# Reads text input directly :
fread("A,B\n1,2\n3,4")
# Reads pasted input directly :
fread("A,B
1,2
3,4
")
# Finds the first data line automatically :
fread("
This is perhaps a banner line or two or ten.
A,B
1,2
3,4
")
# Detects whether column names are present automatically :
fread("
1,2
3,4
")
# Numerical precision :
DT = fread("A\n1.010203040506070809010203040506\n")
# TODO: add numerals=c("allow.loss", "warn.loss", "no.loss") from base::read.table, +"use.Rmpfr"
typeof(DT$A)=="double" # currently "allow.loss" with no option
DT = fread("A\n1.46761e-313\n") # read as 'numeric'
DT[,sprintf("%.15E",A)] # beyond what double precision can store accurately to 15 digits
# For greater accuracy use colClasses to read as character, then package Rmpfr.
# colClasses
data = "A,B,C,D\n1,3,5,7\n2,4,6,8\n"
fread(data, colClasses=c(B="character",C="character",D="character")) # as read.csv
fread(data, colClasses=list(character=c("B","C","D"))) # saves typing
fread(data, colClasses=list(character=2:4)) # same using column numbers
# drop
fread(data, colClasses=c("B"="NULL","C"="NULL")) # as read.csv
fread(data, colClasses=list(NULL=c("B","C"))) #
fread(data, drop=c("B","C")) # same but less typing, easier to read
fread(data, drop=2:3) # same using column numbers
# select
# (in read.csv you need to work out which to drop)
fread(data, select=c("A","D")) # less typing, easier to read
fread(data, select=c(1,4)) # same using column numbers
# select and types combined
fread(data, select=c(A="numeric", D="character"))
fread(data, select=list(numeric="A", character="D"))
# skip blank lines
fread("a,b\n1,a\n2,b\n\n\n3,c\n", blank.lines.skip=TRUE)
# fill
fread("a,b\n1,a\n2\n3,c\n", fill=TRUE)
fread("a,b\n\n1,a\n2\n\n3,c\n\n", fill=TRUE)
# fill with skip blank lines
fread("a,b\n\n1,a\n2\n\n3,c\n\n", fill=TRUE, blank.lines.skip=TRUE)
# check.names usage
fread("a b,a b\n1,2\n")
fread("a b,a b\n1,2\n", check.names=TRUE) # no duplicates + syntactically valid names
if (FALSE) {
# Demo speed-up
n = 1e6
DT = data.table( a=sample(1:1000,n,replace=TRUE),
b=sample(1:1000,n,replace=TRUE),
c=rnorm(n),
d=sample(c("foo","bar","baz","qux","quux"),n,replace=TRUE),
e=rnorm(n),
f=sample(1:1000,n,replace=TRUE) )
DT[2,b:=NA_integer_]
DT[4,c:=NA_real_]
DT[3,d:=NA_character_]
DT[5,d:=""]
DT[2,e:=+Inf]
DT[3,e:=-Inf]
write.table(DT,"test.csv",sep=",",row.names=FALSE,quote=FALSE)
cat("File size (MB):", round(file.info("test.csv")$size/1024^2),"\n")
# 50 MB (1e6 rows x 6 columns)
system.time(DF1 <-read.csv("test.csv",stringsAsFactors=FALSE))
# 5.4 sec (first time in fresh R session)
system.time(DF1 <- read.csv("test.csv",stringsAsFactors=FALSE))
# 3.9 sec (immediate repeat is faster, varies)
system.time(DF2 <- read.table("test.csv",header=TRUE,sep=",",quote="",
stringsAsFactors=FALSE,comment.char="",nrows=n,
colClasses=c("integer","integer","numeric",
"character","numeric","integer")))
# 1.2 sec (consistently). All known tricks and known nrows, see references.
system.time(DT <- fread("test.csv"))
# 0.1 sec (faster and friendlier)
identical(DF1, DF2)
all.equal(as.data.table(DF1), DT)
# Scaling up ...
l = vector("list",10)
for (i in 1:10) l[[i]] = DT
DTbig = rbindlist(l)
tables()
write.table(DTbig,"testbig.csv",sep=",",row.names=FALSE,quote=FALSE)
# 500MB csv (10 million rows x 6 columns)
system.time(DF <- read.table("testbig.csv",header=TRUE,sep=",",
quote="",stringsAsFactors=FALSE,comment.char="",nrows=1e7,
colClasses=c("integer","integer","numeric",
"character","numeric","integer")))
# 17.0 sec (varies)
system.time(DT <- fread("testbig.csv"))
# 0.8 sec
all(mapply(all.equal, DF, DT))
# Real data example (Airline data)
# https://stat-computing.org/dataexpo/2009/the-data.html
download.file("https://stat-computing.org/dataexpo/2009/2008.csv.bz2",
destfile="2008.csv.bz2")
# 109MB (compressed)
system("bunzip2 2008.csv.bz2")
# 658MB (7,009,728 rows x 29 columns)
colClasses = sapply(read.csv("2008.csv",nrows=100,stringsAsFactors=FALSE),class)
# 4 character, 24 integer, 1 logical. Incorrect.
colClasses = sapply(read.csv("2008.csv",nrows=200,stringsAsFactors=FALSE),class)
# 5 character, 24 integer. Correct. Might have missed data only using 100 rows
# since read.table assumes colClasses is correct.
system.time(DF <- read.table("2008.csv", header=TRUE, sep=",",
quote="",stringsAsFactors=FALSE,comment.char="",nrows=7009730,
colClasses=colClasses))
# 24.4 secs
system.time(DT <- fread("2008.csv"))
# 1.9 secs
table(sapply(DT,class))
# 5 character and 24 integer columns. Correct without needing to worry about colClasses
# issue above.
# Reads URLs directly :
fread("https://www.stats.ox.ac.uk/pub/datasets/csb/ch11b.dat")
# Decompresses .gz and .bz2 automatically :
fread("https://stat-computing.org/dataexpo/2009/1987.csv.bz2")
}
Run the code above in your browser using DataLab