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Thursday, November 17, 2011

strsplit in R

The strsplit function in the R statistics package splits a string into a list of substrings based on the separator, just like split in Perl or Ruby. The object returned is a List, one of the core R object types. For example:
> a <- strsplit("x y z", ' ')
> a
[[1]]
[1] "x" "y" "z"
> class(a)
[1] "list"

If you are not that familiar with R, like me, the obvious way to access an element in the list will not work:
> a[1]
[[1]]
[1] "x" "y" "z"
> a[2]
[[1]]
NULL

So what do you do? There seem to be two options:

1: You can 'dereference' the element (for want of a better word) by using the multiple sets of brackets
> a[[1]][1]
[1] "x"
> a[[1]][2]
[1] "y"
... but I'm not going to write code that looks like that !!

2: You can unlist the List to create a Vector and then access elements directly
> b < unlist(a)
[1] FALSE FALSE FALSE
> b <- unlist(a)
> b
[1] "x" "y" "z"
> class(b)
[1] "character"
> b[1]
[1] "x"
> b[2]
[1] "y"
Much nicer !





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