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apply_by_pages <- function(x, FUN, pagesize, verbose, ...){
stopifnot(is.data.frame(x))
nr <- nrow(x)
npages <- nr %/% pagesize;
lastpage <- nr %% pagesize;
out <- as.list(rep(NA, npages + as.logical(lastpage)))
for(i in seq_len(npages)){
from <- pagesize * (i-1) + 1;
to <- pagesize * i
out[[i]] <- FUN(x[from:to, ,drop = FALSE], ...)
if(verbose) cat("\rProcessed", i * pagesize, "rows...")
}
if(lastpage){
from <- nr - lastpage + 1;
out[[npages + 1]] <- FUN(x[from:nr, ,drop = FALSE], ...)
}
if(verbose) cat("\rComplete! Processed total of", nr, "rows.\n")
out
}
#this is another slightly slower implementation
apply_by_pages2 <- function(x, FUN, pagesize, verbose, ...){
x2 <- split(x, seq_len(nrow(x)) %/% pagesize)
for(page in x2){
if(verbose) message("Writing ", nrow(page), " lines (", ").")
FUN(page)
}
invisible()
}
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