-or/pulp)
• [x] Other: ___ (pycharm)

Did you also

• [ ] Tried out the latest github version: https://github.com/coin-or/pulp
• [ ] Searched for an existing similar issue: https://github.com/coin-or/pulp/issues?utf8=%E2%9C%93&q=is%3Aissue%20

[kmeng]

if the third constraint is changed to:

model += x1 + 3*x2 + x3 <= 12

The objective value was wrong either. it should be 9.57142851

[stumitchell]

Did you check the solution status?

I answered this on my phone. Please excuse any typos and brevity

On Wed, 27 Sep 2023, 10:26 pm Kangjian Meng, @.***> wrote:

if the third constraint is changed to:

model += x1 + 3*x2 + x3 <= 12

The objective value was wrong either. it should be 9.57142851

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[kmeng]

Thanks. It seems I got a wrong sample