How can I save multiple jpegs from raster data in R? - r

Question

I have about 40 spatial rasters in the .tiff format in a folder. I'm trying to generate histograms from each of the rasters in R, and save each histogram as a jpeg in a separate folder. I wrote code to loop through each of the raster, create a histogram and save it using the 'jpeg' package.
setwd("G:/Research/MODIS Albedo Oct 08-July 09/Test")
library(raster)
library(jpeg)
files <- list.files(path="G:/Research/MODIS Albedo Oct 08-July 09/Test", pattern=".tif",all.files=T, full.names=F, no.. = T) #generate a list of rasters in the folder
number<-length(files) #count the number of rasters
for(r in 1:number) #loop over each raster in the folder
{
x<-raster(files[r], header=F) #load one raster file
jpeg("G:/Research/MODIS Albedo Oct 08-July 09/test_histplots/r.jpg") #create jpeg using the name 'r' generated by loop
hist(x) #generate histogram
dev.off()
}
I want each of the generated jpeg to have a different name, ideally a subset of the original raster name. For example, if the original name of the raster is 'MODIS101_265', the jpeg's name should be 265. Here, 265 is the Julian date in the year. I'm assuming that this might involve using a format specifier like %d in C, but I'm not sure how this works in R.
When I run the above code, I get only one histogram. It seems the code is correctly looping over the original rasters, but saving all resultant histograms to a single jpeg.
Any advice will be helpful! Thanks!

Answer 1

Regular expression using gsub are your friend for getting the number out of the name. Assuming that all of your files are named the way your example is ("MODIS101_265.tif"), then the code below will work for you.
Also, welcome to R where for loops are slow and can usually be replaced by the faster lapply.
saveMyHist <- function(fileName) {
fileNum <- as.numeric(gsub(".*_(\\d+)\\.tif", "\\1", fileName))
x <- raster(fileName, header=F)
jpeg(sprintf("G:/Research/MODIS Albedo Oct 08-July 09/test_histplots/%03d.jpg",
fileNum))
hist(x)
dev.off()
}
files <- list.files("/Users/home/Documents/Development/Rtesting",
pattern=".tif",all.files=T, full.names=F, no.. = T)
lapply(files, saveMyHist)