Multiply two rasters in R, one raster has stack of 25 images, and another raster has one image

Question

I want to multiply two rasters (one has 25 images, and the second raster has 1 image). I am using the loop function for multiplication. But I am not getting the results.

library(raster)
library(rgdal)

x = raster("D:/MODIS/img.tif") # 25images
y = raster("D:/urban.tif")     # 1 image
z <- function(x,y){x*y}

beginCluster(25) # your availables cores
for (i in 1:length(x)) {
  ndvi <- (x[[i]])
  urban <- (y)
  ndvi_urban <- clusterR(stack(ndvi,urban), calc, args = list(fun = z))
  writeRaster(ndvi_urban,output_name[i])
} 
endCluster()

Answer 1

You can use sapply in combination with lapp ({terra} package, since it's faster than {raster}):

sapply(1:25, function(X) lapp(c(x[[X]],y), function(x,y){x*y}))

Por parallelization, you can use cores argument:

library(parallel)
n = detectCores()
sapply(1:25, function(X) lapp(c(x[[X]],y), function(x,y){x*y}, cores = n))

You can even export each result:

sapply(1:25, function(X) lapp(c(x[[X]],y), function(x,y){x*y}, filename = paste0('path/to/file', X, '.tif')))

Reproducible example:

library(terra)

set.seed(123)

x = rast()
y = rast()

x = rast(sapply(1:25, function(X){setValues(x,rnorm(ncell(x)))}))
y = setValues(y,rnorm(ncell(y)))
  
print(x)
# class       : SpatRaster 
# dimensions  : 180, 360, 25  (nrow, ncol, nlyr)
# resolution  : 1, 1  (x, y)
# extent      : -180, 180, -90, 90  (xmin, xmax, ymin, ymax)
# coord. ref. : lon/lat WGS 84 
# source(s)   : memory
# names       :     lyr.1,     lyr.1,     lyr.1,    lyr.1,     lyr.1,     lyr.1, ... 
# min values  : -4.129135, -4.382098, -4.289319, -4.23503, -4.132421, -4.351687, ... 
# max values  :  4.322815,  4.206090,  4.124220,  4.52151,  4.599884,  4.759086, ... 

print(y)
# class       : SpatRaster 
# dimensions  : 180, 360, 1  (nrow, ncol, nlyr)
# resolution  : 1, 1  (x, y)
# extent      : -180, 180, -90, 90  (xmin, xmax, ymin, ymax)
# coord. ref. : lon/lat WGS 84 
# source(s)   : memory
# name        :     lyr.1 
# min value   : -3.985896 
# max value   :  4.165254 

rast(sapply(1:25, function(X) lapp(c(x[[X]],y), function(x,y){x*y})))
# class       : SpatRaster 
# dimensions  : 180, 360, 25  (nrow, ncol, nlyr)
# resolution  : 1, 1  (x, y)
# extent      : -180, 180, -90, 90  (xmin, xmax, ymin, ymax)
# coord. ref. : lon/lat WGS 84 
# source(s)   : memory
# names       :      lyr1,       lyr1,      lyr1,      lyr1,      lyr1,      lyr1, ... 
# min values  : -7.865725, -12.337091, -9.967996, -8.919808, -9.330696, -9.985127, ... 
# max values  :  7.787720,   9.870736, 11.646272,  9.325943, 10.125534, 10.062626, ... 

Answer 2

In R, shorter objects are recycled to match longer objects, so you can directly multiply the two rasters.

library(terra)
x <- rast("D:/MODIS/img.tif") # 25images
y <- rast("D:/urban.tif")     # 1 image
z <- x * y

With some example data

library(terra)
x <- rast(ncol=10, nrow=10, nlyr=25, vals=1:2500)
y <- rast(ncol=10, nrow=10, nlyr=1, vals=1:100)
z <- x * y

z
#class       : SpatRaster 
#dimensions  : 10, 10, 25  (nrow, ncol, nlyr)
#resolution  : 36, 18  (x, y)
#extent      : -180, 180, -90, 90  (xmin, xmax, ymin, ymax)
#coord. ref. : lon/lat WGS 84 
#source(s)   : memory
#names       : lyr.1, lyr.2, lyr.3, lyr.4, lyr.5, lyr.6, ... 
#min values  :     1,   101,   201,   301,   401,   501, ... 
#max values  : 10000, 20000, 30000, 40000, 50000, 60000, ...