How do I create a 1,2,3 3x1 Matrix?

New to Julia
1

While introducing Arrays, I want to explain the difference in Julia between a normal column vector and a matrix with a single column… Is there a way to create a one-column matrix like b = collect([1 2 3]') in a simpler way (and without using rand, zeros, the Array constructor with undef/nothing…) ?

2 
reshape([1:3;],3,1)
3

Or even

reshape([1:3;], :, 1)

if you do not want to care about number of elements.

1 Like
4

There is no way to create it with just using the brackets syntax without introducing other functions, including reshape ?

5

[ i for i=1:3, j=1:1] but it is not compact.

6

FYI, probably still not what you looking for though.

[1:3 1:3][:, [1]]
7

Matrix{Float64}(undef, 3, 1)
Does that work for you?

8

normally I write hcat(1:3)

4 Likes
9

Or maybe fill(0,(3,1)).

10

yeah, thanks everyone, I think the answer is that it is not possible to use some simple a = [1 ; 2 ; 3 ;] like syntax to create a one-column matrix, as it is reinterpreted as a column vector… I’ll use one of the methods you posted, it is just that they all use already some other concepts…

11

a=[1 2 3]'

2 Likes
12 
[1, 2, 3][:,:]
3×1 Matrix{Int64}:
 1
 2
 3

And if @DNF was online, he would likely suggest:

permutedims([1 2 3])
4 Likes
13

The limitation is that in bracket notation a space is used to indicate hcat and it is ignored if you have only one argument passed.

1 Like
14

One wouldn’t recommend this, but it is interesting:

using LinearAlgebra
julia> [1, 2, 3]*I
3×1 Matrix{Int64}:
 1
 2
 3
15

Note that there is a PR Syntax for multidimensional arrays by BioTurboNick · Pull Request #33697 · JuliaLang/julia · GitHub, which, if I understand it correctly, means that a = [1 ; 2 ; 3 ;;] may be a future syntax for this. Specifically see the proposed NEWS entry.

4 Likes
16

I thought a Vector was a synonym for a 1 Column Matrix? Why would there be a need to differentiate?

1 Like
17

There are many reasons, e.g.:

  • they have different ndims
  • you can resize Vector, but you cannot resize Matrix
  • there are functions that accept only Vector or only Matrix (especially in LinearAlgebra)
  • Tables.jl tables usually require AbstractVector, but disallow 1-column AbstractMatrix as columns
1 Like
18

Mathematically they are the same, but they are different objects in Julia. [1,2,3] is a uni-dimensional array, collect([1 2 3]') is a two- dimensional array where “it happens” that the second dimension has size 1, i.e. there is only one column. So, if a function requires AbstractArray{T,1} the first will fit, but the second will not.
Note that this possible confusion doesn’t arise with row vectors, as all row vectors in Julia have 2 dimensions.

19

You can also use

Matrix([1,2,3]')

if that’s clearer than collect

20

@tomerarnon, you probably meant:

Matrix([1 2 3]')
3×1 Matrix{Int64}:
1 Like