# How to write a constraint in JuMP where the index has if-else structure

I have a little question in how to write a constraint in Jump language. The constraint is:
sum_{i} sum_{r=0}^{9} x[i,j,t-r] <= 1, for all t=1,…,12 and j=1,…,9
if t - r < 0, change by t - r + 9.
How can I use a if-else structure inside a sum() to incorporate the condition above?
Thanks!

Could you explain a bit more? What is the range of i? Note that there is a term t-r=0 which is basically the zeroth index of the variable. Julia array starts from 1, so that would be an error.

Option 1: use Julia’s inplace `cond ? true : false` syntax

``````for t in 1:12, j in 1:9
@constraint(model, sum(x[i, j, t-r < 0 ? t-r : t-r+9] i in 1:N, r in 0:9) <= 1)
end
``````

Option 2: make a list of the third indices (can be much more general)

``````for t in 1:12, j in 1:9
third_index =Int[]
for r in 0:9
if t - r < 0
push!(third_index, t - r + 9)
else
push!(third_index, t - r)
end
end
@constraint(model, sum(x[i, j, k] i in 1:N, k in third_index) <= 1)
end
``````