convert character to date *quickly* in R
Simon Urbanek's fasttime library is very fast for a subset of parseable datetimes:
R> now <- Sys.time()
R> now
[1] "2012-10-15 10:07:28.981 CDT"
R> fasttime::fastPOSIXct(format(now))
[1] "2012-10-15 05:07:28.980 CDT"
R> as.Date(fasttime::fastPOSIXct(format(now)))
[1] "2012-10-15"
R>
However, it only parse ISO formats and assume UTC as timezone.
Edit after 3 1/2 years: Some commenters appear to think that the fasttime package is difficult to install. I beg to differ. Here is (once again) use install.r
which is just a simple wrapper using littler (and also shipped as an example with):
edd@max:~$ install.r fasttime
trying URL 'https://cran.rstudio.com/src/contrib/fasttime_1.0-1.tar.gz'
Content type 'application/x-gzip' length 2646 bytes
==================================================
downloaded 2646 bytes
* installing *source* package ‘fasttime’ ...
** package ‘fasttime’ successfully unpacked and MD5 sums checked
** libs
ccache gcc -I/usr/share/R/include -DNDEBUG -fpic -g -O2 -fstack-protector-strong -Wformat -Werror=format-security -D_FORTIFY_SOURCE=2 -g -O3 -Wall -pipe -pedantic -std=gnu99 -c tparse.c -o tparse.o
ccache gcc -shared -L/usr/lib/R/lib -Wl,-Bsymbolic-functions -Wl,-z,relro -o fasttime.so tparse.o -L/usr/lib/R/lib -lR
installing to /usr/local/lib/R/site-library/fasttime/libs
** R
** preparing package for lazy loading
** help
*** installing help indices
** building package indices
** testing if installed package can be loaded
* DONE (fasttime)
The downloaded source packages are in
‘/tmp/downloaded_packages’
edd@max:~$
As you can see, the package has zero external dependencies, one source file and builds without the slightest hitch. We can also see that fasttime is now on CRAN which was not the case when the answer was written. With that, Windows and OS X binaries now do exist at that page and the installation will be as easy as it was for me even when you do not install from source.
convert string date to R Date FAST for all dates
I can get a little speedup by using the date
package:
library(date)
set.seed(21)
x <- as.character(Sys.Date()-sample(40000, 1e6, TRUE))
system.time(dDate <- as.Date(x))
# user system elapsed
# 6.54 0.01 6.56
system.time(ddate <- as.Date(as.date(x,"ymd")))
# user system elapsed
# 3.42 0.22 3.64
You might want to look at the C code it uses and see if you can modify it to be faster for your specific situation.
Convert date-time string to class Date
You may be overcomplicating things, is there any reason you need the stringr package? You can use as.Date
and its format
argument to specify the input format of your string.
df <- data.frame(Date = c("10/9/2009 0:00:00", "10/15/2009 0:00:00"))
as.Date(df$Date, format = "%m/%d/%Y %H:%M:%S")
# [1] "2009-10-09" "2009-10-15"
Note the Details section of ?as.Date
:
Character strings are processed as far as necessary for the format specified: any trailing characters are ignored
Thus, this also works:
as.Date(df$Date, format = "%m/%d/%Y")
# [1] "2009-10-09" "2009-10-15"
All the conversion specifications that can be used to specify the input format
are found in the Details section in ?strptime
. Make sure that the order of the conversion specification as well as any separators correspond exactly with the format of your input string.
More generally and if you need the time component as well, use as.POSIXct
or strptime
:
as.POSIXct(df$Date, "%m/%d/%Y %H:%M:%S")
strptime(df$Date, "%m/%d/%Y %H:%M:%S")
I'm guessing at what your actual data might look at from the partial results you give.
How to convert character to date with two different types of date formats in R?
You can do something like:
format_ymd <- as.Date(dates, format = "%Y-%m-%d")
format_dmy <- as.Date(dates, format = "%d/%m/%Y")
as.Date(ifelse(is.na(format_ymd), format_dmy, format_ymd), origin = "1970-01-01")
# [1] "2022-04-08" "2021-01-26" "2021-07-14" "2021-12-27"
Converting character to date R
You should think of using 2 step process. First remove the End
part from the ProjectDate
using sub
.
Now you can apply yearmon
from zoo
library to convert to month year
date format.
library(zoo)
as.yearmon(sub("^End ", "", df$ProjectDate), "%b %Y")
#[1] "Aug 2014" "Sep 2014"
Why is as.Date slow on a character vector?
I think it's just that as.Date
converts character
to Date
via POSIXlt
, using strptime
. And strptime
is very slow, I believe.
To trace it through yourself, type as.Date
, then methods(as.Date)
, then look at the character
method.
> as.Date
function (x, ...)
UseMethod("as.Date")
<bytecode: 0x2cf4b20>
<environment: namespace:base>
> methods(as.Date)
[1] as.Date.character as.Date.date as.Date.dates as.Date.default
[5] as.Date.factor as.Date.IDate* as.Date.numeric as.Date.POSIXct
[9] as.Date.POSIXlt
Non-visible functions are asterisked
> as.Date.character
function (x, format = "", ...)
{
charToDate <- function(x) {
xx <- x[1L]
if (is.na(xx)) {
j <- 1L
while (is.na(xx) && (j <- j + 1L) <= length(x)) xx <- x[j]
if (is.na(xx))
f <- "%Y-%m-%d"
}
if (is.na(xx) || !is.na(strptime(xx, f <- "%Y-%m-%d",
tz = "GMT")) || !is.na(strptime(xx, f <- "%Y/%m/%d",
tz = "GMT")))
return(strptime(x, f))
stop("character string is not in a standard unambiguous format")
}
res <- if (missing(format))
charToDate(x)
else strptime(x, format, tz = "GMT") #### slow part, I think ####
as.Date(res)
}
<bytecode: 0x2cf6da0>
<environment: namespace:base>
>
Why is as.POSIXlt(Date)$year+1900
relatively fast? Again, trace it through :
> as.POSIXct
function (x, tz = "", ...)
UseMethod("as.POSIXct")
<bytecode: 0x2936de8>
<environment: namespace:base>
> methods(as.POSIXct)
[1] as.POSIXct.date as.POSIXct.Date as.POSIXct.dates as.POSIXct.default
[5] as.POSIXct.IDate* as.POSIXct.ITime* as.POSIXct.numeric as.POSIXct.POSIXlt
Non-visible functions are asterisked
> as.POSIXlt.Date
function (x, ...)
{
y <- .Internal(Date2POSIXlt(x))
names(y$year) <- names(x)
y
}
<bytecode: 0x395e328>
<environment: namespace:base>
>
Intrigued, let's dig into Date2POSIXlt. For this bit we need to grep main/src to know which .c file to look at.
~/R/Rtrunk/src/main$ grep Date2POSIXlt *
names.c:{"Date2POSIXlt",do_D2POSIXlt, 0, 11, 1, {PP_FUNCALL, PREC_FN, 0}},
$
Now we know we need to look for D2POSIXlt :
~/R/Rtrunk/src/main$ grep D2POSIXlt *
datetime.c:SEXP attribute_hidden do_D2POSIXlt(SEXP call, SEXP op, SEXP args, SEXP env)
names.c:{"Date2POSIXlt",do_D2POSIXlt, 0, 11, 1, {PP_FUNCALL, PREC_FN, 0}},
$
Oh, we could have guessed datetime.c. Anyway, so looking at latest live copy :
datetime.c
Search in there for D2POSIXlt
and you'll see how simple it is to go from Date (numeric) to POSIXlt. You'll also see how POSIXlt is one real vector (8 bytes) plus seven integer vectors (4 bytes each). That's 40 bytes, per date!
So the crux of the issue (I think) is why strptime
is so slow, and maybe that can be improved in R. Or just avoid POSIXlt
, either directly or indirectly.
Here's a reproducible example using the number of items stated in question (3,000,000) :
> Range = seq(as.Date("2000-01-01"),as.Date("2012-01-01"),by="days")
> Date = format(sample(Range,3000000,replace=TRUE),"%m/%d/%Y")
> system.time(as.Date(Date, "%m/%d/%Y"))
user system elapsed
21.681 0.060 21.760
> system.time(strptime(Date, "%m/%d/%Y"))
user system elapsed
29.594 8.633 38.270
> system.time(strptime(Date, "%m/%d/%Y", tz="GMT"))
user system elapsed
19.785 0.000 19.802
Passing tz
appears to speed up strptime
, which as.Date.character
does. So maybe it depends on your locale. But strptime
appears to be the culprit, not data.table
. Perhaps rerun this example and see if it takes 90 seconds for you on your machine?
Convert character to Date (Thu Jun 14 *** 2018-05-14) in r
This sure is not the easiest way to do it, But I just wanted you to have a quick answer.
library(stringr)
library(dplyr)
Data=data.frame(date=c("Thu Jun 14","Fri Jun 15","Tue Jun 19","Wed Jun 20"),match_name=c("a","b","c","d"),price=c(1,2,3,4))
Data$date=as.character(Data$date)
regexp <- "[[:digit:]]+"
Data=mutate(Data,datenum=str_extract(Data$date, regexp))
Data=mutate(Data,monthnum=str_extract(Data$date, regexp))
Data=mutate(Data,monthname=str_extract(Data$date,"Jan|Feb|Mar|Apr|May|Jun|Jul|Aug|Sep|Oct|Nov|Dec"))
Data=mutate(Data,monthnum=if(Data$monthname=="Jan")
"01"
else if(Data$monthname=="Feb")
"02"
else if(Data$monthname=="Mar")
"03"
else if(Data$monthname=="Apr")
"04"
else if(Data$monthname=="May")
"05"
else if(Data$monthname=="Jun")
"06"
else if(Data$monthname=="Jul")
"07"
else if(Data$monthname=="Aug")
"08"
else if(Data$monthname=="Sep")
"09"
else if(Data$monthname=="Oct")
"10"
else if(Data$monthname=="Nov")
"11"
else if(Data$monthname=="Dec")
"12"
)
mutate(Data,Final_Date=paste0("2018-",monthnum,"-",datenum))
Resulting in
date match_name price datenum monthnum monthname Final_Date
1 Thu Jun 14 a 1 14 06 Jun 2018-06-14
2 Fri Jun 15 b 2 15 06 Jun 2018-06-15
3 Tue Jun 19 c 3 19 06 Jun 2018-06-19
4 Wed Jun 20 d 4 20 06 Jun 2018-06-20
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