An error from stacking a new array with an initialization array (empty)

General Usage
1

Here is my code:

A = [];
for ...
A = vcat(A, b);
end

I’m getting the below error. Before A is assigned any values, it is an empty array, and the program is complaining that:
number of columns of each array must match

size(A) = (0,)
size(b) = (26, 29)

Clearly, this is a bad habit I brought here from Matlab, so how do Julian folks get around this issue? Many thanks!

2

Maybe you want to push! into A? Note that growing arrays like that is not very performant.

1 Like
3

Many thanks for the reply!

So I should do push!(A, b)? Unfortunately, it does not work either. How do I specify the fact that I want the item being pushed to be appended vertically?

4

can you provide a simple example of the code?

5

The problem is that you can’t vcat a matrix and a vector. And even if your A were a matrix, you still can’t vcat a 26x29 matrix to a 0x0 matrix (for a vertical concatenation, the number of rows must match).

You could create a 0x29 matrix and then use vcat, but it should be easier to avoid this entirely using reduce:

julia> my_matrices = [[1 2; 3 4], [5 6; 7 8], [9 10; 11 12]]
3-element Vector{Matrix{Int64}}:
 [1 2; 3 4]
 [5 6; 7 8]
 [9 10; 11 12]

julia> reduce(vcat, my_matrices)
6×2 Matrix{Int64}:
  1   2
  3   4
  5   6
  7   8
  9  10
 11  12

reduce combines elements of a collection pairwise, and vcat does vertical concatenation, so reduce(vcat, ...) combines all the matrices in the collection by vertical concatenation. Julia actually has a special implementation (still in Julia, just written carefully) that makes that particular combination of operations more efficient than a naive approach. If you want to see how it works, you can do: @edit reduce(vcat, my_matrices)

1 Like
6

Actually, it did not work either.

Here is my code:

using MAT
# Read the bathymetric values out of the mat file:
F1 = matopen("/test.mat");
A = read(F1, "A");
b = read(F1, "b");
close(F1);
A = reduce(vcat, [A, b])

Here is a link to download the “test.mat” file storing my data:

7

Looks like it will work if I define my A as the below, instead of using A=;

A = Array{Float64}(undef, 0, 29);