random - simple way to randomly subset a raster in R -

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i work in r. have raster (raster_entrop). want automatically generate random subset of raster. issue extent not rectangular , want subsets completly enclosed in extent. solution check if there not "nan" (corresponding pixel outside extent of raster_entrop) in coordinate of "subextent" randomly chosen inside extent of original raster. 

ext<-extent(raster_entrop) repeat{   rnd.x<-runif(1,ext@xmin, ext@xmax)   rnd.y<-runif(1,ext@ymin, ext@ymax)    #test if sub extent within "big extent"    test<-is.na(extract(raster_entrop, cbind(rnd.x,rnd.y)))+   is.na(extract(raster_entrop, cbind(rnd.x,rnd.y+0.02)))+   is.na(extract(raster_entrop, cbind(rnd.x+(0.02),rnd.y)))+   is.na(extract(raster_entrop, cbind(rnd.x+(0.02),rnd.y+0.02)))    if (test==0){     break   } } ext_sub<-extent(c(rnd.x, rnd.x+0.02,rnd.y,rnd.y+0.02)) raster_entrop_sub <- crop(raster_entrop, ext_sub)

my question: there simpler solution not use loop ?


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