CODEWITHSUNDEEP
juliaequation-solving
Caspar
Caspar

Reputation: 21

Julia NLsolve gives DimensionMismatch-Error when dimension of codomain is larger than dimension of domain

using NLsolve


function g!(G,x)
    G .= [x[1]^2-x[2] for i in 1:3]
end

nlsolve(g!,[0.1, 0.1])

This code gives a DimensionMismatch("array could not be broadcast to match destination")

When I instead use G .= [x[1]^2-x[2] for i in 1:2] I don't get the error. It seems like NLsolve wants th input to be the same size as the output. Is this a bug or am I doing something wrong?

Upvotes: 2

Views: 240

Answers (2)

Avik Pal
Avik Pal

Reputation: 36

Note that the problem that you are trying to solve is a Nonlinear Least Squares Problem, not Nonlinear Equations. The main difference between them is you are allowed to have under-determined or over-determined systems in the former, which makes getting the size of your output vector tricky. I recommend going over https://docs.sciml.ai/NonlinearSolve/stable/tutorials/getting_started/, which describes the main differences and how to solve them.

Now, coming to your specific problem.

using NonlinearSolve

function g!(G, x, p)  # Note that we need to pass a parameter p for NonlinearSolve.jl
    G[1] = x[1]^2 - x[2]
    G[2] = x[1]^2 - x[2]
    G[3] = x[1]^2 - x[2]
    return nothing
end

Now we will creat the function passing resid_prototype as zeros(3) which tells NonlinearSolve.jl that the residual vector is of length 3

nlf = NonlinearFunction(g!; resid_prototype=zeros(3))
prob = NonlinearLeastSquaresProblem(nlf, [0.1, 0.1])

Finally solve it. solve by default will choose a robust polyalgorithm, else you can also pass in a solver of your choice from https://docs.sciml.ai/NonlinearSolve/stable/solvers/nonlinear_least_squares_solvers/

solve(prob)

Upvotes: 0

Nils Gudat
Nils Gudat

Reputation: 13800

This isn't an NLsolve issue, in a fresh Julia session:

julia> function g!(G,x)
           G .= [x[1]^2-x[2] for i in 1:3]
       end
g! (generic function with 1 method)

julia> g!([0.0, 0.0], [1.0, 1.0])
ERROR: DimensionMismatch("array could not be broadcast to match destination")

Here I am passing G with length 2, and then create an array of length 3 on the right hand side. .= then tries to write the values created on the right hand side into the left hand side array element-by-element, but the sizes of the arrays are incompatible. Even shorter, you are essentially doing:

julia> [0, 0] .= [1, 2, 3]
ERROR: DimensionMismatch("array could not be broadcast to match destination")

Upvotes: 0

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