On Jul 12, 2011, at 4:17 PM, Sam Steingold wrote:
when do I need to use which()?
a <- c(1,2,3,4,5,6)
a
[1] 1 2 3 4 5 6
a[a==4]
[1] 4
a[which(a==4)]
[1] 4
which(a==4)
[1] 4
a[which(a>2)]
[1] 3 4 5 6
a[a>2]
[1] 3 4 5 6
seems unnecessary...
It is unnecessary when `a` is a toy case and has no NA's. And you will
find some of the cognoscenti trying to correct you when you do use
which().
a <- c(1,2, NA ,3,4, NaN, 5,6)
> data.frame(lets= letters[1:8], stringsAsFactors=FALSE)[a>0, ]
[1] "a" "b" NA "d" "e" NA "g" "h"
> data.frame(lets= letters[1:8], stringsAsFactors=FALSE)[which(a>0), ]
[1] "a" "b" "d" "e" "g" "h"
If you have millions of records and tens of thousands of NA's (say ~
1% of the data), imagine what your console looks like if you try to
pick out records from one day and get 10,000 where you were expecting
100. A real PITA when you are doing real work.
--
David Winsemius, MD
West Hartford, CT
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