CODEWITHSUNDEEP
r
Joe King
Joe King

Reputation: 3021

Add only to negative entries in a vector

I would like to take a vector, for example:

X <-  c(1,2,3,-1,-2,-3)

and add 1 (or any other number fixed in advance) to the entries in which the values are negative, to produce

1,2,3,0,-1,-2

Is there a slick way to do this ?

Upvotes: 3

Views: 139

Answers (5)

user1021713
user1021713

Reputation: 2203

One can use which command also : 

X <-  c(1,2,3,-1,-2,-3)
X[which(X < 0)] <- X[which(X < 0)] +1

Upvotes: 0

IRTFM
IRTFM

Reputation: 263362

Submitted in the R Golf competition for this problem:

X+(X<0)

Works because (X<0) will get coerced 1 for any desired case and 0 for others.

Upvotes: 8

Gavin Simpson
Gavin Simpson

Reputation: 174813

An alternative that is vectorised but slightly more verbose is:

> X <-  c(1,2,3,-1,-2,-3)
> want <- X < 0
> X[want] <- X[want] + 1
> X
[1]  1  2  3  0 -1 -2

The key step is to generate indices where X is negative (want). We then use this to index X and add 1 to these only.

Whether the vectorisation outweighs the number of function calls involved above I leave to others to discuss (I doubt it make any difference for most problems).

Upvotes: 7

MvG
MvG

Reputation: 60868

Perhaps not the most readable solution, but I'll still post it for the fun of it:

> pmax(X, X+sign(X)*(-1))
[1]  1  2  3  0 -1 -2

Upvotes: 3

Andrie
Andrie

Reputation: 179428

use ifelse:

x + ifelse(x<0, 1, 0)
[1]  1  2  3  0 -1 -2

Upvotes: 7

Related Questions