large data - How to read only lines that fulfil a condition from a csv into R?

Question

I am trying to read a large csv file into R. I only want to read and work with some of the rows that fulfil a particular condition (e.g. Variable2 >= 3). This is a much smaller dataset.

I want to read these lines directly into a dataframe, rather than load the whole dataset into a dataframe and then select according to the condition, since the whole dataset does not easily fit into memory.

Answer 1

You could use the read.csv.sql function in the sqldf package and filter using SQL select. From the help page of read.csv.sql:

library(sqldf)
write.csv(iris, "iris.csv", quote = FALSE, row.names = FALSE)
iris2 <- read.csv.sql("iris.csv", 
    sql = "select * from file where `Sepal.Length` > 5", eol = "
")