equalizing matrices for combining in abind R

问题

I believe that a similar question was asked here, but I can't seem to find it anymore.

I have two matrices with different dimensions, and I want to equalise them so that I can combine them in an array.

for example, I have the following two matrices:

a <- matrix(1:6, 3, 2)
b <- matrix(1:12, 4, 3)
a

     [,1] [,2]
[1,]    1    4
[2,]    2    5
[3,]    3    6

b
     [,1] [,2] [,3]
[1,]    1    5    9
[2,]    2    6   10
[3,]    3    7   11
[4,]    4    8   12

Because I am working with time series data, I would like the added rows/colums to have NAs in them. In my example, matrix a would get an extra column and an extra row only containing NAs like this:

     [,1] [,2] [,3]
[1,]    1    4   NA
[2,]    2    5   NA
[3,]    3    6   NA
[4,]   NA   NA   NA

In my dataset I will have 79 matrices with unequal dimensions, and I need to make them as big as the matrix with the largest dimensions.

回答1:

If b is the largest matrix, you can create a matrix with the same dimensions as b, filled with NA, and replace the rows and columns corresponding to the smaller matrix a with the values of a:

a2 <- "[<-"(x = matrix(NA, nrow = nrow(b), ncol = ncol(b)),
            i = 1:nrow(a), j = 1:ncol(a),
            value = a)
a2
#      [,1] [,2] [,3]
# [1,]    1    4   NA
# [2,]    2    5   NA
# [3,]    3    6   NA
# [4,]   NA   NA   NA

Example with several matrices, where we find the largest matrix and pad all matrices with NA to match the dimension of the largest.

# create some matrices of different size
a <- matrix(1:6, nrow = 3, ncol = 2)
b <- matrix(1:12, nrow = 4, ncol = 3)
c <- matrix(1:4, nrow = 2, ncol = 2)    

# put them in a list
l <- list(a, b, c)

# index of largest (here, max number of rows) matrix in the list
id <- which.max(unlist((lapply(l, nrow))))

# pad matrices with NA
l2 <- lapply(l, function(x){
  x <- "[<-"(x = matrix(NA, nrow = nrow(l[[id]]), ncol = ncol(l[[id]])),
             i = 1:nrow(x), j = 1:ncol(x),
             value = x)
})
l2

# [[1]]
#      [,1] [,2] [,3]
# [1,]    1    4   NA
# [2,]    2    5   NA
# [3,]    3    6   NA
# [4,]   NA   NA   NA
# 
# [[2]]
#      [,1] [,2] [,3]
# [1,]    1    5    9
# [2,]    2    6   10
# [3,]    3    7   11
# [4,]    4    8   12
# 
# [[3]]
#      [,1] [,2] [,3]
# [1,]    1    3   NA
# [2,]    2    4   NA
# [3,]   NA   NA   NA
# [4,]   NA   NA   NA

回答2:

As you only want to extend the small matrix with NA, we can use a simple approach such as:

create a matrix as big as b, with only NA. Code:

extended.a = matrix(NA,nrow(b),ncol(b))
fill this matrix with the values from a. Code:

extended.a[cbind(rep(1:nrow(a),ncol(a)), rep(1:ncol(a),each=nrow(a)))] = a

edit:

As per Roland's suggestion, you can also get the vector of indices with which(..., arr.ind=TRUE).

For example, which(TRUE | a, arr.ind=TRUE)

Or even: which(matrix(TRUE,nrow(a),ncol(a), arr.ind=TRUE)

Or far better, using the expand.grid function: expand.grid(1:nrow(a), 1:ncol(a))

来源：https://stackoverflow.com/questions/22513862/equalizing-matrices-for-combining-in-abind-r

标签

arrays

matrix