How can i add interaction of two variables as a constraint in PuLP-CodePudding

I have an optimization problem in my hand and i want to add some constraints

maa_count = LpVariable("ManAtArmsCount", int(archer_count) - 1, None, LpInteger)
archer_count = LpVariable("ArcherCount", int(mangonel_count)   1 , int(maa_count) - 1, LpInteger)
mangonel_count = LpVariable("MangonelCount", int(cavalry_count)   1, int(archer_count) - 1, LpInteger)
cavalry_count = LpVariable("CavalryCount", 0, int(mangonel_count) - 1, LpInteger)
army_count = archer_count   maa_count   cavalry_count   mangonel_count

This code results in TypeError: int() argument must be a string, a bytes-like object or a number, not 'LpVariable'

I tried to define variables with standart bounds and add a constraint with

prob  = maa_count > archer_count > mangonel_count > cavalry_count

but this resulted in a type error stating that > operator cannot be used between lpvariables.

How can i fix this?

CodePudding user response：

Break up what you are trying to do. You cannot reference a variable as a part of the construct of another variable. The correct thing to do is to put upper/lower bounds (if it makes sense in the context of the problem) in the construction and then state any further relationships in constraints. For instance if I want 2 integer variables and I want y to be greater than x, I just need to state that relationship in a constraint. Also, do not cast things as int()... just declare the variable as an integer type. As such:

import pulp

prob = pulp.LpProblem('example', pulp.LpMinimize)

x = pulp.LpVariable('x', lowBound=0, cat=pulp.LpInteger)
y = pulp.LpVariable('y', lowBound=0, cat=pulp.LpInteger)

# state the relationship of the variables in a linear constraint...  And add it to the problem

prob  = y >= x

print(prob)

Yields:

example:
MINIMIZE
None
SUBJECT TO
_C1: - x   y >= 0

VARIABLES
0 <= x Integer
0 <= y Integer