# JuMP: how to find which constraints a variable appears in

**URL:** <https://discourse.julialang.org/t/jump-how-to-find-which-constraints-a-variable-appears-in/103014>\
**Category:** Optimization (Mathematical)\
**Tags:** jump\
**Created:** [August 20, 2023, 8:11pm UTC](https://discourse.julialang.org/t/jump-how-to-find-which-constraints-a-variable-appears-in/103014 "2023-08-20T20:11:12Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![rjuly7](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/rjuly7/32/52042_2.png) [@rjuly7](https://discourse.julialang.org/u/rjuly7)\
**Post date:** [August 20, 2023, 8:11pm UTC](https://discourse.julialang.org/t/jump-how-to-find-which-constraints-a-variable-appears-in/103014/1 "2023-08-20T20:11:12Z")

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I am using JuMP to formulate a large optimization problem. Is there any function which allows me to see all constraints in which a specific variable appears? Or if I have the reference to a specific constraint, is there a method for me to check whether or not a certain variable appears in that constraint?

So if I have:

```julia
using JuMP 
model = Model()
@variable(model, x[1:3])
my_constraint = @constraint(model, x[1] + x[2] >= 0)

```

I am looking for a function that I could ask if x[1] appears in my\_constraint, and have it return true.

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<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:** [August 20, 2023, 9:03pm UTC](https://discourse.julialang.org/t/jump-how-to-find-which-constraints-a-variable-appears-in/103014/2 "2023-08-20T21:03:53Z")

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> Is there any function which allows me to see all constraints in which a specific variable appears?

There isn’t explicitly in JuMP for this, but if your constraints are linear, then one option is this:

```Julia
julia> function in_constraint(constraint, variable)
           object = constraint_object(constraint)
           return !iszero(coefficient(object.func, variable))
       end
in_constraint (generic function with 1 method)

julia> in_constraint(my_constraint, x[1])
true

julia> in_constraint(my_constraint, x[2])
true

julia> in_constraint(my_constraint, x[3])
false

```

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<div class="post-metadata">

**Author:** ![rjuly7](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/rjuly7/32/52042_2.png) [@rjuly7](https://discourse.julialang.org/u/rjuly7)\
**Post date:** [August 21, 2023, 2:37pm UTC](https://discourse.julialang.org/t/jump-how-to-find-which-constraints-a-variable-appears-in/103014/3 "2023-08-21T14:37:14Z")

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OK, thank you, that’s very helpful! I have both linear and nonlinear constraints, but I realized the variable I’m interested in only appears in the linear constraints, so this will work great. (I did realize I could also just check for that variable as I’m formulating constraints and save the references to those constraints, but I was hoping for a method that wouldn’t require adding that additional code to the source, but rather to examine the model after it’s complete. So this is just what I was looking for.) Thanks!