mapping-with-an-index.md

Mapping With An Index

If you're mapping over a collection and you need an index for each item, you can reach for map-indexed. The map-indexed function can be used like so:

> (def foods ["pizza" "hotdog" "chicken alfredo"])
#'cljs.user/foods
> (map-indexed (fn [idx item] (str idx " - " item)) foods)
("0 - pizza" "1 - hotdog" "2 - chicken alfredo")

Alternatively, map can take multiple sequences, and each becomes a new argument to the mapping function. By giving an infinite sequential list of numbers starting at 0 as the first sequence, you can pretend they're indices, like so:

> (map (fn [idx item] (str idx " - " item)) (range) foods)
("0 - pizza" "1 - hotdog" "2 - chicken alfredo")