[R] How to modify data frame stored in a list

[William Dunlap]

 >Thanks!
>Why have to add “x” at the end of function, which was what I missed.

You can see for yourself with some tests:

f1 <- function(x) { x[1] <- 10 }
f2 <- function(x) { x[1] <- 10 ; x }

print(f1(1:3)) # 10
print(f2(1:3)) # 10 2 3

An assignment returns the value of its right hand side but you want to
return the altered input.

Bill Dunlap
TIBCO Software
wdunlap tibco.com

On Mon, Jun 18, 2018 at 2:58 PM, Honkit Wong <stephen66 using gmail.com> wrote:

> Thanks!
> Why have to add “x” at the end of function, which was what I missed.
>
> > On Jun 18, 2018, at 2:53 PM, Bert Gunter <bgunter.4567 using gmail.com> wrote:
> >
> > It depends on whether you wish to refer to the column to be logged by
> name or index.
> >
> > Reprex:
> >
> > set.seed(1234)
> > dat <- lapply(1:3, function(i)data.frame(a = runif(5), b =
> sample(letters,5)))
> >
> > ## by numerical index of column
> > d <- lapply(dat,function(x){x[,"logged"]<- log10(x[,1]); x})
> >
> > ## by name of column
> > dd <- lapply(dat,function(x){x[,"logged"]<- log10(x[,"a"]); x})
> >
> > There are also slight variations on how you can do the "[" indexing that
> others may post.
> > Note that you have to return the modified data frame in the function.
> >
> > Cheers,
> > Bert
> >
> >
> >
> >
> >
> >
> > Bert Gunter
> >
> > "The trouble with having an open mind is that people keep coming along
> and sticking things into it."
> > -- Opus (aka Berkeley Breathed in his "Bloom County" comic strip )
> >
> >> On Mon, Jun 18, 2018 at 1:46 PM, Honkit Wong <stephen66 using gmail.com>
> wrote:
> >> Dear R community,
> >> I have a question seems very simple but have trouble to do it.
> >> I have a list which stores many data frames. Now, I want to perform
> log10 on one column in each data frame in the list and save the value as a
> new column back to the original data frame in the list. How do I quickly do
> that with lapply function ?
> >>
> >> Many thanks.
> >> ______________________________________________
> >> R-help using 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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>
> ______________________________________________
> R-help using 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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