Why does it provide two different outputs with if2/if3?

Question

I am testing the m.if3 function in gekko by using conditional statements of if-else but I get two different outputs. The optimal number i get from below code is 12. I plug that in the next code with if-else statement to ensure that the cost matches up but it does not. Am I using if3/if2 incorrectly? The rate is 0.1 for the first 5 days and it switches to 0.3 for the remaining 45 days.

I get different outputs even though I am doing the same thing in both of them.

I tried everything from using if-else statements to using if2 statements.

Answer 1

The if2 or if3 function isn't needed because the switching argument duration-5 is a constant value that is not a function of a Gekko variable. Just like the validation script, the two segments can be calculated separately and added together to get a total cost and patient count.

from gekko import GEKKO
m = GEKKO(remote=False)

# parameters
cost_p = 9  
cost_s = 12 
var1 = 50 
duration = 50  
x = m.Var(integer=True, lb=1)  

rate1 = 0.3
rate2 = 0.1

cost1 = m.Intermediate((rate1 * cost_p * 5 + cost_s) * x)
cost2 = m.Intermediate((rate2 * cost_p * (duration-5) + cost_s) * x)
cost = m.Intermediate(cost1+cost2)

countp1 = m.Intermediate(rate1 * 5 * x)
countp2 = m.Intermediate(rate2 * (duration-5) * x)
p_count = m.Intermediate(countp1+countp2)

m.Minimize(cost)
m.Equation(p_count >= var1)
m.options.SOLVER = 1 # for MINLP solution
m.solve(disp=False)
num_sites = x.value[0]
print(f'num_s = {num_s}')
print(f'cost: {cost.value[0]}')
print(f'p_count: {p_count.value[0]}')

The optimal solution is:

num_s = 9.0
cost: 810.0
p_count: 54.0

The solution validation agrees with this answer:

# Solution validation
# Parameters
cost_s = 9  
cost_p = 12  
num_p = 50  
duration = 50 

if duration > 5:
    rate = 0.1
else:
    rate = 0.3
x = 9 


cost1 = (0.1 * cost_p * 45 + cost_s) * x
cost2 = (0.3 * cost_p * 5 + cost_s) * x
cost = cost1 + cost2


countp1 = 0.3 * 5 * x
countp2 = 0.1 * 45 * x
countp = countp1 + countp2

print(f'cost (validation): {cost}')
print(f'count (validation): {countp}')