Index of a grid

General Usage
1

My goal is to find the index of a grid that meet certain criteria. For example, I have an X Y that is meshgridded together using the below function:
X, Y = ndgrid([-180:1:180], [-90:1:90];

So my Y will be like a matrix like the below:

Y = [-90, -89, -88, ... 90;
...
-90, -89, -88, ... 90];

How do I find the Index of the grid with Y>80, so that Y[Index] will give me a grid where Y>80? Why didn’t the below work? It gives me a vector, instead of a matrix.
Ind = Y .> 80;

Many thanks!

2

Can you make an MWE? I don’t think Y is a matrix because:

julia> Y = rand(5, 5);

julia> Y .> 0.5
5×5 BitMatrix:
 0  1  0  1  1
 0  1  1  0  0
 0  0  0  1  0
 0  1  1  1  0
 0  0  0  0  1
2 Likes
3

Hi there, I tried the following and got a matrix:

julia> Y = rand(1:20, 5,5)
5×5 Matrix{Int64}:
  6  10   1  10   3
 16  11  12   3  11
 13  20   1   4   3
  1   9   1  12   1
 15  13  15   7   6
julia> Y .> 10
5×5 BitMatrix:
 0  0  0  0  0
 1  1  1  0  1
 1  1  0  0  0
 0  0  0  1  0
 1  1  1  0  0

However, if you want indices in another format, try:

julia> findall(Y .> 10)
10-element Vector{CartesianIndex{2}}:
 CartesianIndex(2, 1)
 CartesianIndex(3, 1)
 CartesianIndex(5, 1)
 CartesianIndex(2, 2)
 CartesianIndex(3, 2)
 CartesianIndex(5, 2)
 CartesianIndex(2, 3)
 CartesianIndex(5, 3)
 CartesianIndex(4, 4)
 CartesianIndex(2, 5)
1 Like
4

Thanks for the reply.

It’s true that the Index is a matrix, but when you want to get the real values of the Y by doing the below:
Y[Y .> 0.5]
it will become a vector.

5

Unfortunately, I got the same issue:

Ind = findall(Y .> 10);
Y_new = Y[Ind];

will give me a vector as well.

6

if I have this matrix:

julia> a = rand(0:1, 3,3)
3×3 Matrix{Int64}:
 1  0  0
 0  0  1
 1  0  0

what is the “grid” you expect/want to get by doing

a[a .> 0]

?

2 Likes
7

Hmm do you want something like a matrix of only the selected terms and everything else zero? If so, then:

julia> Ynew = zeros(5,5)
5×5 Matrix{Float64}:
 0.0  0.0  0.0  0.0  0.0
 0.0  0.0  0.0  0.0  0.0
 0.0  0.0  0.0  0.0  0.0
 0.0  0.0  0.0  0.0  0.0
 0.0  0.0  0.0  0.0  0.0

julia> Ynew[inds] .= Y[inds];

julia> Ynew
5×5 Matrix{Float64}:
  0.0   0.0   0.0   0.0   0.0
 16.0  11.0  12.0   0.0  11.0
 13.0  20.0   0.0   0.0   0.0
  0.0   0.0   0.0  12.0   0.0
 15.0  13.0  15.0   0.0   0.0

Otherwise, what Y[inds] does is return a vector of all the values in Y that match your criterion. That is, inds is a vector of indices that match your criterion and then you get a vector out of Y of those corresponding indices. Does this make sense?

1 Like
8

Guess I can just do a reshape and get what I want.

Many thanks All.

9

it’s about the intention being ambiguous. if your answer is it should return

[1 1
 1 0]

then why not

[1 0
 1 1]

if you know exactly what you will get, just do a reshape, but it’s not clear you can always get back a “matrix” without ambiguity

2 Likes