[R] which.max2()

[Esmail Bonakdarian]

Hello,

which.max() only returns one index value, the one for the
maximum value. If I want the two index values for the two
largest values, is this a decent solution, or is there a
nicer/better R'ish way?

max2 <-function(v)
{
m=which.max(v)
v[m] = -v[m]
m2=which.max(v)
result=c(m, m2)
result
}

Seems to work ok.

Thanks,
Esmail

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[Dimitris Rizopoulos]

try this:

v <- rnorm(10)
v
order(v, decreasing = TRUE)[1:2]

I hope it helps.

Best,
Dimitris

----
Dimitris Rizopoulos
Biostatistical Centre
School of Public Health
Catholic University of Leuven

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[Marc Schwartz]

on 05/09/2008 08:07 AM Esmail Bonakdarian wrote:
> Hello,
>
> which.max() only returns one index value, the one for the
> maximum value. If I want the two index values for the two
> largest values, is this a decent solution, or is there a
> nicer/better R'ish way?
>
> max2 <-function(v)
> {
> m=which.max(v)
> v[m] = -v[m]
> m2=which.max(v)
> result=c(m, m2)
> result
> }
>
> Seems to work ok.
>
> Thanks,
> Esmail

I might be tempted to take a more generic approach, where one can
provide an argument to the function to indicate that I want the 'top x'
maximum values and to give the user the option of returning the indices
or the values themselves.

Perhaps:

which.max2 <- function(x, top = 1, values = FALSE)
{
if (values)
rev(sort(x))[1:top]
else
order(x, decreasing = TRUE)[1:top]
}

set.seed(1)
Vec <- rnorm(10)

> Vec
[1] -0.6264538 0.1836433 -0.8356286 1.5952808 0.3295078 -0.8204684
[7] 0.4874291 0.7383247 0.5757814 -0.3053884

> which.max2(Vec, 2)
[1] 4 8

> which.max2(Vec, 2, values = TRUE)
[1] 1.5952808 0.7383247

HTH,

Marc Schwartz

[Esmail Bonakdarian]

Dimitris Rizopoulos wrote:
> try this:
>
> v <- rnorm(10)
> v
> order(v, decreasing = TRUE)[1:2]

Wow .. that is slick! First I thought, wait .. I don't want to
reorder the elements, but this doesn't - it just returns the index
values in order. I don't really get that from reading the documentation,
it's probably there, but not that clear to me.

Thanks for showing me something more "R'ish".

[Esmail Bonakdarian]

Marc Schwartz wrote:
>
> I might be tempted to take a more generic approach, where one can
> provide an argument to the function to indicate that I want the 'top x'
> maximum values and to give the user the option of returning the indices
> or the values themselves.
>
> Perhaps:
>
> which.max2 <- function(x, top = 1, values = FALSE)
> {
> if (values)
> rev(sort(x))[1:top]
> else
> order(x, decreasing = TRUE)[1:top]
> }

Very cool too! .. Thanks Marc. Again, I did not get this from the
order documentation (ie that it manipulate index values rather than
the values themselves). Great to see examples.

Best,
Esmail