# Show and use expressions

**URL:** <https://discourse.julialang.org/t/show-and-use-expressions/100584>\
**Category:** Optimization (Mathematical)\
**Created:** [June 20, 2023, 1:12am UTC](https://discourse.julialang.org/t/show-and-use-expressions/100584 "2023-06-20T01:12:11Z")\
**Posts on this page:** 2\
**Page:** 1

<div class="post-metadata">

**Author:** ![horvetz](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/horvetz/32/31993_2.png) [@horvetz](https://discourse.julialang.org/u/horvetz)\
**Post date:** [June 20, 2023, 1:12am UTC](https://discourse.julialang.org/t/show-and-use-expressions/100584/1 "2023-06-20T01:12:11Z")

</div>

I am having a trouble 1- printing the expression Ad, and 2- using it in a nonlinear constraint.

> using LinearAlgebra  
> using JuMP  
> model = Model( )
> 
> n = 8  
> u = ones(n)
> 
> @variable(model, A[1:n, 1:n], Symmetric)  
> A = LowerTriangular(A)  
> @variable(model, b[1:n])
> 
> d = @expression(model, A \* u)  
> @expression(model, Ad, (sum(A[i,j]\*d[j] for j in 1:i-1) for i in 1:n) )  
> @NLconstraint(model, sum(b[i]\*Ad[i] for i in 1:n) == 1/3);

For the former, when I type Ad in the prompt I get

`Base.Generator{UnitRange{Int64}, var"#37#39"}(var"#37#39"(), 1:8)`

which I don’t know what it means. For the constraint it gives this error

```julia
MethodError: no method matching getindex(::Base.Generator{UnitRange{Int64}, var"#37#39"}, ::Int64)

Stacktrace:
 [1] macro expansion
   @ ~/.julia/packages/JuMP/60Bnj/src/macros.jl:1974 [inlined]
 [2] top-level scope
   @ ./In[4]:15
 [3] eval
   @ ./boot.jl:373 [inlined]
 [4] include_string(mapexpr::typeof(REPL.softscope), mod::Module, code::String, filename::String)
   @ Base ./loading.jl:1196

```

---

<div class="post-metadata">

**Author:** ![odow](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/odow/32/28685_2.png) [@odow](https://discourse.julialang.org/u/odow)\
**Post date:** [June 20, 2023, 1:42am UTC](https://discourse.julialang.org/t/show-and-use-expressions/100584/2 "2023-06-20T01:42:16Z")

</div>

You didn’t have the syntax for `Ad` quite right. It needs to be:

```julia
@expression(model, Ad[i=1:n], sum(A[i,j]*d[j] for j in 1:i-1))

```

Here’s how I’d write your model:

```plaintext
using JuMP
import LinearAlgebra

n = 8
model = Model()
@variable(model, A[1:n, 1:n], Symmetric)
@variable(model, b[1:n])
A = LinearAlgebra.LowerTriangular(A)
u = ones(n)
d = @expression(model, A * u)
@expression(model, Ad[i=1:n], sum(A[i,j]*d[j] for j in 1:i-1))
@NLconstraint(model, sum(b[i] * Ad[i] for i in 1:n) == 1 / 3)

```