Question

The match(x, y) function is perfect to search the elements of the vector x within the elements of vector y. But what is an efficient and easy way to do the similar job when y is a list of vectors – of possibly different lengths?

I mean the result should be a vector of the same length as x, and the i-th element should be the first member of y that contains the i-th element of x, or NA.

Answer 1

To find the element of y in which each element of x (first) occurs, try this:

## First, a reproducible example
set.seed(44)
x <- letters[1:25]
y <- replicate(4, list(sample(letters, 8)))
y
# [[1]]
# [1] "t" "h" "m" "n" "a" "d" "i" "b"
# 
# [[2]]
# [1] "c" "l" "z" "a" "s" "d" "i" "u"
# 
# [[3]]
# [1] "b" "k" "e" "g" "o" "i" "h" "j"
# 
# [[4]]
# [1] "g" "i" "f" "r" "h" "w" "l" "o"

## Find the element of y first containing the letters a-j
breaks <- c(0, cumsum(sapply(y, length))) + 1 
findInterval(match(x, unlist(y)), breaks)
# [1]  1  1  2  1  3  4  3  1  1  3  3  2  1  1  3 NA NA  4  2  1  2 NA  4 NA NA