Why does focal() reduce the rows and columns of my raster?

Hi guys,

This is my first attemp using focal() in raster package.

Looking at the raster dimension after running focal(), it seems that
everything is OK until I plot the raster and found  out that the first and
last column/row has been change to NA.

Any help guys?

Thanks in advance.

r <- raster(ncols=5, nrows=5, xmn=0)
r[] <- runif(ncell(r))
r

r

class       : RasterLayer
dimensions  : 10, 10, 100  (nrow, ncol, ncell)
resolution  : 18, 18  (x, y)
extent      : 0, 180, -90, 90  (xmin, xmax, ymin, ymax)
coord. ref. : +proj=longlat +datum=WGS84 +ellps=WGS84 +towgs84=0,0,0
data source : in memory
names       : layer
values      : 0.01734865, 0.9977548  (min, max)

# 3x3 mean filter
r3 <- focal(r, w=matrix(1/9,nrow=3,ncol=3))
r3

r3

class       : RasterLayer
dimensions  : 10, 10, 100  (nrow, ncol, ncell)
resolution  : 18, 18  (x, y)
extent      : 0, 180, -90, 90  (xmin, xmax, ymin, ymax)
coord. ref. : +proj=longlat +datum=WGS84 +ellps=WGS84 +towgs84=0,0,0
data source : in memory
names       : layer
values      : 0.346758, 0.6850644  (min, max)

#plot 2 rasters
par(mfrow=c(1,2))
plot(r)
plot(r3)

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