Identify cells that are adjacent to a set of cells on a raster.

# S4 method for class 'BasicRaster'
adjacent(x, cells, directions=4, pairs=TRUE, target=NULL, sorted=FALSE,
include=FALSE, id=FALSE, ...)

## Arguments

x

Raster* object

cells

vector of cell numbers for which adjacent cells should be found. Cell numbers start with 1 in the upper-left corner and increase from left to right and from top to bottom

directions

the number of directions in which cells should be connected: 4 (rook's case), 8 (queen's case), 16 (knight and one-cell queen moves), or 'bishop' to connect cells with one-cell diagonal moves. Or a neighborhood matrix (see Details)

pairs

logical. If TRUE, a matrix of pairs of adjacent cells is returned. If FALSE, a vector of cells adjacent to cells is returned

target

optional vector of target cell numbers that should be considered. All other adjacent cells are ignored

sorted

logical. Should the results be sorted?

include

logical. Should the focal cells be included in the result?

id

logical. Should the id of the cells be included in the result? (numbered from 1 to length(cells)

...

## Details

A neighborhood matrix identifies the cells around each cell that are considered adjacent. The matrix should have one, and only one, cell with value 0 (the focal cell); at least one cell with value 1 (the adjacent cell(s)); All other cells are not considered adjacent and ignored.

## Value

matrix or vector with adjacent cells.

## Author

Robert J. Hijmans and Jacob van Etten

## Examples

r <- raster(nrows=10, ncols=10)
#>       from to
#>  [1,]   55 44
#>  [2,]    1 10
#>  [3,]   55 54
#>  [4,]    1 20
#>  [5,]   55 64
#>  [6,]   55 46
#>  [7,]    1  2
#>  [8,]   55 56
#>  [9,]    1 12
#> [10,]   55 66
#> [11,]   55 45
#> [12,]    1 11
#> [13,]   55 65

a <- adjacent(r, cell = c(1,55,90), directions=4, sorted=TRUE)
a
#>       from  to
#>  [1,]    1   2
#>  [2,]    1  10
#>  [3,]    1  11
#>  [4,]   55  45
#>  [5,]   55  54
#>  [6,]   55  56
#>  [7,]   55  65
#>  [8,]   90  80
#>  [9,]   90  81
#> [10,]   90  89
#> [11,]   90 100

r[c(1,55,90)] <- 1
r[a] <- 2
plot(r)

# same result as above
rook <- matrix(c(NA, 1, NA,
1, 0,  1,
NA, 1, NA), ncol=3, byrow=TRUE)

adjacent(r, cells = c(1,55,90), directions=rook, sorted=TRUE)
#>       from  to
#>  [1,]    1   2
#>  [2,]    1  10
#>  [3,]    1  11
#>  [4,]   55  45
#>  [5,]   55  54
#>  [6,]   55  56
#>  [7,]   55  65
#>  [8,]   90  80
#>  [9,]   90  81
#> [10,]   90  89
#> [11,]   90 100

# Count the number of times that a cell with a certain value
# occurs next to a cell with a certain value
set.seed(0)
r <- raster(ncol=10, nrow=10)
values(r) <- round(runif(ncell(r)) * 5)
a <- adjacent(r, 1:ncell(r), 4, pairs=TRUE)
tb <- table(r[a[,1]], r[a[,2]])
tb
#>
#>      0  1  2  3  4  5
#>   0  2  4  7  4 10  0
#>   1  4 14 14  7 19  6
#>   2  7 14 30 15 35  8
#>   3  4  7 15 14 13  3
#>   4 10 19 35 13 20  5
#>   5  0  6  8  3  5  0
# make a matrix out of the 'table' object
tb <- unclass(tb)
plot(raster(tb, xmn=-0.5, xmx=5.5, ymn=-0.5, ymx=5.5))