CODEWITHSUNDEEP
rvectorization
colinfang
colinfang

Reputation: 21727

How to prevent NULL from killing my ifelse vectorising?

I would like to vectorize such function:

if (i > 0)
   fun1(a[i])
else
   fun2(i)

where fun1, fun2 are already vectorized, and a is a vector.

My attempt:

ifelse(i > 0, fun1(a[i]), fun2(i))

However it is wrong!

> a <- c(1,2,3,4,5)
> i<-c(0,1,2,3)
> ifelse(i > 0, a[i], 0)
[1] 0 2 3 1 # expected 0 1 2 3

Do I have to use sapply? Is there any simple alternative that works?

Upvotes: 1

Views: 532

Answers (1)

Ferdinand.kraft
Ferdinand.kraft

Reputation: 12819

There is nothing wrong. a[i] evaluates to c(1,2,3) since a[0] is ignored. this is recycled to c(1,2,3,1) to match length(i). So you get 0 2 3 1 from your ifelse because the first element of i is FALSE, and the other come from a[i] recycled.

There is a workaroud though: you can replace non-positive indices with NA:

> ifelse(i > 0, a[ifelse(i > 0, i, NA)], 0)
[1] 0 1 2 3

Upvotes: 3

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