CODEWITHSUNDEEP
rstring
pat-s
pat-s

Reputation: 6302

Find the position of a string sequence in other vector

Given the following two character vectors

stringA = c("AA", "BB", "CC", "BB", "DD", "CC")
stringB = c("BB", "CC")

I want to find the positions of stringB within stringA.

The result should be a vector of length stringB, hence for this example: c(2, 3). A vector of length one only containing the start position of the sequence (here: 2) would also be ok (since I know the length of stringB and could operate further with it).

It can be assumed that the sequence of stringB is unique within stringA.

Upvotes: 4

Views: 366

Answers (3)

GKi
GKi

Reputation: 39667

In case you are looking for the position where the whole sequence of stringB starts in stringA you can use:

which(rowSums(!sapply(seq(stringB), function(i) stringB[i] ==
   stringA[i:(length(stringA)-length(stringB)+i)])) == 0)
#[1] 2

Upvotes: 2

Chris Ruehlemann
Chris Ruehlemann

Reputation: 21400

You're after positions of elements in vectors. Positions are equivalent to indices.

To grab TRUE indices (that is, those indices the elements of which evaluate to TRUE on some condition--in your case, that the unique elements of stringA be contained in stringB), you can use which:

which(unique(stringA) %in% stringB)
[1] 2 3

Upvotes: 2

CourtesyBus
CourtesyBus

Reputation: 351

if you're looking every position of stringB in stringA, then you could use this:
lapply(stringB, function(x) which(x == stringA))

alternatively, if you just want a vector containing the first position of stringB in stringA, you could try this:
sapply(stringB, function(x) which(x == stringA)[1])

Upvotes: 0

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