I would like to use fread to pull in only columns with names that match a condition. (In this case, I would like to pull in all columns that contain the label email.) Imagine that you have this data in a file called tempdata.txt in your working directory:
col1,col2,col3,email1,email2,col4,url1,url2,col5
1,2,3,4,5,6,7,8,9
9,8,7,6,5,4,3,2,1
x,x,x,[email protected],[email protected],y,y,y,y
a,a,a,a,a,a,http://google.com,http://stackoverflow.com,a
It is possible to use fread to load a subset of the columns if you know the names:
test <- data.table::fread("tempdata.txt", select=c("email1","email2"))
> test
email1 email2
1: 4 5
2: 6 5
3: [email protected] [email protected]
4: a a
Is it also possible to select using a string match? I am trying to mimic this behavior but within the fread command:
> all <- data.table::fread("tempdata.txt")
> all %>% select(contains("email"))
email1 email2
1: 4 5
2: 6 5
3: [email protected] [email protected]
4: a a
Thanks for any insight.
I don't know that fread doesn't have that capability (though I don't see it in the docs). However, a relatively inexpensive approach would be to read the first row or two, get the column names, grep them, and move on from there.
library(data.table)
fwrite(data.table(a=1:2, email1=c('a','b'), snailmail=c('c','d'), email2=c('e','f')), "test.csv")
fread("test.csv", nrows=1)
# a email1 snailmail email2
# 1: 1 a c e
cols <- colnames(fread("test.csv", nrows=0))
cols
# [1] "a" "email1" "snailmail" "email2"
fread("test.csv", select = grep("^email", cols, value = TRUE))
# email1 email2
# 1: a e
# 2: b f