On Feb 10, 2015, at 3:58 PM, Brian Trautman wrote:
> Hi!
>
> I'm trying to write a custom function that applies SubStr to a string, and
> then depending on the arguments, converts the output to a number.
>
> The substring part of my code works fine, but it's not converting the way I
> want to --
>
> options('stringsAsFactors'=FALSE)
> require(data.table)
>
> substr_typeswitch <- function(x, start, stop, typeto='chr')
> {
> tmpvar <- substr(x=x, start=start, stop=stop)
> tmpvar <- switch(typeto, num=as.numeric(tmpvar), tmpvar)
> return(tmpvar)
> }
> startpos <- c(01, 03)
> endpos <- c(02, 04)
> typelist <- c('chr', 'num')
>
> startdata <- as.data.table(c('aa01', 'bb02'))
>
> enddata_want <- as.data.table(mapply(substr_typeswitch, startdata,
> startpos, endpos, typelist))
>
> If I examine enddata_want --
>
>> str(enddata_want)
> Classes ‘data.table’ and 'data.frame': 2 obs. of 2 variables:
> $ V1: chr "aa" "bb"
> $ NA: chr "1" "2"
> - attr(*, ".internal.selfref")=<externalptr>
>
> "1" and "2" are being stored as character, and not as number.
It appears from you code that you might be expecting a vector in a dataframe
object to have a character mode in the first postition and a numeric mode in
the second position. That wouldn't seem to be a reasonable expectation. But
maybe you were hoping the chr and num types were to be applied to columns. I
was surprised to get something different from as.data.table:
> str(enddata_want)
Classes ‘data.table’ and 'data.frame': 2 obs. of 2 variables:
$ V1: Factor w/ 2 levels "aa","bb": 1 2
$ NA: Factor w/ 2 levels "1","2": 1 2
- attr(*, ".internal.selfref")=<externalptr>
The mapply operation made a matrix which forces all values to be the same mode:
> str( mapply(substr_typeswitch, startdata,
+ startpos, endpos, typelist) )
chr [1:2, 1:2] "aa" "bb" "1" "2"
- attr(*, "dimnames")=List of 2
..$ : NULL
..$ : chr [1:2] "V1" NA
You might have gotten something less homogeneous if you added the SIMPLIFY
argument:
> str( mapply(substr_typeswitch, startdata,
+ startpos, endpos, typelist, SIMPLIFY=FALSE) )
List of 2
$ V1: chr [1:2] "aa" "bb"
$ NA: num [1:2] 1 2
>
> Can anyone help me understand what I'm doing wrong?
>
> Thank you!
>
> [[alternative HTML version deleted]]
>
> ______________________________________________
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> and provide commented, minimal, self-contained, reproducible code.
David Winsemius
Alameda, CA, USA
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