Slice an array — rray

rray_slice() selects positions along each axis of x. The result always has the same dimensionality as x.

rray_slice_axis() is rray_slice() restricted to a single axis, which can be more ergonomic to write. rray_slice_rows() and rray_slice_columns() are shortcuts for axis = 1 and axis = 2.

The _assign versions replace the selected positions with value and return a modified copy of x.

Usage

rray_slice(x, ...)

rray_slice_axis(x, i, ..., axis)

rray_slice_rows(x, i)

rray_slice_columns(x, i)

rray_slice_assign(x, ..., value)

rray_slice_assign_axis(x, i, ..., axis, value)

rray_slice_assign_rows(x, i, value)

rray_slice_assign_columns(x, i, value)

Arguments

x

An array.

...

One unnamed subscript for each axis of x, in axis order. Each subscript is one of:

TRUE, to select the whole axis.
A logical vector the size of the axis. TRUE selects a position.
An integer or double vector of locations. Negative values drop positions, zero is ignored, and duplicates repeat positions.
A character vector of names, matched against the names of the axis. The first match is used when names are duplicated.
NULL, to select nothing.

For rray_slice_axis() and rray_slice_assign_axis(), these dots must be empty.

i

A subscript for the selected axis. It takes any of the forms allowed in ....

axis

A single integer. The axis to slice along.

value

An array to assign to the selected positions. It is cast to the type of x, then broadcast to the dimensions of the selection.

Value

rray_slice(), rray_slice_axis(), rray_slice_rows(), and rray_slice_columns() return an array with the same type and dimensionality as x. The dimension of each axis is the number of positions selected on it. The names of each axis are selected along with the values.
The _assign versions return x with the selected positions replaced. The type, dimensions, and names of x are kept.

Details

NA in a subscript gives missing values in the result. For raw arrays the missing value is as.raw(0), and for list arrays it is NULL. If the axis has names, the name of a missing value is "". For assignment, NA in a subscript leaves x unchanged at that position but uses one element of value.

Unlike [, an empty argument does not select a whole axis. Use TRUE instead, so that every axis is provided explicitly:

x <- array(1:24, c(2L, 3L, 4L))

# Base R
x[1, , , drop = FALSE]

# rray
rray_slice(x, 1, TRUE, TRUE)

# Or, more simply
rray_slice_axis(x, 1, axis = 1)
rray_slice_rows(x, 1)

Examples

x <- array(1:24, c(2L, 3L, 4L))

# The first row
rray_slice(x, 1, TRUE, TRUE)
#> , , 1
#> 
#>      [,1] [,2] [,3]
#> [1,]    1    3    5
#> 
#> , , 2
#> 
#>      [,1] [,2] [,3]
#> [1,]    7    9   11
#> 
#> , , 3
#> 
#>      [,1] [,2] [,3]
#> [1,]   13   15   17
#> 
#> , , 4
#> 
#>      [,1] [,2] [,3]
#> [1,]   19   21   23
#> 

# Reorder and repeat positions
rray_slice(x, c(2, 1, 2), 1, 4)
#> , , 1
#> 
#>      [,1]
#> [1,]   20
#> [2,]   19
#> [3,]   20
#> 

# Drop positions with negative locations
rray_slice(x, TRUE, -2, -(1:2))
#> , , 1
#> 
#>      [,1] [,2]
#> [1,]   13   17
#> [2,]   14   18
#> 
#> , , 2
#> 
#>      [,1] [,2]
#> [1,]   19   23
#> [2,]   20   24
#> 

# Select by name
y <- array(1:6, c(2L, 3L), dimnames = list(c("a", "b"), c("c", "d", "e")))
rray_slice(y, "b", c("e", "c"))
#>   e c
#> b 6 2

# Subscripts can be spliced into `...`
subscripts <- rep(list(TRUE), rray_dimensionality(x))
subscripts[[3]] <- c(4, 1)
rray_slice(x, !!!subscripts)
#> , , 1
#> 
#>      [,1] [,2] [,3]
#> [1,]   19   21   23
#> [2,]   20   22   24
#> 
#> , , 2
#> 
#>      [,1] [,2] [,3]
#> [1,]    1    3    5
#> [2,]    2    4    6
#> 

# Slice a single axis
rray_slice_axis(x, c(4, 1), axis = 3)
#> , , 1
#> 
#>      [,1] [,2] [,3]
#> [1,]   19   21   23
#> [2,]   20   22   24
#> 
#> , , 2
#> 
#>      [,1] [,2] [,3]
#> [1,]    1    3    5
#> [2,]    2    4    6
#> 
rray_slice_rows(y, c("b", "a"))
#>   c d e
#> b 2 4 6
#> a 1 3 5
rray_slice_columns(x, -2)
#> , , 1
#> 
#>      [,1] [,2]
#> [1,]    1    5
#> [2,]    2    6
#> 
#> , , 2
#> 
#>      [,1] [,2]
#> [1,]    7   11
#> [2,]    8   12
#> 
#> , , 3
#> 
#>      [,1] [,2]
#> [1,]   13   17
#> [2,]   14   18
#> 
#> , , 4
#> 
#>      [,1] [,2]
#> [1,]   19   23
#> [2,]   20   24
#> 

# Assign one value to every first row
rray_slice_assign(x, 1, TRUE, TRUE, value = 0L)
#> , , 1
#> 
#>      [,1] [,2] [,3]
#> [1,]    0    0    0
#> [2,]    2    4    6
#> 
#> , , 2
#> 
#>      [,1] [,2] [,3]
#> [1,]    0    0    0
#> [2,]    8   10   12
#> 
#> , , 3
#> 
#>      [,1] [,2] [,3]
#> [1,]    0    0    0
#> [2,]   14   16   18
#> 
#> , , 4
#> 
#>      [,1] [,2] [,3]
#> [1,]    0    0    0
#> [2,]   20   22   24
#> 
rray_slice_assign_rows(x, 1, 0L)
#> , , 1
#> 
#>      [,1] [,2] [,3]
#> [1,]    0    0    0
#> [2,]    2    4    6
#> 
#> , , 2
#> 
#>      [,1] [,2] [,3]
#> [1,]    0    0    0
#> [2,]    8   10   12
#> 
#> , , 3
#> 
#>      [,1] [,2] [,3]
#> [1,]    0    0    0
#> [2,]   14   16   18
#> 
#> , , 4
#> 
#>      [,1] [,2] [,3]
#> [1,]    0    0    0
#> [2,]   20   22   24
#> 

# Or broadcast `value` to the dimensions of the selection
value <- array(c(100L, 200L), c(1L, 2L))
rray_slice_assign(x, TRUE, c(1, 3), 1, value = value)
#> , , 1
#> 
#>      [,1] [,2] [,3]
#> [1,]  100    3  200
#> [2,]  100    4  200
#> 
#> , , 2
#> 
#>      [,1] [,2] [,3]
#> [1,]    7    9   11
#> [2,]    8   10   12
#> 
#> , , 3
#> 
#>      [,1] [,2] [,3]
#> [1,]   13   15   17
#> [2,]   14   16   18
#> 
#> , , 4
#> 
#>      [,1] [,2] [,3]
#> [1,]   19   21   23
#> [2,]   20   22   24
#> 

value <- array(c(100L, 200L), c(2L, 1L))
rray_slice_assign_axis(x, 3, axis = 2, value = value)
#> , , 1
#> 
#>      [,1] [,2] [,3]
#> [1,]    1    3  100
#> [2,]    2    4  200
#> 
#> , , 2
#> 
#>      [,1] [,2] [,3]
#> [1,]    7    9  100
#> [2,]    8   10  200
#> 
#> , , 3
#> 
#>      [,1] [,2] [,3]
#> [1,]   13   15  100
#> [2,]   14   16  200
#> 
#> , , 4
#> 
#>      [,1] [,2] [,3]
#> [1,]   19   21  100
#> [2,]   20   22  200
#>