Finding indices with a certain property

I want the indices  i  for which  x[i] < 0 (say):

x <- c(1, -1, 3, 3, -2)
where.negative(x)

[1] 2 5

Surely  where.negative  is something simple, but how?

which(x<0)


	-thomas

Thomas Lumley
Asssistant Professor, Biostatistics
University of Washington, Seattle

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G?ran Brostr?m <gb at stat.umu.se> writes:

I want the indices  i  for which  x[i] < 0 (say):

x <- c(1, -1, 3, 3, -2)
where.negative(x)

[1] 2 5

Surely  where.negative  is something simple, but how?

How about something like this?

indices<-function(x)seq(along=x)[x]
x <- c(1, -1, 3, 3, -2)
indices(x<0)

[1] 2 5

O__  ---- Peter Dalgaard             Blegdamsvej 3  
  c/ /'_ --- Dept. of Biostatistics     2200 Cph. N   
 (*) \(*) -- University of Copenhagen   Denmark      Ph: (+45) 35327918
~~~~~~~~~~ - (p.dalgaard at biostat.ku.dk)             FAX: (+45) 35327907
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I want the indices  i  for which  x[i] < 0 (say):

x <- c(1, -1, 3, 3, -2)
where.negative(x)

[1] 2 5

Try "which(x<0)".

Jonathan Rougier                       Science Laboratories
Department of Mathematical Sciences    South Road
University of Durham                   Durham DH1 3LE

"[B]egin upon the precept ... that the things we see are to be 
 weighed in the scale with what we know"  (Meredith, 1879, The Egoist)

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Thomas Lumley <thomas at biostat.washington.edu> writes:

Surely  where.negative  is something simple, but how?

which(x<0)

Darn! Forgot about that one....

O__  ---- Peter Dalgaard             Blegdamsvej 3  
  c/ /'_ --- Dept. of Biostatistics     2200 Cph. N   
 (*) \(*) -- University of Copenhagen   Denmark      Ph: (+45) 35327918
~~~~~~~~~~ - (p.dalgaard at biostat.ku.dk)             FAX: (+45) 35327907
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"GB" == Goran Brostrom <gb at stat.umu.se> writes:

GB> I want the indices i for which x[i] < 0 (say):

    >> x <- c(1, -1, 3, 3, -2) where.negative(x)
    GB> [1] 2 5

    GB> Surely where.negative is something simple, but how?

Yes :

which(x < 0)
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Quick answers to my question:

which(x < 0)

but also

indices<-function(x)seq(along=x)[x]
x <- c(1, -1, 3, 3, -2)
indices(x<0)

[1] 2 5

from some.

The last one is essentially the definition of  which, I think.
I had completely forgotten  which  (where?!?) 

Thanks to Thomas Lumley, Jonathan Rougier, Martin Maechler,
Peter Dalgaard, Brian Ripley!

G?ran

Original question:

I want the indices  i  for which  x[i] < 0 (say):

x <- c(1, -1, 3, 3, -2)
where.negative(x)

[1] 2 5

Surely  where.negative  is something simple, but how?

--------------------------------------------------------------
 G?ran Brostr?m
 Department of Statistics              tel: +46 90 786-5223
 Ume? University                       fax: +46 90 786-6614
 S-90187 Ume?, Sweden               e-mail:  gb at stat.umu.se

 http://www.stat.umu.se/~gb          ftp://capa.stat.umu.se
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