# Replace a random sample of elements in a matrix that contain a certain value

## Problem

You want to replace values within a matrix with another value, but only at a random sample of locations where the original value occurs.

## Solution

The following code will find the index of all locations in the matrix that have a given value, then assign a new value to a random sample of those index locations.

#Create an example matrix MTX <- matrix(1:5, nrow=5, ncol=5) #View the matrix MTX #Find the index of all locations where MTX==2, #and assign a new value of 888 to a random sample of 3 of those MTX[sample(which(MTX %in% 2), 3, replace=FALSE)] <- 888 #View the result MTX #Note that this can be adapted for use with raster data, #but doesn't appear to work as-is on rasters #Use the following to turn your raster into a matrix: #MyMatrix <- matrix(getValues(MyRaster))

### Notes

Jason Carlisle contributed to the development of this tool.