Non-constant expressions cannot be multiplied

Question

I am trying to solve a valued n-queens problem, in which queens in black squares worth double of those in white squares. I solved it in AMPL just fine, but I would like to try that in Python using a different approach. I tried to simplify that by using a variable $k$ in an if statement, such that if $k$ is odd then 1 (white square) and if $k$ is even then $2$ (black square) (Obs: $k = N - i - j$, where $N$ is the size of the chessboard, $i$ represents rows and $j$ represents columns). However I am getting this error: Non-constant expressions cannot be multiplied". I understand where the problem is but do not know how to overcome it.

Objective function $$ \max z = \sum x_{ij} w_{ij} $$

Subject to:

1 queen per row $$ \sum x_{ij} = 1 \; \forall j \; \{j \in \mathbb{N}, j \leq 8 \} $$ 1 queen per column $$ \sum x_{ij} = 1 \; \forall i \; \{i \in \mathbb{N}, i \leq 8 \} $$ 1 queen per diagonal type 1 $$ \sum x_{ij} \leq 1 \; \forall k \; \{ k=i+j | k \in \mathbb{N}, k < 16 \} $$ 1 queen per diagonal type 2 $$ \sum x_{ij} \leq 1 \; \forall k \; \{ k=i-j | k \in \mathbb{Z}, -7 < k < 7 \} $$

$$ x_{ij} \in \{0,1\} $$

from pulp import *
N = 8
nums = list(range(1, N+1)) #list from 1 to N",
numsC = list(range(1, N+1)) #list from 1 to N",
numsL = list(range(1, N+1)) #list from 1 to N",

vars = {}
r = {}

model = LpProblem('Damas', LpMaximize)

# Decison Variables
for i in nums:
    for j in nums: # create a binary variable
        vars[i, j] = LpVariable('x{},{}'.format(i, j), cat='Binary')

for i in numsC:
    for j in numsL:
        k = N - i - j
        if k % 2 == 0:
            r[i,j] = 2
            r[i, j] = LpVariable('r', 'LowBound=0', cat='Integer')
        else:
            r[i,j] = 1
            r[i,j] = LpVariable('r','LowBound=0', cat='Integer')

# Objective function
model += sum(vars[i, j] for i in nums for j in nums) * sum(r[i,j] for i in numsC for j in numsL)

# Restrições
# 1 queen per row
for i in nums:
    model += sum(vars[i, j] for j in nums) <= 1
# 1 queen per column
for j in nums:
    model += sum(vars[i, j] for i in nums) <= 1
# 1 queen per diagonal 1
for k in range(2, 2*N+1):
    model += sum(vars[i, j] for i in nums for j in nums if i+j == k) <= 1
# 1 queen per diagonal 2
for k in range(-(N-2),(N-2)+1):
    model += sum(vars[i, j] for i in nums for j in nums if i-j == k) <= 1

Answer 1

Here are some problems in your code:

1. I don't think you need

   r[i, j] = LpVariable('r', 'LowBound=0', cat='Integer')
   

in your code. $r$ has been defined to be either $1$ or $2$ in the line above.

2. Also,

   model += sum(vars[i, j] for i in nums for j in nums) * sum(r[i,j] for i in numsC for j in numsL
   

is not correct, you are performing the sum separately and then multiplied them together. Try the following list comprehension.

sum(vars[i, j] * r[i,j] for i in nums for j in nums)

---

Edit:

I have fixed your code, I will leave the final formatting to you:

from pulp import *
N = 8
nums = list(range(1, N+1)) #list from 1 to N",
numsC = list(range(1, N+1)) #list from 1 to N",
numsL = list(range(1, N+1)) #list from 1 to N",

vars = {}
r = {}

model = LpProblem('Damas', LpMaximize)

# Decison Variables
for i in nums:
    for j in nums: # create a binary variable
        vars[i, j] = LpVariable('x{},{}'.format(i, j), cat='Binary')

for i in numsC:
    for j in numsL:
        k = N - i - j
        if k % 2 == 0:
            r[i,j] = 2
        else:
            r[i,j] = 1

# Objective function
model += sum(vars[i, j] * r[i,j] for i in nums for j in nums) 

# Restrições
# 1 queen per row
for i in nums:
    model += sum(vars[i, j] for j in nums) <= 1
# 1 queen per column
for j in nums:
    model += sum(vars[i, j] for i in nums) <= 1
# 1 queen per diagonal 1
for k in range(2, 2*N+1):
    model += sum(vars[i, j] for i in nums for j in nums if i+j == k) <= 1
# 1 queen per diagonal 2
for k in range(-(N-2),(N-2)+1):
    model += sum(vars[i, j] for i in nums for j in nums if i-j == k) <= 1

model.solve()
for variable in model.variables():
    print("{} = {}".format(variable.name, variable.varValue))