Find the position of a string sequence in other vector

Question

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.

might be relevant: https://stackoverflow.com/questions/48660606/get-indexes-of-a-vector-of-numbers-in-another-vector — chinsoon12, May 12 '20 at 09:55

Chris Ruehlemann · Answer 1 · 2020-05-12T10:40:36.803

2

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

edited May 12 '20 at 10:40

answered May 12 '20 at 10:13

Chris Ruehlemann

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score 2 · Accepted Answer · answered May 12 '20 at 10:43

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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

answered May 12 '20 at 10:43

GKi

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I ended up using this answer since it is the only one that worked for my practical problem. – pat-s May 14 '20 at 19:57

score 0 · Answer 3 · answered May 12 '20 at 10:03

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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])

answered May 12 '20 at 10:03

CourtesyBus

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or use this: `lapply(stringB, function(x) which(x == unique(stringA)))` – Chris Ruehlemann May 12 '20 at 10:42

Find the position of a string sequence in other vector

3 Answers3