Fill values for multiple dimension array in R

Question

I want to fill values into a multiple dimension array in R. I know how to implement them in a loop. See below

a <- array(seq(1, 8), dim = rep(2, 3))

Case 1

b <- a
for (i in seq(length = dim(a)[2]))
{
    for (j in seq(length = dim(a)[3]))
    {
        b[,i,j] <- b[,1,1]
    }
}

i.e - fill first column of first stratum in all other columns:

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

Case 2

b <- a
for (i in seq(length = dim(a)[2]))
{
    b[,i,] <- b[,1,]
}

i.e. fill first column of each strata in remaining columns of the strata:

#, , 1
#
#     [,1] [,2]
#[1,]    1    1
#[2,]    2    2
#
#, , 2
#
#     [,1] [,2]
#[1,]    5    5
#[2,]    6    6

Case 3

b <- a
for (i in seq(length = dim(a)[3]))
{
    b[,,i] <- b[,,1]
}

i.e. fill first strata contents into all other strata:

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

How could I vectorize it? Thanks for any suggestions.

Jthorpe · Accepted Answer · 2015-02-11 03:42:03Z

4

Here are the generalizations of your for loops using index vectors.

a <- array(seq(1, 24), dim = 2:4)

# a couple of handy index vectors
s2 <- seq(length = dim(a)[2])
s3 <- seq(length = dim(a)[3])

Case 1

B  <-  b <- a
for (i in seq(length = dim(a)[2]))
{
    for (j in seq(length = dim(a)[3]))
    {
        b[,i,j] <- b[,1,1]
    }
}


B[,s2,s3] <- B[,1,1]
identical(B,b)

Case 2

B  <-  b <- a
for (i in seq(length = dim(a)[2]))
{
    b[,i,] <- b[,1,]
}

# this is the tricky one.  The assignment works
# because we perumte the the matrix B so that we're 
# assigning to the last index.
B<- aperm(B,c(1,3,2)) 
B[,,s2] <- B[,,1]
B<- aperm(B,c(1,3,2)) 
identical(B,b)

Case 3

B  <-  b <- a
for (i in seq(length = dim(a)[3]))
{
    b[,,i] <- b[,,1]
}

B[,,s3] <- B[,,1]
identical(B,b)

edited Feb 11, 2015 at 3:42

answered Feb 11, 2015 at 3:16

Jthorpe

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1 Comment

thelatemail Over a year ago

Case 1 and Case 3 could just be a[] <- a[,1,1] and a[] <- a[,,1]

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