Thanks Michael:
You are essentially doing the eval and parsing by hand instead of
letting eval(parse()) do the work. I prefer the latter.
However, your code did something that I did not expect and for which I
can find no documentation -- I would have thought it shouldn't work.
... and that is, the return of your sapply is n1:n2 where n1 and n2
are _character values_ (because that's what gsub returns, of course).
I would have thought this would give an error, but in fact it gives
the "correct" result. That is, to my complete surprise:
> "3":"5"
[1] 3 4 5
> seq(from= "3", to= "5")
[1] 3 4 5
> seq.int( "3", "5")
[1] 3 4 5
> "3":5
[1] 3 4 5
all work! Is this behavior documented anywhere and I've missed it or
is it a secret "feature." And to what extent does it work, noting
that:
seq(from="3.5",to="5.5",by="1")
Error in to - from : non-numeric argument to binary operator
Cheers,
Bert
On Fri, Aug 20, 2010 at 4:39 PM, Michael Hannon <jm_han...@yahoo.com> wrote:
>> For regular expression afficianados, I'd like a cleverer solution to
>> the following problem (my solution works just fine for my needs; I'm
>> just trying to improve my regex skills):
>>
>> Given the string (entered, say, at a readline prompt):
>>
>> "1 2 -5, 3- 6 4 8 5-7 10" ## only integers will be entered
>>
>> parse it to produce the numeric vector:
>>
>> c(1, 2, 3, 4, 5, 3, 4, 5, 6, 8, 5, 6, 7, 10)
>>
>> Note that "-" in the expression is used to indicate a range of values
>> instead of ":"
>>
>> Here's my UNclever solution:
>>
>> First convert more than one space to a single space and then replace
>> "<any spaces>-<any spaces>" by ":" by:
>>
>> > x1 <- gsub(" *- *",":",gsub(" +"," ",resp)) #giving
>> > x1
>> [1] "1 2:5, 3:6 4 8 5:7 10" ## Note that the comma remains
>>
>> Next convert the single string into a character vector via strsplit by
>> splitting on anything but ":" or a digit:
>>
>> > x2 <- strsplit(x1,split="[^:[:digit:]]+")[[1]] #giving
>> > x2
>> [1] "1" "2:5" "3:6" "4" "8" "5:7" "10"
>>
>> Finally, parse() the vector, eval() each element, and unlist() the
>> resulting list of numeric vectors:
>>
>> > unlist(lapply(parse(text=x2),eval)) #giving, as desired,
>> [1] 1 2 3 4 5 3 4 5 6 4 8 5 6 7 10
>>
>>
>> This seems far too clumsy and circumlocuitous not to have a more
>> elegant solution from a true regex expert.
>>
>> (Special note to Thomas Lumley: This seems one of the few instances
>> where eval(parse..)) may actually be appropriate.)
>
> Howdy. I don't know that I can produce anything less circumlocutory, but I
> note that your "x2" form has a simple-enough structure that it can be further
> parsed with regular expressions, i.e., as opposed to using parse and eval. I
> don't know that this is an improvement -- just a variation on the theme.
>
> I've appended an example.
>
> -- Mike
>
> #### Original vector
> x <- "1 2 -5, 3- 6 4 8 5-7 10"; x
>
> #### Convert ranges to standard R form
> x1 <- gsub("[ ]*-[ ]*", ":", x); x1
>
> #### Get rid of the comma
> x2 <- gsub(",", " ", x1); x2
>
> #### Remove extra spaces
> x3 <- gsub("[ ]+", " ", x2); x3
>
> #### Split off elements, now in standard form
> x4 <- unlist(strsplit(x3, " ")); x4
>
> #### Use regular expression for simple parse of elements
> x5 <- sapply(x4, function(a) {
> n1 <- gsub("([[:digit:]]):[[:digit:]]", "\\1", a)
> n2 <- gsub("[[:digit:]]:([[:digit:]])", "\\1", a)
> n1:n2}, USE.NAMES=FALSE); x5
> x6 <- unlist(x5); x6
>
> ##########################################################
>
>> #### Original vector
>> x <- "1 2 -5, 3- 6 4 8 5-7 10"; x
> [1] "1 2 -5, 3- 6 4 8 5-7 10"
>>
>> #### Convert ranges to standard R form
>> x1 <- gsub("[ ]*-[ ]*", ":", x); x1
> [1] "1 2:5, 3:6 4 8 5:7 10"
>>
>> #### Get rid of the comma
>> x2 <- gsub(",", " ", x1); x2
> [1] "1 2:5 3:6 4 8 5:7 10"
>>
>> #### Remove extra spaces
>> x3 <- gsub("[ ]+", " ", x2); x3
> [1] "1 2:5 3:6 4 8 5:7 10"
>>
>> #### Split off elements, now in standard form
>> x4 <- unlist(strsplit(x3, " ")); x4
> [1] "1" "2:5" "3:6" "4" "8" "5:7" "10"
>>
>> #### Use regular expression for simple parse of elements
>> x5 <- sapply(x4, function(a) {
> + n1 <- gsub("([[:digit:]]):[[:digit:]]", "\\1", a)
> + n2 <- gsub("[[:digit:]]:([[:digit:]])", "\\1", a)
> + n1:n2}, USE.NAMES=FALSE); x5
> [[1]]
> [1] 1
>
> [[2]]
> [1] 2 3 4 5
>
> [[3]]
> [1] 3 4 5 6
>
> [[4]]
> [1] 4
>
> [[5]]
> [1] 8
>
> [[6]]
> [1] 5 6 7
>
> [[7]]
> [1] 10
>
>> x6 <- unlist(x5); x6
> [1] 1 2 3 4 5 3 4 5 6 4 8 5 6 7 10
>>
>
>
>
>
--
Bert Gunter
Genentech Nonclinical Biostatistics
467-7374
http://devo.gene.com/groups/devo/depts/ncb/home.shtml
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