CODEWITHSUNDEEP
multidimensional-arrayjulia
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a06e

Reputation: 20794

How to construct a matrix in Julia sequentially?

I need to construct a matrix in Julia by appending rows. The idea is this:

mat = []     # initialize empty mat
for i = 1:5
    x, y = f(i), g(i)    # here f and g are previously defined functions
    mat = [mat; [x y]]
end

This doesn't work, I get an error:

ERROR: DimensionMismatch("mismatch in dimension 2 (expected 2 got 1)")

in the line where I try to append to mat. I am new to Julia. How can I accomplish what I want?

Upvotes: 1

Views: 1150

Answers (1)

Dan Getz
Dan Getz

Reputation: 18227

The initial mat = [] creates a 1-dim array. The proper way to initialize would be mat = reshape([],0,2). Perhaps typing the array is also recommended. For example, mat = Array{Float64}(0,2).

Concatenating rows this way is costly because of the column-first ordering of arrays in memory used by Julia. Consider defining the full array and assigning elements in the loop. Possibly mat = Array{Float64}(5,2).

Also, a commenter suggested Matrix(0,2) which is another method to initialize a 0x2 matrix.

Upvotes: 4

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