R: Find indices of each unique item in list

Question

I have a long list of repeating strings- here is some toy data,

unique_items <- c('shoes', 'shirt', 'pants', 'socks')
l <- unique_items[sample(1:4, 100000, replace = TRUE)]

I would now like to find ALL of the indices of each of the items in unique_items.

A naive solution would be:

item_indices <- vector(mode = 'list', length = length(unique_items))
names(item_indices) <- unique_items
for (idx in 1:length(unique_items))
{
  item <- unique_items[idx]
  item_indices[[item]] <- which(l == item)
}

However, that is incredibly slow when I have a huge list.

Is there a faster way to do this?

Answer 1

item_indices <- split(seq_along(l), l)