How to create sub-arrays access the i-th dimension of an array within for()?

Question

In a for-loop, I run in i over an array which I would like to sub-index in dimension i. How can this be done? So a minimal example would be

(A <- array(1:24, dim = 2:4))
A[2,,] # i=1
A[,1,] # i=2
A[,,3] # i=3

where I index 'by foot'. I tried something along the lines of this but wasn't successful. Of course one could could create "2,," as a string and then eval & parse the code, but that's ugly. Also, inside the for loop (over i), I could use aperm() to permute the array such that the new first dimension is the former ith, so that I can simply access the first component. But that's kind of ugly too and requires to permute the array back. Any ideas how to do it more R-like/elegantly?

The actual problem is for a multi-dimensional table() object, but I think the idea will remain the same.

Update

I accepted Rick's answer. I just present it with a for loop and simplified it further:

subindex <- c(2,1,3) # in the ith dimension, we would like to subindex by subindex[i]
for(i in seq_along(dim(A))) {
    args <- list(1:2, 1:3, 1:4)
    args[i] <- subindex[i]
    print(do.call("[", c(list(A), args)))
}

Rick · Accepted Answer · 2015-03-06 23:28:49Z

2

#Build a multidimensional array
A <- array(1:24, dim = 2:4)
# Select a sub-array 
indexNumber = 2
indexSelection = 1

# Build a parameter list indexing all the elements of A
parameters <- list(A, 1:2, 1:3, 1:4)
# Modify the appropriate list element to a single value
parameters[1 + indexNumber] <- indexSelection   
# select the desired subarray
do.call("[", parameters)

answered Mar 6, 2015 at 23:28

Rick

8988 silver badges10 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Rick · Accepted Answer · 2015-03-07 02:41:19Z

0

# Now for something completely different!
#Build a multidimensional array
A <- array(1:24, dim = 2:4)
# Select a sub-array 
indexNumber = 2
indexSelection = 1

reduced <- A[slice.index(A, indexNumber) == indexSelection]
dim(reduced) <- dim(A)[-indexNumber]

# Also works on the left-side
A[slice.index(A, 2)==2] <- -1:-8

edited Mar 7, 2015 at 2:41

answered Mar 7, 2015 at 2:17

Rick

8988 silver badges10 bronze badges

Collectives™ on Stack Overflow

How to create sub-arrays access the i-th dimension of an array within for()?

2 Answers 2

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related