How to slice a n dimensional array with a m*(n-i) dimensional matrix keeping some dimensions free?

问题

If I have a n dimensional array it can be sliced by a m * n matrix like this

a <- array(1:27,c(3,3,3))

b <- matrix(1:3,3,3)

# This will return the index a[1,1,1] a[2,2,2] and a[3,3,3]
# That is one result for each row in the b matrix
a[b]

# Output
[1]  1 14 27

In the above example the array a is sliced the same way as if it was sliced by each row in the b matrix and the result was combined.

Is there any "effective and easy" way to do a similar slice but to keep some dimensions free? That is slice a n dimensional array with a m * (n-i) dimensional array and get a i+1 dimensional array as result.

# sliceem
# Function for slicing 
# Parameters
#   a - The n dimensional array to slice
#   b - A m*(n-i) matrix 
#   freeDimension - A vector of length i with dimensions to keep free 
#                    during the slicing
# Returns
#   A i dimensional array where the first dimension have m elements and
#   each element in the first dimension correspond to a slice using
#   a row in the b matrix

#### Examples of desired behavior ####

a <- array(1:27,c(3,3,3))

b <- matrix(1:2,2,2)


sliceem(a,b,freeDimension=2)
# Output (a[1,,1] and a[2,,2])
     [,1] [,2] [,3]
[1,]    1    4    7
[2,]   11   14   17


sliceem(a,b,freeDimension=1)
# Output a[,1,1] and a[,2,2]
     [,1] [,2] [,3]
[1,]    1    2    3
[2,]   13   14   15




b <- matrix(1:2,2,1)
sliceem(a,b,freeDimension=c(2,3))

# Output (a[1,,] and a[2,,]
     [,1] [,2] [,3]
[1,]    1    4    7
[2,]    2    5    8

, , 2

     [,1] [,2] [,3]
[1,]   10   13   16
[2,]   11   14   17

, , 3

     [,1] [,2] [,3]
[1,]   19   22   25
[2,]   20   23   26

来源：https://stackoverflow.com/questions/31947700/how-to-slice-a-n-dimensional-array-with-a-mn-i-dimensional-matrix-keeping-som

标签

slice