return indices of duplicated elements corresponding to the unique elements in R

问题

anyone know if there's a build in function in R that can return indices of duplicated elements corresponding to the unique elements? For instance I have a vector a <- ["A","B","B","C","C"] unique(a) will give ["A","B","C"] duplicated(a) will give [F,F,T,F,T] is there a build-in function to get a vector of indices for the same length as original vector a, that shows the location a's elements in the unique vecor (which is [1,2,2,3,3] in this example)?

i.e., something like the output variable "ic" in the matlab function "unique". (which is, if we let c = unique(a), then a = c(ic,:)). http://www.mathworks.com/help/matlab/ref/unique.html

Thank you!

回答1:

We can use match

match(a, unique(a))
#[1] 1 2 2 3 3

Or convert to factor and coerce to integer

as.integer(factor(a, levels = unique(a)))
#[1] 1 2 2 3 3

data

a <- c("A","B","B","C","C")

回答2:

This should work:

cumsum( !duplicated( sort( a)) )  # one you replace  Mathlab syntax with R syntax.

Or just:

as.numeric(factor(a) )

来源：https://stackoverflow.com/questions/37495559/return-indices-of-duplicated-elements-corresponding-to-the-unique-elements-in-r