Changing raster values in specific zone only

Question

I would like to replace raster values only in specific zone of the raster. Say this is my example raster:

library(raster)
r <- raster(ncol=1000, nrow=1000)
r[] <- runif(ncell(r),0,1)
extent(r) <- matrix(c(0, 0, 1000, 1000), nrow=2)

What I would like to do now, is to change all values that equal to val <- 0.8and replace with valnew <- 0.2. The extent of raster where my function should search and replace values is following (only this square of raster should be searched for val values in raster and replaced with newval)

extentnewvals <- matrix(c(300, 300, 500, 500), nrow=2)

Any ideas how to approach it?

Answer 1

I will present a workaround with two pairs of brackets, maybe there is another method more efficient. I rounded values to make this example easiest. Use the first pair of brackets to select values inside the specific extent and the second pair of brackets to change the desired values. Check histogram and plot to view differences:

library(raster)

r <- raster(ncol=1000, nrow=1000)
r[] <- round(runif(ncell(r),0,1),1)
extent(r) <- matrix(c(0, 0, 1000, 1000), nrow=2)

extentnewvals <- extent(matrix(c(300, 300, 500, 500), nrow=2))

hist(r)

plot(r)

# Change values
r[extentnewvals][r[extentnewvals] == 0.8] <- 0.2

hist(r)

plot(r)