CODEWITHSUNDEEP
r
Sathish
Sathish

Reputation: 12723

Find sequence between elements in a vector

I would like to get the sequence between elements in a vector. Here is a reproducible example.

vec <- c( 'a', letters[2:7], 'a', letters[9:14], 'a', letters[16:21] ) # sample data
ind_a <- which( grepl( 'a', vec ))  # indices matching the character 'a'
ind_a <- c( ind_a, length(vec)+1 )  # concatenate the length of 'vec' + 1 with indices
ind_a
# [1]  1  8 15 22

Now, how to compute the sequence between the elements of ind_a. For example, seq(from = 2, to = 7, by = 1) and the same for others.

Preferably, I would like to know any function in base R for this task.

Expected Output

# List of 3
# $ : int [1:6] 2 3 4 5 6 7
# $ : int [1:6] 9 10 11 12 13 14
# $ : int [1:6] 16 17 18 19 20 21

Upvotes: 4

Views: 812

Answers (2)

d.b
d.b

Reputation: 32558

lapply(2:length(ind_a), function(i) setdiff(sequence(ind_a[i] - 1), sequence(ind_a[i-1])))
#OR    
lapply(2:length(ind_a), function(i) (ind_a[i-1]+1):(ind_a[i]-1))

#[[1]]
#[1] 2 3 4 5 6 7

#[[2]]
#[1]  9 10 11 12 13 14

#[[3]]
#[1] 16 17 18 19 20 21

Upvotes: 2

akrun
akrun

Reputation: 887951

We create two vectors from 'ind_a' by removing the last and first observations, then get the sequence of corresponding elements with Map

Map(seq, ind_a[-length(ind_a)]+1, ind_a[-1]-1)
#[[1]]
#[1] 2 3 4 5 6 7

#[[2]]
#[1]  9 10 11 12 13 14

#[[3]]
#[1] 16 17 18 19 20 21

Or as @Zelazny7 suggested, removing the first and last element can be done with head and tail function

Map(`:`, head(ind_a, -1) + 1, tail(ind_a, -1) - 1)

Upvotes: 4

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