counting only truthy values in a collection [duplicate]

Possible Duplicate:
how to return only truthy values as the result of a map operation

I have a collection which has falsy and truthy values. I would like to only count the truthy values, is there a way to do that ?

(count (1 2 3 nil nil)) => 5

If you want to keep the truthy values just need to use the identity function:

   (count (filter identity '(1 2 3 nil nil false true)))

I would recommend doing this with reduce as follows:

(defn count-truthy [coll]
  (reduce (fn [cnt val] (if val (inc cnt) cnt)) 0 coll))

Reasons for using reduce in this way:

It is likely to be more efficient, and will benefit from Clojure's new reducers functionality that enables fact reduces on many collections
It avoids creating an intermediate sequence (which would happen if you used a lazy sequence function like filter)

If you already have a realised sequence, then the following is also a good option, as it will benefit from primitive arithmetic in the loop:

(defn count-truthy [coll]
  (loop [s (seq coll) cnt 0]
    (if s
      (recur (next s) (if (first s) (inc cnt) cnt))
      cnt)))

Just remove values that you do not want to count.

(count (remove nil? [1 2 3 nil nil])) => 3

(defn truthy-count [coll] 
   (reduce + 0 
     (map #(if % 1 0) coll)))

Although I admit I like dAni's solution better.

the genral pattern is filter the sequence and count the results

(count (filter #(if % %) [1 2 3 nil nil false]))
3

the #(if % %) is just a short test for truthyness that returns an item only if it is truthy or something falsy (nil) otherwise

来源：https://stackoverflow.com/questions/12518320/counting-only-truthy-values-in-a-collection

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