Stream processing large csv file in R - r

Question

I need to make a couple of relatively simple changes to a very large csv file (c.8.5GB). I tried initially using various reader functions: read.csv, readr::read.csv, data.table::fread. However: they all run out of memory.
I'm thinking I need to use a stream processing approach instead; read a chunk, update it, write it, repeat. I found this answer which is on the right lines; however I don't how to terminate the loop (I'm relatively new to R).
So I have 2 questions:
What's the right way to make the while loop work?
Is there a better way (for some definition of 'better')? e.g. is there some way to do this using dplyr & pipes?
Current code as follows:
src_fname <- "testdata/model_input.csv"
tgt_fname <- "testdata/model_output.csv"
#Changes needed in file: rebase identifiers, set another col to constant value
rebase_data <- function(data, offset) {
data$'Unique Member ID' <- data$'Unique Member ID' - offset
data$'Client Name' <- "TestClient2"
return(data)
}
CHUNK_SIZE <- 1000
src_conn = file(src_fname, "r")
data <- read.csv(src_conn, nrows = CHUNK_SIZE, check.names=FALSE)
cols <- colnames(data)
offset <- data$'Unique Member ID'[1] - 1
data <- rebase_data(data, offset)
#1st time through, write the headers
tgt_conn = file(tgt_fname, "w")
write.csv(data,tgt_conn, row.names=FALSE)
#loop over remaining data
end = FALSE
while(end == FALSE) {
data <- read.csv(src_conn, nrows = CHUNK_SIZE, check.names=FALSE, col.names = cols)
data <- rebase_data(data, offset)
#write.csv doesn't support col.names=FALSE; so use write.table which does
write.table(data, tgt_conn, row.names=FALSE, col.names=FALSE, sep=",")
# ??? How to test for EOF and set end = TRUE if so ???
# This doesn't work, presumably because nrow() != CHUNK_SIZE on final loop?
if (nrow(data) < CHUNK_SIZE) {
end <- TRUE
}
}
close(src_conn)
close(tgt_conn)
Thanks for any pointers.

Answer 1

Sorry to poke a 2-year-old thread, but now with readr::read_csv_chunked (auto-loaded along with dplyr when loading tidyverse), we could also do like:
require(tidyverse)
## For non-exploratory code, as #antoine-sac suggested, use:
# require(readr) # for function `read_csv_chunked` and `read_csv`
# require(dplyr) # for the pipe `%>%` thus less parentheses
src_fname = "testdata/model_input.csv"
tgt_fname = "testdata/model_output.csv"
CHUNK_SIZE = 1000
offset = read_csv(src_fname, n_max=1)$comm_code %>% as.numeric() - 1
rebase.chunk = function(df, pos) {
df$comm_code = df$comm_code %>% as.numeric() - offset
df$'Client Name' = "TestClient2"
is.append = ifelse(pos > 1, T, F)
df %>% write_csv(
tgt_fname,
append=is.append
)
}
read_csv_chunked(
src_fname,
callback=SideEffectChunkCallback$new(rebase.chunk),
chunk_size = chunck.size,
progress = T # optional, show progress bar
)
Here the tricky part is to set is.append based on parameter pos, which indicates the start row number of the data frame df within original file. Within readr::write_csv, when append=F the header (columns name) will be written to file, otherwise not.

Answer 2

Try this out:
library("chunked")
read_chunkwise(src_fname, chunk_size=CHUNK_SIZE) %>%
rebase_data(offset) %>%
write_chunkwise(tgt_fname)
You may need to fiddle a bit with the colnames to get exactly what you want.
(Disclaimer: haven't tried the code)
Note that there is no vignette with the package but the standard usage is described on github: https://github.com/edwindj/chunked/

Answer 3

OK I found a solution, as follows:
# src_fname <- "testdata/model_input.csv"
# tgt_fname <- "testdata/model_output.csv"
CHUNK_SIZE <- 20000
#Changes needed in file: rebase identifiers, set another col to constant value
rebase_data <- function(data, offset) {
data$'Unique Member ID' <- data$'Unique Member ID' - offset
data$'Client Name' <- "TestClient2"
return(data)
}
#--------------------------------------------------------
# Get the structure first to speed things up
#--------------------------------------------------------
structure <- read.csv(src_fname, nrows = 2, check.names = FALSE)
cols <- colnames(structure)
offset <- structure$'Unique Member ID'[1] - 1
#Open the input & output files for reading & writing
src_conn = file(src_fname, "r")
tgt_conn = file(tgt_fname, "w")
lines_read <- 0
end <- FALSE
read_header <- TRUE
write_header <- TRUE
while(end == FALSE) {
data <- read.csv(src_conn, nrows = CHUNK_SIZE, check.names=FALSE, col.names = cols, header = read_header)
if (nrow(data) > 0) {
lines_read <- lines_read + nrow(data)
print(paste0("lines read this chunk: ", nrow(data), ", lines read so far: ", lines_read))
data <- rebase_data(data, offset)
#write.csv doesn't support col.names=FALSE; so use write.table which does
write.table(data, tgt_conn, row.names=FALSE, col.names=write_header, sep = ",")
}
if (nrow(data) < CHUNK_SIZE) {
end <- TRUE
}
read_header <- FALSE
write_header <- FALSE
}
close(src_conn)
close(tgt_conn)