CODEWITHSUNDEEP
r
J S
J S

Reputation: 75

Select array entries based on 2D matrix values

I have a simple question that I seem to not be able to wrap my head around. I have a 3D array (an image stack) and I am trying to do a projection onto 2D array (e.g. a maximum intensity projection for an image stack). To do this, I have a matrix of indices which indicate the z-stack to use for each pixel.

For example, I have a 3D array which looks like this:

, , 1

     [,1] [,2]
[1,]    1    3
[2,]    2    4

, , 2

     [,1] [,2]
[1,]    5    7
[2,]    6    8

And a "selection matrix" which looks like this:

     [,1] [,2]
[1,]    1    1
[2,]    2    2

The result of this selection should then be

     [,1] [,2]
[1,]    1    3
[2,]    6    8

i.e. the matrix indicates from which "z-columns" of the array to extract the values.

I know this should be a stupid simple thing but I'm drawing a blank on how to do this. Thank you in advance!

Upvotes: 2

Views: 46

Answers (2)

aichao
aichao

Reputation: 7445

One way to do this is to use multi-dimensional array indexing via cbind:

Here is your data and selection matrix:

d <- array(1:8,c(2,2,2))
selec <- matrix(c(1,2,1,2),2,2)

First construct a grid of your selection matrix indices:

selec.ind <- expand.grid(1:nrow(selec),1:ncol(selec))

Then use this with the selection matrix values to access d:

out <- matrix(d[cbind(selec.ind$Var1,selec.ind$Var2,as.vector(selec))], nrow(selec), ncol(selec))
##     [,1] [,2]
##[1,]    1    3
##[2,]    6    8

Finally, reshape the result back to the size of the selection matrix.

This will work with any size selection matrix and any number of layers in z.

Upvotes: 1

Haboryme
Haboryme

Reputation: 4761

This only works for array with 2 third dimensions (dim=c(...,...,2)) The data:

ar=array(data=c(1,2,3,4,5,6,7,8),dim = c(2,2,2))
, , 1

     [,1] [,2]
[1,]    1    3
[2,]    2    4

, , 2

     [,1] [,2]
[1,]    5    7
[2,]    6    8
selec=matrix(c(1,2,1,2),nrow = 2,ncol = 2)
     [,1] [,2]
[1,]    1    1
[2,]    2    2

We use ifelse

ifelse(selec==1,ar[,,1],ar[,,2])
     [,1] [,2]
[1,]    1    3
[2,]    6    8

Upvotes: 0

Related Questions