CODEWITHSUNDEEP
rvectorsubset
Anna Plaxienko
Anna Plaxienko

Reputation: 33

R Wrong logical subsetting for a vector

I have two logical vectors.

a <- c(1, 0, 0, 0, 1, 1, 0, 0, 1, 0, 0, 0, 1, 0, 0, 1, 0, 1, 1, 1, 1, 0, 1, 1, 0, 1, 0, 1, 1, 1, 0, 1, 1, 0, 0, 1, 1, 1, 0, 0, 1, 1, 0, 1, 1, 1, 1, 1, 1, 0, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0)
b <- c(0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0)

When I try to subset b to a like this

b[which(a == 1)]

everything is as I expect. However, like this

b[a]

I get different result.

Why? Probably the explanation is obvious but I completely don't understand...

Upvotes: 1

Views: 97

Answers (2)

ThomasIsCoding
ThomasIsCoding

Reputation: 101317

As @akrun explained, you need logical values for subseting b based on a.

Below is another option to turn numeric a to a logical one, e.g.,

> b[!!a]
 [1] 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 1
[39] 1 0 0 0

Upvotes: 0

akrun
akrun

Reputation: 887088

a is a binary vector. It can be converted to boolean with as.logical and then it will work as expected i.e. it will get the elements in 'b' that corresponds to TRUE values in 'a'

b[as.logical(a)]

because 0 is considered as FALSE whereas all other values can be TRUE

as.logical(c(1, 0, 2, 3))
#[1]  TRUE FALSE  TRUE  TRUE

Otherwise, indexing starts in R from 1 and the 1 value is selecting the first element based on the position, where as which(a == 1), gets the position of sequence from logical vector

b[c(1, 1, 0, 1)]

selects the first element of 'b' three times

whereas

b[which(c(1, 1, 0, 1) == 1)]

gets the position as 1, 2, 4 for selecting the 'b'

which(c(1, 1, 0, 1) == 1)
#[1] 1 2 4

Upvotes: 0

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