Check for consecutive elements in vector

user_15598346 0
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I have a vector of K elements and I need to check if it contains a precise subset of 2 consecutive elements.

Assume K = 6

vec = c(4,4,3,1,2,2)

How can I get an indicator that checks for the subset (4,2) as consecutive elements? Should return FALSE in this example while (3,1) should return TRUE.

subset indicator vector r
4 Answers

One another approach could be:

grepl("42", paste(vec, collapse = ""))

[1] FALSE

grepl("31", paste(vec, collapse = ""))

[1] TRUE
user_59645570

1) Use rollapply with the identical function.

library(zoo)

vec <- c(4,4,3,1,2,2)
x <- c(4, 2)
any(rollapply(vec, length(x), identical, x))
## [1] FALSE

x <- c(3, 1)
any(rollapply(vec, length(x), identical, x))
## [1] TRUE

2) Create strings out of vec and x and use grepl:

x <- c(4, 2)
grepl(sprintf("\\b%s\\b", toString(x)), toString(vec))
## [1] FALSE

x <- c(3, 1)
grepl(sprintf("\\b%s\\b", toString(x)), toString(vec))
## [1] TRUE

3) If x always has two elements then:

x <- c(4, 2)
any(head(vec, -1) == x[1] & tail(vec, -1) == x[2])
## [1] FALSE

x <- c(3, 1)
any(head(vec, -1) == x[1] & tail(vec, -1) == x[2])
## [1] TRUE
user_5165480

Especially if you have more than 2 elements to check, this might be worthwhile:

vec <-= c(4,4,3,1,2,2)
chk <- c(3, 1)

as.logical(length(Reduce(intersect, lapply(seq_along(chk), function(x) which(chk[x] == lead(vec, x - 1))))))
# [1] TRUE

chk <- c(4, 1)
as.logical(length(Reduce(intersect, lapply(seq_along(chk), function(x) which(chk[x] == lead(vec, x - 1))))))
# [1] FALSE
user_61620110

Using base R:

find_subsequence = function (vec, needle) {
    sub_idx = seq_along(needle) - 1L
    Filter(
        function (i) all(needle == vec[sub_idx + i]),
        seq_len(length(vec) - length(needle) + 1L)
    )
}

find_subsequence(vec, c(3, 1))
# [1] 4
find_subsequence(vec, c(4, 2))
# integer(0)
user_19680
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