CODEWITHSUNDEEP
rmatrixvectorindexingoperation
bastien chassagnol
bastien chassagnol

Reputation: 25

How can we do operations inside indexing operations in R?

For example, let's imagine following vector in R:

a <- 1:8; k <- 2

What I would like to do is getting for example all elements between 2k and 3k, namely:

interesting_elements <- a[2k:3k]
Erreur : unexpected symbol in "test[2k"
interesting_elements <- a[(2k):(3k)]
Erreur : unexpected symbol in "test[2k"

Unfortunately, indexing vectors in such a way in R does not work, and the only way I can do such an operation seems to create a specific variable k' storing result of 2k, and another k'' storing result of 3k.

Is there another way, without creating each time a new variable, for doing operations when indexing?

Upvotes: 0

Views: 42

Answers (1)

LMc
LMc

Reputation: 18692

R does not interpret 2k as scalar multiplication as with other languages. You need to use explicit arithmetic operators.

If you are trying to access the 4 to 6 elements of a then you need to use * and parentheses:

a[(2*k):(3*k)]
[1] 4 5 6

If you leave off the parentheses then the sequence will evaluate first then the multiplication:

2*k:3*k
[1]  8 12

Is the same as

(k:3)*2*k
[1]  8 12

Upvotes: 1

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