R help - Oct 2019 - decomposing a string representing a valid mathematical expression?

Hello,

I would like to decompose a string i.e.

"a - b"
or "a +b"

into an array or list
["a","-","b"]

Since R knows how to parse expressions these type of expressions I
would like to reuse some existing functions and not to use gsub or
strsplit etc.

Thank you
Witek

-- 
Witold Eryk Wolski

Not sure what you're looking for, but see ?deparse for deparsing
**expressions.**

-- 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 Tue, Oct 29, 2019 at 5:56 AM Witold E Wolski <wewolski at gmail.com>
wrote:
> Hello,
>
> I would like to decompose a string i.e.
>
> "a - b"
> or "a +b"
>
> into an array or list
> ["a","-","b"]
>
> Since R knows how to parse expressions these type of expressions I
> would like to reuse some existing functions and not to use gsub or
> strsplit etc.
>
> Thank you
> Witek
>
> --
> Witold Eryk Wolski
>
> ______________________________________________
> 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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On Tue, 29 Oct 2019 13:55:27 +0100
Witold E Wolski <wewolski at gmail.com> wrote:
> Since R knows how to parse expressions these type of expressions I
> would like to reuse some existing functions and not to use gsub or
> strsplit etc.
You might be interested in the `parse` function:

x <- "(a+b) * c/(d * (e - f))"
str(parse(text = x))
# length 1 expression((a + b) * c/(d * (e - f)))
#  - attr(*, "srcref")=List of 1
#   ..$ : 'srcref' int [1:8] 1 1 1 23 1 23 1 1
#   .. ..- attr(*, "srcfile")=Classes 'srcfilecopy',
'srcfile' <environment: 0x55ebecdac210>
#  - attr(*, "srcfile")=Classes 'srcfilecopy',
'srcfile' <environment:0x55ebecdac210>
#  - attr(*, "wholeSrcref")= 'srcref' int [1:8] 1 0 2 0 0 0 1
2
#   ..- attr(*, "srcfile")=Classes 'srcfilecopy',
'srcfile'<environment:0x55ebecdac210>
parse(text = x)[[1]]
# (a + b) * c/(d * (e - f))
parse(text = x)[[1]][1]
# `/`()
parse(text = x)[[1]][2]
# ((a + b) * c)()
parse(text = x)[[1]][3]
# (d * (e - f))()

Quoting ?expression,
>> As an object of mode ?"expression"? is a list, it can be
subsetted
>> by ?[?, ?[[? or ?$?, the latter two extracting individual calls etc.
-- 
Best regards,
Ivan

Does something like the following make any sense for your context?
> z <- "(xy+ s)/ (ab +log(4.13))"
>
> lapply(parse(text = z), FUN = function(x)strsplit(deparse(x), split
"[[:space:]]+"))
[[1]]
[[1]][[1]]
[1] "(xy"        "+"          "s)/(ab"    
"+"          "log(4.13))"


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 Tue, Oct 29, 2019 at 9:23 AM Ivan Krylov <krylov.r00t at gmail.com>
wrote:
> On Tue, 29 Oct 2019 13:55:27 +0100
> Witold E Wolski <wewolski at gmail.com> wrote:
>
> > Since R knows how to parse expressions these type of expressions I
> > would like to reuse some existing functions and not to use gsub or
> > strsplit etc.
>
> You might be interested in the `parse` function:
>
> x <- "(a+b) * c/(d * (e - f))"
> str(parse(text = x))
> # length 1 expression((a + b) * c/(d * (e - f)))
> #  - attr(*, "srcref")=List of 1
> #   ..$ : 'srcref' int [1:8] 1 1 1 23 1 23 1 1
> #   .. ..- attr(*, "srcfile")=Classes 'srcfilecopy',
'srcfile'
> <environment: 0x55ebecdac210>
> #  - attr(*, "srcfile")=Classes 'srcfilecopy',
'srcfile'
> <environment:0x55ebecdac210>
> #  - attr(*, "wholeSrcref")= 'srcref' int [1:8] 1 0 2 0 0
0 1 2
> #   ..- attr(*, "srcfile")=Classes 'srcfilecopy',
> 'srcfile'<environment:0x55ebecdac210>
> parse(text = x)[[1]]
> # (a + b) * c/(d * (e - f))
> parse(text = x)[[1]][1]
> # `/`()
> parse(text = x)[[1]][2]
> # ((a + b) * c)()
> parse(text = x)[[1]][3]
> # (d * (e - f))()
>
> Quoting ?expression,
>
> >> As an object of mode ?"expression"? is a list, it can be
subsetted
> >> by ?[?, ?[[? or ?$?, the latter two extracting individual calls
etc.
>
> --
> Best regards,
> Ivan
>
> ______________________________________________
> 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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