Is there an R function finding a vector within another vector?

user_17189426 0
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For example, I have two vectors:

x1 <- c(0,1,2)
x2 <- c(1,2,0,1,2,3,4,5,0,1,0,1,2)

I would like to find the position where x1 is first present x2, which is in this case is 3. I have tried which(), match(), but both seems to find elements, instead of vectors. Is there a R function that can achieve this or a way to construct one?

Thank you!

match vector position r
4 Answers

If you are looking for the first match of 0:

match(x1[1], x2)

But for the whole vector:

> which(apply(t(embed(x2, length(x1))) == rev(x1), 2, all))[1]
[1] 3
>

Edit:

To give NA on no-match:

cond <- apply(t(embed(x2, length(x1))) == rev(x1), 2, all)
if (any(cond)) which(cond)[1] else NA

Explanation:

This uses embed to split x2 into chunks by the length of x1, and it applies it and detects whether it is equivalent to x1, and gives a boolean vector (or TRUEs and FALSEs). Then finally it uses which to get the index of the occurrences of TRUE, and I use [1] to get the first value.

For the edit, I detect if in the conditioned boolean vector contains any TRUEs, if so, do the same thing as mentioned above, if not, give NA.

user_87083640

One option would be to do rolling comparison.

x1 <- c(0,1,2) 
x2 <- c(1,2,0,1,2,3,4,5,0,1,0,1,2)

which(zoo::rollapply(x2, length(x1), function(x) all(x == x1)))
#[1]  3 11

To get the 1st occurrence -

which(zoo::rollapply(x2, length(x1), function(x) all(x == x1)))[1]
#[1] 3
user_39629140

For all elements for vector x1.

sapply(x1, function(x) match(x,x2))
[1] 3 1 2
user_107227520

Another approach with stringr;

   library(tidyverse)

x1_ <- paste0(x1,collapse="")
x2_ <- paste0(x2,collapse="")
str_locate_all(x2_, x1_)[[1]]

gives,

     start end
[1,]     3   5
[2,]    11  13

which returns empty if there is no match.

user_72243540
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