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.)
