R devel - Aug 2019 - Feature request: non-dropping regmatches/strextract

I do think keeping the default behavior is desirable for backwards
compatibility; my suggestion is not to change default behavior but to add an
optional argument that allows a different behavior. Although this can be
implemented in a user-defined function, retaining empty matches facilitates
programmatic use, and seems to be something that should be available in base R.
It is available, for example, in MATLAB, a comparable array language.

Alternatively, perhaps a nomatch (or maybe emptymatch) argument in the spirit of
`[.data.table`? That is, an argument nomatch where nomatch = NULL (the default)
results in drops for vector outputs and character(0) for list outputs and
nomatch = NA results in insertion of NA_character_, and nomatch = ''
results in insertion of empty string.

I can submit proposed patch code if others think this is a good idea.

What are your thoughts on the proposed alteration to (currently nonexported)
strextract? I assume (maybe wrongly) that the plan is to eventually export that
function.

Thank you,
CG

I don't care much for regmatches and haven't tried strextract, but I
think
replacing the character(0) by NA_character_ is almost always inappropriate
if the match information comes from gregexpr.

I think strcapture() does a pretty good job of what I think you are trying
to do.  Perhaps adding an argument to map no match to NA instead of ""
would give you just what you wanted.
> x <- c("Groucho <groucho at marx.com>", "<chico
at marx.com>", "Harpo")
> d <- strcapture("([[:alpha:]]+)?( *<([[:alpha:]. ]+@[[:alpha:].
]+)>)?",
x, proto=data.frame(Name=character(), Junk=character(),
Address=character(), stringsAsFactors=FALSE))
> d[c("Name", "Address")]
     Name          Address
1 Groucho groucho at marx.com
2           chico at marx.com
3   Harpo
> str(.Last.value)
'data.frame':   3 obs. of  2 variables:
 $ Name   : chr  "Groucho" "" "Harpo"
 $ Address: chr  "groucho at marx.com" "chico at marx.com"
""
Bill Dunlap
TIBCO Software
wdunlap tibco.com


On Thu, Aug 15, 2019 at 11:31 AM Cyclic Group Z_1 <cyclicgroup-z1 at
yahoo.com>
wrote:
> I do think keeping the default behavior is desirable for backwards
> compatibility; my suggestion is not to change default behavior but to add
> an optional argument that allows a different behavior. Although this can be
> implemented in a user-defined function, retaining empty matches facilitates
> programmatic use, and seems to be something that should be available in
> base R. It is available, for example, in MATLAB, a comparable array
> language.
>
> Alternatively, perhaps a nomatch (or maybe emptymatch) argument in the
> spirit of `[.data.table`? That is, an argument nomatch where nomatch = NULL
> (the default) results in drops for vector outputs and character(0) for list
> outputs and nomatch = NA results in insertion of NA_character_, and nomatch
> = '' results in insertion of empty string.
>
> I can submit proposed patch code if others think this is a good idea.
>
> What are your thoughts on the proposed alteration to (currently
> nonexported) strextract? I assume (maybe wrongly) that the plan is to
> eventually export that function.
>
> Thank you,
> CG
>
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Using a non-capturing group, "(?:...)" instead of "(...)",
simplifies my
example a bit
> x <- c("Groucho <groucho at marx.com>", "<chico
at marx.com>", "Harpo")
> strcapture("([[:alpha:]]+)?(?: *<([[:alpha:]. ]+@[[:alpha:].
]+)>)?", x,
proto=data.frame(Name=character(), Address=character(),
stringsAsFactors=FALSE))
     Name          Address
1 Groucho groucho at marx.com
2           chico at marx.com
3   Harpo

Bill Dunlap
TIBCO Software
wdunlap tibco.com


On Thu, Aug 15, 2019 at 1:04 PM William Dunlap <wdunlap at tibco.com>
wrote:
> I don't care much for regmatches and haven't tried strextract, but
I think
> replacing the character(0) by NA_character_ is almost always inappropriate
> if the match information comes from gregexpr.
>
> I think strcapture() does a pretty good job of what I think you are trying
> to do.  Perhaps adding an argument to map no match to NA instead of
""
> would give you just what you wanted.
>
> > x <- c("Groucho <groucho at marx.com>",
"<chico at marx.com>", "Harpo")
> > d <- strcapture("([[:alpha:]]+)?( *<([[:alpha:].
]+@[[:alpha:]. ]+)>)?",
> x, proto=data.frame(Name=character(), Junk=character(),
> Address=character(), stringsAsFactors=FALSE))
> > d[c("Name", "Address")]
>      Name          Address
> 1 Groucho groucho at marx.com
> 2           chico at marx.com
> 3   Harpo
> > str(.Last.value)
> 'data.frame':   3 obs. of  2 variables:
>  $ Name   : chr  "Groucho" "" "Harpo"
>  $ Address: chr  "groucho at marx.com" "chico at
marx.com" ""
> Bill Dunlap
> TIBCO Software
> wdunlap tibco.com
>
>
> On Thu, Aug 15, 2019 at 11:31 AM Cyclic Group Z_1 <
> cyclicgroup-z1 at yahoo.com> wrote:
>
>> I do think keeping the default behavior is desirable for backwards
>> compatibility; my suggestion is not to change default behavior but to
add
>> an optional argument that allows a different behavior. Although this
can be
>> implemented in a user-defined function, retaining empty matches
facilitates
>> programmatic use, and seems to be something that should be available in
>> base R. It is available, for example, in MATLAB, a comparable array
>> language.
>>
>> Alternatively, perhaps a nomatch (or maybe emptymatch) argument in the
>> spirit of `[.data.table`? That is, an argument nomatch where nomatch =
NULL
>> (the default) results in drops for vector outputs and character(0) for
list
>> outputs and nomatch = NA results in insertion of NA_character_, and
nomatch
>> = '' results in insertion of empty string.
>>
>> I can submit proposed patch code if others think this is a good idea.
>>
>> What are your thoughts on the proposed alteration to (currently
>> nonexported) strextract? I assume (maybe wrongly) that the plan is to
>> eventually export that function.
>>
>> Thank you,
>> CG
>>
>
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