R: how do I count the number of times a value has already appeared in a vector (or the number of times it appears to the left of that value)?

Question

I am using R. Say I have the vector x:

x <- c(1, 4, 3, 1, 5, 3, 4, 1, 2, 3)

And I want to count, for each value, the number of times that value has already appeared (or the number of times it appears to the left of that value).

In this example, the output would be:

1, 1, 1, 2, 1, 2, 2, 3, 1, 3

How do you do that?

Answer 1

An option with ave i.e. grouped by 'x', get the sequence

ave(seq_along(x), x, FUN = seq_along)
#[1] 1 1 1 2 1 2 2 3 1 3

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Or using rowid from data.table

library(data.table)
rowid(x)