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Question

How to return 5 topmost values from vector in R?

Answer 1

A:

Yes, head( X, 5) where X is your sorted vector.

Dirk Eddelbuettel 2010-09-11 20:44:00

Answer 2

+1 A:

> a <- c(1:100)
> tail(sort(a),5)
[1]  96  97  98  99 100

chrisamiller 2010-09-11 21:23:43

Or `head(sort(a, decreasing=TRUE), 5)`

Marek 2010-09-11 21:35:07

tail is slightly faster than head and decreasing = TRUE> x <- rnorm(50000000)> system.time(tail(sort(x), 5)) user system elapsed 22.64 0.25 22.95 > system.time(head(sort(x, decreasing = TRUE), 5)) user system elapsed 23.26 0.20 23.51

Thierry 2010-09-11 22:28:40

@Thierry You should run this more then once and take average time. Cause I think there is no difference (statistically speaking), based on my simulations.

Marek 2010-09-13 08:06:33

I get on average 2% faster times for user.self and elapse. The gain on sys.self is 8%. But the relevance on the gain depends on the application.

Thierry 2010-09-19 21:59:20

Answer 3

+1 A:

x[order(x)[1:5]]

hadley 2010-09-11 23:07:39

Answer 4

A:

tail(sort.int(x, partial=length(x) - 4), 5)

Using sort.int with partial has the advantage of being (potentially) faster by (potentially) not doing a full sort. But in reality, my implementation appears a little slower. Maybe this is because with parameter partial != NULL, shell sort is used rather than quick sort?

> x <- 1:1e6
> system.time(replicate(100, tail(sort.int(x, partial=length(x) - 4), 5)))
   user  system elapsed 
  4.782   0.846   5.668
> system.time(replicate(100, tail(sort(x), 5)))
   user  system elapsed 
  3.643   0.879   4.854

David F 2010-09-12 08:07:32

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How to return 5 topmost values from vector in R?

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