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rvectoruniqueindexof

R: how to find index of all repetition vector values order by unique vector without using loop?

I have a vector of integers like this: 

a <- c(2,3,4,1,2,1,3,5,6,3,2)
values<-c(1,2,3,4,5,6)

I want to list, for every unique value in my vector (the unique values being ordered), the position of their occurences. My desired output:

rep_indx<-data.frame(c(4,6),c(1,5,11),c(2,7,10),c(3),c(8),c(9))
Solution

  • split fits pretty well here, which returns a list of indexes for each unique value in a:

    indList <- split(seq_along(a), a)
indList
# $`1`
# [1] 4 6
# 
# $`2`
# [1]  1  5 11
# 
# $`3`
# [1]  2  7 10
# 
# $`4`
# [1] 3
# 
# $`5`
# [1] 8
# 
# $`6`
# [1] 9

    And you can access the index by passing the value as a character, i.e.:

    indList[["1"]]
# [1] 4 6