For loop error

Specific Domains Optimization (Mathematical)
1

Hi,
I’m still new to Julia, and am having problems with a for loop. I’m trying to create an array where each value is the former value minus another value. This is the code:

for i = 1:T 
     bal[i] = bal[i-1] - A[i]
end

where bal and A are arrays of the same size. The error I get is “MethodError: no method matching -(::VariableRef, ::Array{VariableRef,1})”

Please help, thanks!

2

It would be great if you could provide an MWE - the error is telling you that for the type of variables you’re using, a subtraction operation is not defined, but without more context it’s hard to say where these types come from and why (if at all) you’d need them.

1 Like
3

Oh alright, thanks for the link to the page as well. Here’s an MWE, I hope this would be enough.

using JuMP

m = Model()
T = 300

@variables(m, begin
    delta0 >= 0
    amort[i=1:T] >= 0
	bal[0:T] >= 0
end)

amort = delta0/T
bal[0] = delta0

for i = 1:T
	bal[i] = bal[i-1] - amort
end

If there is a better way to do this please let me know.

Thanks!

4

Running your code seems to fail with a different error for me

for i = 1:T 
    bal[i] = bal[i-1] - amort
end

ERROR: MethodError: Cannot `convert` an object of type GenericAffExpr{Float64,VariableRef} to an object of type VariableRef
Closest candidates are:
  convert(::Type{T}, ::T) where T at essentials.jl:154
  VariableRef(::Any, ::Any) at C:\Users\####\.julia\packages\JuMP\ibcEh\src\variables.jl:99

So what’s happening here is:

julia> typeof(bal)
JuMP.Containers.DenseAxisArray{VariableRef,1,Tuple{UnitRange{Int64}},Tuple{Dict{Int64,Int64}}}

julia> typeof(bal[0])
VariableRef

julia> typeof(amort)
GenericAffExpr{Float64,VariableRef}

julia> typeof(bal[0] - amort)
GenericAffExpr{Float64,VariableRef}

Subtracting amort from an element in your bal array produces an object of type GenericAffExpr{Float64, VariableRef}, which then can’t be assigned to the JuMP DenseAxisArray which is expecting an object of type VariableRef

Unfortunately I’ve never really used JuMP so I’m not sure what a workaround might be. Maybe you could explain what you’re trying to do?

5

Oh alright, yeah I’m getting the same error too actually, and I sort of understand why it gives an error, but I’m not sure what else to do. I don’t think this particular issue is related to JuMP tho. So here’s basically what I want.

Let’s say I have an amount of $10 at time period t = 0, and at the beginning of each period I want to deduct $1. So the formula would be at time t, amount owned would be the amount at period (t-1) minus the $1, correct? How would I do this in Julia? In my code, bal is the amount and amort is the amount to be subtracted.

Would really appreciate any advice, thanks!

6

That’s simple enough, although I’m not sure how it relates to what you’re trying to do in JuMP - what you’re trying to do works if you do it with regular numbers (e.g. Float64) rather than references:

balance = zeros(10)
balance[1] = 10 # initial amount owed
amort = 1 # repayment per period

julia> for i in 2:length(balance)
           balance[i] = balance[i-1] - amort
       end

julia> balance
10-element Array{Float64,1}:
 10.0
  9.0
  8.0
  7.0
  6.0
  5.0
  4.0
  3.0
  2.0
  1.0
2 Likes
7

Thank you! I completely forgot about trying regular numbers instead of JuMP variables.

1 Like
8

That is an impressive commitment to optimization - glad it works now!

1 Like