if key exists: update, otherwise: assoc

Question

consider this inside a reduce loop:

(if (contains? m k)
  (update m k conj v)
  (assoc m k [v]))

Is there a way to get rid of the if statement?

Daniel Compton · Accepted Answer · 2016-02-11T10:31:59.330

23

Use fnil to handle the nil value of v when k doesn't exist in the map:

(update m k (fnil conj []) v)

edited Feb 11 '16 at 10:31

answered Feb 11 '16 at 00:37

Daniel Compton

1

took me some time to grasp it, but yes! got it now, great answer! – Anton Harald Feb 11 '16 at 01:01
1

shazam, nice answer! – Timothy Pratley Feb 11 '16 at 04:37

score 3 · Answer 2 · answered Nov 09 '17 at 14:36

While the fnil answer is more spectacular, I find the following easier to read, especially if unfamiliar with fnil:

(assoc m k (conj (m k []) v))

where (m k []) returns the value of k in m or defaults to [] if k does not exist in m.

If k is a symbol, (k m []) would also work.

2 Answers2