Jump.jl: 'fixate' function for convenience

[martinbiel]

In some problem formulations, for example MPC problems, it would be convenient to be able to specify the state variables as
@variable(m, x[1:5])
and then fix x[1,1] to some given initial value. Currently, it is achievable through setlowerbound(x[1,1],x_init) and setupperbound(x[1,1],x_init), but I also found that by allowing :Fixed in setcategory I was able to do

@variable(m, x[1:5])
setcategory(x[1,1],:Fixed)
setvalue(x[1,1],1)

which on print yields

Subject to
 x[1] = 1
 x[2] free
 x[3] free
 x[4] free
 x[5] free

Would it be possible to allow for this behavior for convenience and readability? for example through some fixate function:

function fixate(v::Variable, val::Number)
    v.m.colVal[v.col] = val
    v.m.colCat[v.col] = :Fixed
    v.m.colLower[v.col] = val
    v.m.colUpper[v.col] = val
end

[joehuchette]

I'm not opposed to this, though I'm not crazy about the name. @mlubin?

[mlubin]

I'm open to a PR. Maybe JuMP.fix?

[blegat]

Would it make more sense as a macro or as a function ?

[mlubin]

I don't see much value in the additional complexity of using a macro.

[joaquimg]

Has anybody done something? If not, I volunteer... the function was even written in this post...

[joehuchette]

Go for it!

On Tue, Dec 20, 2016 at 12:11 PM -0500, "Joaquim Dias Garcia" [e-mail] wrote:

Has anybody done something? If not, I volunteer... the function was even written in this post...

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[blegat]

@joaquimg I was planning to do it but I didn't find the time :( Thanks !

[mlubin]

Fixed by https://github.com/JuliaOpt/JuMP.jl/pull/929. Thanks @joaquimg!