What is causing this undefined variable Error message

New to Julia
1

Define parameters

operating_cost = [4000, 4200, 4800, 5800, 7200, 9000]
salvage_value = [10500, 7300, 5100, 3600, 2500, 1600]
replacement_cost = 15000
maintenance_cost = 3500
years = 5

Initialize DP table

C[t][a] will store the minimum cost for year t with drone age a

C = Array{Float64}(undef, years + 1, length(operating_cost))

Initialize end of planning horizon

for a in 1:length(operating_cost)
C[years + 1, a] = 0 # Boundary condition: No future costs after year 5
end

DP recursion to fill the table from the last year backwards

for t in years:-1:1
for a in 1:length(operating_cost)
# Cost of keeping the drone
keep_cost = operating_cost[a] + C[t + 1, min(a + 1, length(operating_cost))]

    # Cost of performing maintenance
    new_age_after_maintenance = max(1, a - 2)
    total_maintenance_cost = maintenance_cost + operating_cost[new_age_after_maintenance]
    maintenance_option_cost = total_maintenance_cost + C[t + 1, new_age_after_maintenance]

    # Cost of replacing the drone
    replace_cost = replacement_cost - salvage_value[a] + C[t + 1, 1]

    # Store the minimum cost for year t and age a
    C[t, a] = minimum([keep_cost, maintenance_option_cost, replace_cost])
end

end

Output the minimum cost to start from year 1 with the current age of 2 years

optimal_cost = C[1, 3] # Drone starts at age 2, so we look at the third column (age 2)
println("The minimum cumulative cost for the optimal replacement policy is: ", optimal_cost)

Display the DP table with intermediate calculations

println(“Dynamic Programming Table (Minimum Costs):”)
for t in 1:years+1
println("Year $t: ", C[t, :])
end

Traceback for Optimal Path

println(“\nTracing Optimal Replacement Strategy:”)

Define initial age at the start of the traceback section

current_age = 3 # Initial index for age 2 (third column in DP table)

for t in 1:years
keep_cost = operating_cost[current_age] + C[t + 1, min(current_age + 1, length(operating_cost))]
new_age_after_maintenance = max(1, current_age - 2)
maintenance_option_cost = maintenance_cost + operating_cost[new_age_after_maintenance] + C[t + 1, new_age_after_maintenance]
replace_cost = replacement_cost - salvage_value[current_age] + C[t + 1, 1]

# Determine the action taken at this stage
optimal_action_cost = C[t, current_age]
if optimal_action_cost == keep_cost
    println("Year $t: Keep the drone (age ", current_age, ")")
    current_age = min(current_age + 1, length(operating_cost))
elseif optimal_action_cost == maintenance_option_cost
    println("Year $t: Perform maintenance (reset to age ", new_age_after_maintenance, ")")
    current_age = new_age_after_maintenance
else
    println("Year $t: Replace the drone (new age 1)")
    current_age = 1
end

end

whenever I try this code running it gives error for age/ current_age not defined

2

Hi,

Currently the code you posted is not properly formatted, making it hard to read. Please edit it, cf. instructions here:

Additionally, it would also help if you would post the full error message, and reduce the code as much as possible to obtain a minimal working example (resulting in the same error). Check out

1 Like
3

For what it’s worth, your code runs for me with output

Year 1: Perform maintenance (reset to age 1)
Year 2: Replace the drone (new age 1)
Year 3: Replace the drone (new age 1)
Year 4: Keep the drone (age 1)
Year 5: Keep the drone (age 2)

A couple of style suggestions:

  • wrap each step in a function with clear inputs and outputs (to avoid side effects and clarify intent)
  • avoid nonconstant globals
1 Like