R help - Dec 2015 - Create a data.table by looping over variable names

I apologise for this very basic question, but I found no answers on the web.
I want to create a data.table with n columns, named i1 ... i'n', with
only
one row with value = 100 for every variable.

I can do it "by hand":

M.dt <- data.table(i1=100,i2=100,i3=100)

but I would like to make it in a loop, as 'n' is large in my case.


Thanks.

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Dear Matteo,
I found a one-liner solution to this problem:
n <- 100
data.table(i0 = 100)[, (paste0("i", 1:n)) := 100]

the data.table(i0=100) creates a one column-one row data.table,

Between the [] brakets i create a column name vector with paste, and
surround it with () to make it recognised as names (for details, look at
the Dateails of ?`:=`), and it will append columns to the i0 data.table.

This cannot be done with a 0-column data.table.

Good luck:
Andr?s Tajti




On 16 December 2015 at 12:53, Matteo Richiardi <matteo.richiardi at
gmail.com>
wrote:
> I apologise for this very basic question, but I found no answers on the
> web.
> I want to create a data.table with n columns, named i1 ... i'n',
with only
> one row with value = 100 for every variable.
>
> I can do it "by hand":
>
> M.dt <- data.table(i1=100,i2=100,i3=100)
>
> but I would like to make it in a loop, as 'n' is large in my case.
>
>
> Thanks.
>
>         [[alternative HTML version deleted]]
>
> ______________________________________________
> R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
> http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>
	[[alternative HTML version deleted]]

Maybe there?s a cleverer way, but that?s what I can think off the top of my
head:

n <- 100
M.dt <- data.table(matrix(ncol = 100))
M.dt[] <- 100
names(M.dt) <- sapply(1:100, function(x) paste0("i", x))

The sapply is your loop.
> Am 16.12.2015 um 12:53 schrieb Matteo Richiardi <matteo.richiardi at
gmail.com>:
> 
> I apologise for this very basic question, but I found no answers on the
web.
> I want to create a data.table with n columns, named i1 ... i'n',
with only
> one row with value = 100 for every variable.
> 
> I can do it "by hand":
> 
> M.dt <- data.table(i1=100,i2=100,i3=100)
> 
> but I would like to make it in a loop, as 'n' is large in my case.
> 
> 
> Thanks.
> 
> 	[[alternative HTML version deleted]]
> 
> ______________________________________________
> R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.

Hi

Not sure about data table but

dat<-rep(100, 100)
dim(dat) <- c(1,100)
dat <- data.frame(dat)
names(dat) <-paste("i", 1:100, sep="")

gives you data.frame. I presume you can change it to data.table.

I cannot resist to comment that maybe list is better option. But as I do not
know what you want to do with your data.table I may be completely out.

Cheers
Petr
> -----Original Message-----
> From: R-help [mailto:r-help-bounces at r-project.org] On Behalf Of Matteo
> Richiardi
> Sent: Wednesday, December 16, 2015 12:53 PM
> To: r-help at r-project.org
> Subject: [R] Create a data.table by looping over variable names
>
> I apologise for this very basic question, but I found no answers on the
> web.
> I want to create a data.table with n columns, named i1 ... i'n',
with
> only one row with value = 100 for every variable.
>
> I can do it "by hand":
>
> M.dt <- data.table(i1=100,i2=100,i3=100)
>
> but I would like to make it in a loop, as 'n' is large in my case.
>
>
> Thanks.
>
>       [[alternative HTML version deleted]]
>
> ______________________________________________
> R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide http://www.R-project.org/posting-
> guide.html
> and provide commented, minimal, self-contained, reproducible code.
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Hi,
thanks a lot to everybody for your help. Very nice suggestions!
Matteo

On 16 December 2015 at 12:53, Matteo Richiardi <matteo.richiardi at
gmail.com>
wrote:
> I apologise for this very basic question, but I found no answers on the
> web.
> I want to create a data.table with n columns, named i1 ... i'n',
with only
> one row with value = 100 for every variable.
>
> I can do it "by hand":
>
> M.dt <- data.table(i1=100,i2=100,i3=100)
>
> but I would like to make it in a loop, as 'n' is large in my case.
>
>
> Thanks.
>
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