CODEWITHSUNDEEP
rdataframe
SolessChong
SolessChong

Reputation: 3407

How to get the index of data block in a data.frame?

I have a data.frame like this:

    a b
1   1 2
2   1 3
3   2 3
4   2 5

It's sorted by a and I need the indices of different a's. Now I'm using for loop, however it's not elegant.

Upvotes: 0

Views: 92

Answers (1)

Ricardo Saporta
Ricardo Saporta

Reputation: 55420

Try the following: 

# this will give you the row indices
lapply(unique(dat$a), function(a) which(dat$a==a))

If you want your results to be named, use:

U <- unique(dat$a)
names(U) <- U
lapply(U, function(a) which(dat$a==a))

# Produces: 
#  $`1`
#  [1] 1 2
#
#  $`2`
#  [1] 3 4

Upvotes: 2

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