[R] Extracting the first element of a list

[John Fox]

Dear Kevin,

At 10:48 PM 3/27/2002 +1200, Ko-Kang Kevin Wang wrote:

> > For the first element of a list L:
> >  L[[1]]
> > For the first elements of all elements (vector) of the list:
> >  lapply(L, function(x) x[1])
>
>So, suppose I've got a vector called foo, which has elements as follows:
>   305 159 251 215 101 224 306 199 194 325 329 221 318 238  17 ....
>(the rest omittied)
>
>And suppose I want to extract out the first digit of each number, so like:
>   3 1 2 2 1 2 3 1 1 3 3 2 3 2 1...
>
>Do I still use
>   lapply(foo, function(x) x[1])
>it doesn't seem to work...

The lapply function takes a list as its first argument, and a vector of 
numbers isn't a list; moreover, each number isn't a vector a individual 
digits that can be extracted by subscripting.

You could use something like the following:

         > x <- c(1, 23, 456, 7890)
         > x %/% 10^floor(log(x, 10))
         [1] 1 2 4 7

If there are negative entries in x, use abs(x).

I hope that this helps,
  John


-----------------------------------------------------
John Fox
Department of Sociology
McMaster University
Hamilton, Ontario, Canada L8S 4M4
email: jfox at mcmaster.ca
phone: 905-525-9140x23604
web: www.socsci.mcmaster.ca/jfox
-----------------------------------------------------

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