在函数中使用R替换for循环

Question

I have created a function which requires a raster and a matrix.Every row of the matrix holds the coordinates of a cell in the raster. After finishing my function returns a vector which contains the number of every cell that is saved in my matrix via it's coordinates:

library(raster)
library(gdistance)
r <- raster(nrow=100,ncol=15)
r[] <- 1:ncell(r)
plot(r)
pts <- matrix(c(-144,72.9,-168,45.9,-144,13.5),ncol=2,nrow=3,byrow=TRUE)

get_cells <- function(raster,points_matrix) {
Cells <- c()
for (i in 1:nrow(points_matrix))
    {
    Cells[i] <- cellFromXY(raster,c(points_matrix[i,]))
    }
    return(Cells)

}

Now I would like to restructure my function so that I can use it with apply() . So in the end I want to be able to type something like:

apply(pts,1,get_cells)

and get the same result, but I can't figure out how to do it. Any kind of help is highly appreciated.

Answer 1

You can define one line function within apply function. Try this

apply(pts,1,function(x) return(cellFromXY(raster,x)))

EDIT: @snoops apply does takes user defined functions also like yours. Your Problem is you have two input arguments in your function while your code with apply you are providing only one argument. And you do not need loop inside your function.

get_cells <- function(x,raster) {
    Cells <- cellFromXY(raster,x)
    return(Cells)
}

Now this will work. You can give other arguments of your function inside apply function itself like this.

apply(pts,1,getCells,raster = r)