efficient way to find neighbors to specified cell? - r

Question

Looking for sensible code to solve the following problem without a stack of "if" comparisons:
dput(acell)
structure(c(0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0,
0, 0, 0, 0, 0, 0, 0, 0), .Dim = c(5L, 5L))
dput(bcell)
structure(c(0, 0, 1, 0, 1, 0, 1, 0, 1, 0, 0, 0, 1, 0, 0, 0, 0,
1, 0, 1, 0, 0, 0, 0, 1), .Dim = c(5L, 5L))
It's guaranteed by construction (elsewhere in my code) that there is at least one nonzero entry in bcell in the four locations left,right,up,down one cell from the location of the nonzero value ([2,3]) in acell . Is there some better way to return the indices of all such nonzero cells? I'm currently using (pseudocode), for coords i,j referring to the nonzero element of acell ,
if( bcell[i-1,j]>0 ) found_values<-rbind(found_values,c(i-1,j))
if (bcell[i+1,j]>0 ) found_values<-rbind(found_values,c(i+1,j))
and so on.

Answer 1

Another possibility
# index of non-zero in acell
id <- which(acell == 1, arr.ind = TRUE)
# index of neighbors
rows <- id[1] + c(0, 0, -1, 1)
cols <- id[2] + c(-1, 1, 0, 0)
idx <- cbind(rows, cols)
# values of neighbors in bcell
vals <- bcell[idx]
# index of non-zero neighbors
idx[vals != 0, ]
# rows cols
# [1,] 2 2
# [2,] 3 3
Update: An alternative
# index of non-zero in acell
id <- which(acell == 1, arr.ind = TRUE)
id
# create a matrix with cells adjacent to 'id'
# 'non-neighbor' cells are multiplied by zero
m <- matrix(c(0, 1, 0,
1, 0, 1,
0, 1, 0), ncol = 3) *
bcell[id[1] + (-1:1), id[2] + (-1:1)]
# index of non-zero neighbours
idx <- which(m != 0, arr.ind = TRUE)
# 'de-center' the centered indices
idx2 <- sapply(1:2, function(x) id[x] + (idx[ , x] - 2))
colnames(idx2) <- c("rows", "cols")
idx2
# rows cols
# [1,] 2 2
# [2,] 3 3

Answer 2

Here is a start:
dim <- dim(acell)
lookfor <- which(acell != 0)
lookup <- which(bcell != 0)
if (!((lookfor %% dim[1]) %in% c(1L,0L)) &
!((lookfor %% dim[2]) %in% c(1L,0L)) ) {
neighborsR <- lookup[abs(lookfor-lookup) == 1L]
neighborsC <- lookup[(lookfor %% dim[1] == lookup %% dim[1]) &
abs(lookfor %/% dim[1] - lookup %/% dim[1]) == 1L]
}
neighbors <- c(neighborsR, neighborsC)
res <- cbind(neighbors %% dim[1], neighbors %/% dim[1]+1)
colnames(res) <- c("row", "col")
# row col
#[1,] 3 3
#[2,] 2 2
This can only handle one lookfor value, which is not in the first/last row/column.

Answer 3

neighbors <- function(mat) {
nr <- nrow(mat)
nc <- ncol(mat)
ones <- which(mat == 1)
adjacent <- c(
Filter(function(x) x %% nr != 0, ones - 1) ## above
, Filter(function(x) x %% nr != 1, ones + 1) ## below
, ones - nr ## left
, ones + nr ## right
)
adjacent <- unique(Filter(function(x) x > 0 && x <= nr * nc, adjacent))
out <- matrix(FALSE, nr, nc)
out[adjacent] <- TRUE
out
}
which(neighbors(acell) & bcell, arr.ind = TRUE)
## rows cols
## [1,] 2 2
## [2,] 3 3