How to Convert Matrix Indexing Code from Matlab to R
I Have a One-Dimensional Matrix Called H1 and a 3-Dimensional Matrix Called Nv in My Matlab Code. I Then Run the Code H1(Nv) to Create a New 3 Column Matrix...
I have a one-dimensional matrix called h1 and a 3-dimensional matrix called nv in my MATLAB code. I then run the code h1(nv) to create a new 3 column matrix hc. I need to convert this code into R. I am not sure how to do this sort of matrix indexing in R. I thought you used brackets for matrix indexing so I tried just doing h1[nv] but that gave me a 1-dimensional array instead of 3 columns like my MATLAB output so it definitely isn't doing the same thing.
1 Answer
Frustratingly (personal opinion), R's arrays drop a dimension on indexing if the indexing reduces any of its dimensions to length-1.
For example:
m <- matrix(1:6, nrow=2)
m[1,]
# [1] 1 3 5
m[,2]
# [1] 3 4
This can be seen a little more clearly (perhaps) when looking at a 3-dim array:
ary <- array(1:24, dim=c(3,4,2))
ary
# , , 1
# [,1] [,2] [,3] [,4]
# [1,] 1 4 7 10
# [2,] 2 5 8 11
# [3,] 3 6 9 12
# , , 2
# [,1] [,2] [,3] [,4]
# [1,] 13 16 19 22
# [2,] 14 17 20 23
# [3,] 15 18 21 24
ary[1,,]
# [,1] [,2]
# [1,] 1 13
# [2,] 4 16
# [3,] 7 19
# [4,] 10 22
ary[,2,]
# [,1] [,2]
# [1,] 4 16
# [2,] 5 17
# [3,] 6 18
This can be avoided by adding ,drop=FALSE within the [-indexing brackets:
m[1,,drop=FALSE]
# [,1] [,2] [,3]
# [1,] 1 3 5
m[,2,drop=FALSE]
# [,1]
# [1,] 3
# [2,] 4
ary[1,,,drop=FALSE]
# , , 1
# [,1] [,2] [,3] [,4]
# [1,] 1 4 7 10
# , , 2
# [,1] [,2] [,3] [,4]
# [1,] 13 16 19 22
(Note that you must include any intermediate unused commas, as n ary[1,,,drop=FALSE]. The drop= argument must always be in the (n+1)th position where the array has n dimensions.)
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