R help - Apr 2011 - Getting many substrings but only loading the original string one time.

Hi All,
    I'm looking for a way to get many substrings from a longer string and
then stitch them together.  But, since the longer string is really, really
long (like 250 MB long), I don't want to do this in a loop and load and
re-load the longer string many times.  Does anybody have an idea?

Maybe I could pass in two vectors (the first would have the starting
coordinates, and the second would have the stopping coordinates), so it
would be like a vectorized version of substr, where start and stop would be
vector instead of single integers.

Example (I'm reducing the size of the string for the example) of how this
might work:
> longerString <- 'HelloThisIsMyLongerString"
> startVector <-  c(2,6,4)
> stopVector <- c(4,10,5)
> substrings <- vectorized_substr(longerString, startVector, stop Vector)
> longerString
[1] "ell" "ThisI" "lo"

Then I'd like to concatenate them (there will be many of them)
> result <- paste(longerString,collapse='')
> result
[1] "ellThisIlo"

(perhaps the paste command as I've done it is the best way, but depending on
how the substrings are reported there may be different ways). Thanks!

Jonathan

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On 11/04/2011 3:48 PM, Jonathan wrote:
> Hi All,
>      I'm looking for a way to get many substrings from a longer string
and
> then stitch them together.  But, since the longer string is really, really
> long (like 250 MB long), I don't want to do this in a loop and load and
> re-load the longer string many times.  Does anybody have an idea?
>
> Maybe I could pass in two vectors (the first would have the starting
> coordinates, and the second would have the stopping coordinates), so it
> would be like a vectorized version of substr, where start and stop would be
> vector instead of single integers.
>
> Example (I'm reducing the size of the string for the example) of how
this
> might work:
>
> >  longerString<- 'HelloThisIsMyLongerString"
> >  startVector<-  c(2,6,4)
> >  stopVector<- c(4,10,5)
>
> >  substrings<- vectorized_substr(longerString, startVector, stop
Vector)
> >  longerString
> [1] "ell" "ThisI" "lo"
Use substring(), not substr().  It is vectorized:

 > substring(longerString, startVector, stopVector)
[1] "ell"   "ThisI" "lo"

It does this by replicating the longerString, but that doesn't mean 
actual copies are made:  just multiple pointers to the same big one.

Duncan Murdoch
> Then I'd like to concatenate them (there will be many of them)
>
> >  result<- paste(longerString,collapse='')
> >  result
> [1] "ellThisIlo"
>
> (perhaps the paste command as I've done it is the best way, but
depending on
> how the substrings are reported there may be different ways). Thanks!
>
> Jonathan
>
> 	[[alternative HTML version deleted]]
>
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