# Issue providing start/hint values to variables with Gurobi
**URL:** https://discourse.julialang.org/t/issue-providing-start-hint-values-to-variables-with-gurobi/77079
**Category:** Optimization (Mathematical)
**Tags:** question, jump, gurobi
**Created:** [February 25, 2022, 3:39pm UTC](https://discourse.julialang.org/t/issue-providing-start-hint-values-to-variables-with-gurobi/77079 "2022-02-25T15:39:21Z")
**Posts on this page:** 5
**Page:** 1
### Author:  [@umbe](https://discourse.julialang.org/u/umbe)
#### Post date: [February 25, 2022, 3:39pm UTC](https://discourse.julialang.org/t/issue-providing-start-hint-values-to-variables-with-gurobi/77079/1 "2022-02-25T15:39:21Z")
Hello to everyone!
I am trying to provide initial guess values or hint values to some variables of an optimization problem, which is intended to be solved using Gurobi. I have read [this](https://discourse.julialang.org/t/how-to-use-gurobi-variable-hints-in-jump-0-21/42764) previous topic for the assignment of inital guesses/hints, and I am using the same command, which is actually accepted during the definition of the optimization model.
Below is reported a minimum example:
```julia
using Gurobi
using JuMP
model = Model(Gurobi.Optimizer)
@variable(model, 0.0 <= x1 <= 60.0)
@variable(model, 0.0 <= x2 <= 30.0)
@variable(model, y, Bin)
MOI.set.(model, Gurobi.VariableAttribute("VarHintVal"), x1, 5.0)
@objective(model, Min, 5*x1 - 3*x2)
@constraint(model, 2*x1 + x2 <= 20)
@constraint(model, 10*y <= x2)
JuMP.optimize!(model)
```
However, when I try to execute the optimization Julia returns this error:
```julia
ERROR: ArgumentError: Attribute VarHintVal is Real but Nothing provided.
Stacktrace:
[1] set(::Gurobi.Optimizer, ::Gurobi.VariableAttribute, ::MathOptInterface.VariableIndex, ::Nothing) at C:\Users\umbe\.julia\packages\Gurobi\7YNJV\src\MOI_wrapper.jl:2928
[2] _broadcast_getindex_evalf at .\broadcast.jl:631 [inlined]
[3] _broadcast_getindex at .\broadcast.jl:604 [inlined]
[4] getindex at .\broadcast.jl:564 [inlined]
[5] macro expansion at .\broadcast.jl:910 [inlined]
[6] macro expansion at .\simdloop.jl:77 [inlined]
[7] copyto! at .\broadcast.jl:909 [inlined]
[8] copyto! at .\broadcast.jl:864 [inlined]
[9] copy at .\broadcast.jl:840 [inlined]
[10] materialize at .\broadcast.jl:820 [inlined]
[11] set(::Gurobi.Optimizer, ::Gurobi.VariableAttribute, ::Array{MathOptInterface.VariableIndex,1}, ::Array{Union{Nothing, Float64},1}) at C:\Users\umbe\.julia\packages\MathOptInterface\ZJFKw\src\attributes.jl:369
[12] set(::MathOptInterface.Bridges.LazyBridgeOptimizer{Gurobi.Optimizer}, ::Gurobi.VariableAttribute, ::Array{MathOptInterface.VariableIndex,1}, ::Array{Union{Nothing, Float64},1}) at C:\Users\umbe\.julia\packages\MathOptInterface\ZJFKw\src\Bridges\bridge_optimizer.jl:842
[13] _pass_attributes(::MathOptInterface.Bridges.LazyBridgeOptimizer{Gurobi.Optimizer}, ::MathOptInterface.Utilities.UniversalFallback{MathOptInterface.Utilities.Model{Float64}}, ::MathOptInterface.Utilities.IndexMap, ::Array{MathOptInterface.AbstractVariableAttribute,1}, ::Tuple{DataType}, ::Tuple{Array{MathOptInterface.VariableIndex,1}}, ::Tuple{Array{MathOptInterface.VariableIndex,1}}, ::Function) at C:\Users\umbe\.julia\packages\MathOptInterface\ZJFKw\src\Utilities\copy.jl:193
[14] pass_attributes at C:\Users\umbe\.julia\packages\MathOptInterface\ZJFKw\src\Utilities\copy.jl:160 [inlined]
[15] pass_attributes at C:\Users\umbe\.julia\packages\MathOptInterface\ZJFKw\src\Utilities\copy.jl:154 [inlined]
[16] default_copy_to(::MathOptInterface.Bridges.LazyBridgeOptimizer{Gurobi.Optimizer}, ::MathOptInterface.Utilities.UniversalFallback{MathOptInterface.Utilities.Model{Float64}}, ::Bool, ::Nothing) at C:\Users\umbe\.julia\packages\MathOptInterface\ZJFKw\src\Utilities\copy.jl:468
[17] #automatic_copy_to#137 at C:\Users\umbe\.julia\packages\MathOptInterface\ZJFKw\src\Utilities\copy.jl:21 [inlined]
[18] #copy_to#3 at C:\Users\umbe\.julia\packages\MathOptInterface\ZJFKw\src\Bridges\bridge_optimizer.jl:293 [inlined]
[19] attach_optimizer(::MathOptInterface.Utilities.CachingOptimizer{MathOptInterface.AbstractOptimizer,MathOptInterface.Utilities.UniversalFallback{MathOptInterface.Utilities.Model{Float64}}}) at C:\Users\umbe\.julia\packages\MathOptInterface\ZJFKw\src\Utilities\cachingoptimizer.jl:150
[20] optimize!(::MathOptInterface.Utilities.CachingOptimizer{MathOptInterface.AbstractOptimizer,MathOptInterface.Utilities.UniversalFallback{MathOptInterface.Utilities.Model{Float64}}}) at C:\Users\umbe\.julia\packages\MathOptInterface\ZJFKw\src\Utilities\cachingoptimizer.jl:211
[21] optimize!(::Model, ::Nothing; bridge_constraints::Bool, ignore_optimize_hook::Bool, kwargs::Base.Iterators.Pairs{Union{},Union{},Tuple{},NamedTuple{(),Tuple{}}}) at C:\Users\umbe\.julia\packages\JuMP\e0Uc2\src\optimizer_interface.jl:130
[22] optimize! at C:\Users\umber\.julia\packages\JuMP\e0Uc2\src\optimizer_interface.jl:106 [inlined] (repeats 2 times)
[23] top-level scope at none:0
```
Does anyone have any idea of how can I solve this issue?
---
### Author:  [@odow](https://discourse.julialang.org/u/odow)
#### Post date: [February 25, 2022, 4:56pm UTC](https://discourse.julialang.org/t/issue-providing-start-hint-values-to-variables-with-gurobi/77079/2 "2022-02-25T16:56:59Z")
> [@umbe](#):
>
> `@variable(model, 0.0 <= x1 <= 60.0)`
Use the `start` keyword:
```Julia
@variable(model, 0.0 <= x1 <= 60.0, start = 5.0)
```
or `set_start_value(x1, 5.0)`.
Docs: [Variables · JuMP](https://jump.dev/JuMP.jl/stable/manual/variables/#Start-values)
---
### Author:  [@umbe](https://discourse.julialang.org/u/umbe)
#### Post date: [February 25, 2022, 5:30pm UTC](https://discourse.julialang.org/t/issue-providing-start-hint-values-to-variables-with-gurobi/77079/3 "2022-02-25T17:30:51Z")
Thank you for the answer, I’ll use one of these commands then.
But please, forgive me for insisting, do you think that exists another turnaround to provide the value of the VarHintVal parameter?
Because, according to the [Gurobi documentation](https://www.gurobi.com/documentation/9.5/refman/varhintval.html), this parameter does not imposes an initial guess value, but it guides the optimization process such that the hint values are considered as suggestions to evaluate more carefully with respect to other values.
> Hints will affect the heuristics that Gurobi uses to find feasible solutions, and the branching decisions that Gurobi makes to explore the MIP search tree.
And this is something that could be very useful to my case.
---
### Author:  [@Henrique\_Becker](https://discourse.julialang.org/u/Henrique_Becker)
#### Post date: [February 25, 2022, 5:49pm UTC](https://discourse.julialang.org/t/issue-providing-start-hint-values-to-variables-with-gurobi/77079/4 "2022-02-25T17:49:59Z")
Yes, both things are distinct, I think using `start` will end up in [this line](https://github.com/jump-dev/Gurobi.jl/blob/master/src/MOI_wrapper/MOI_wrapper.jl#L3267) so it sets the “Start” parameter, not “VarHintVal”. For solver specific variable attributes your probably will need to unwrap your variables to get the solver-specific reference (I know you need to start using`JuMP.index` for this but I do not remember the whole shebang) and then call the low-level functions of `Gurobi.jl` over them.
---
### Author:  [@odow](https://discourse.julialang.org/u/odow)
#### Post date: [February 25, 2022, 6:54pm UTC](https://discourse.julialang.org/t/issue-providing-start-hint-values-to-variables-with-gurobi/77079/5 "2022-02-25T18:54:22Z")
Two options:
1. Use `model = direct_model(Gurobi.Optimizer())`. This gets you closer to the Gurobi model without some of the overhead of JuMP.
2. Your example exposed a bug in Gurobi.jl To work-around it, set `VarHintVal` for every variable (not just `x1`). I’ll open an issue in Gurobi.jl to see if we can fix this.