如何在Clojure中测试空向量

Question

What's the best way to test for an empty vector in Clojure? I expected that this would print false:

(if [] "true" "false")

but it doesn't. This does:

(if (> (count []) 0) "true" "false")

but is unwieldy - is there a shorter construct?

Answer 1

使用标准谓词(empty? list)

Answer 2

The most common way I see in Clojure code to check for a non-empty list is to use seq . This returns nil if the collection is empty, or a valid seq object otherwise.

Example of usage:

(seq [])
=> nil

(seq nil)
=> nil

(seq [1 2 3])
=> (1 2 3)         ;; note this is a "true value"

(if (seq [1 4 6]) "true" "false")
=> "true"

(if (seq []) "true" "false")
=> "false"

You can also use empty? to test the opposite (ie test for an empty set). Note that empty? is implemented in the clojure source code as (not (seq coll)) so you can be safe in the knowledge that the two approaches are fundamentally equivalent.

Answer 3

In addition to Mishadoff's answer (which is more readable), you could also compare against an empty list itself (and save two characters in length):

(if (= () []) "true" "false")

Hmmm... I still think I prefer Mishadoff's answer.