clojure.core/doubles

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Type: function Added: Clojure 1.0 Examples: 2 Runnable: 1

([xs])

Casts to double[]

Examples

by tomdl89 on 2022-07-01 . May have evaluation errors. ▶

;; double-array will convert where possible (double-array [1 2 3]);; => [1.0, 2.0, 3.0] ;; doubles will not (try (doubles [1 2 3]) (catch ClassCastException e (ex-message e))) ;; => "clojure.lang.PersistentVector cannot be cast to [D"

by tomdl89 on 2023-04-23 ▶

;; Casting avoids expensive reflection, so to see the benefit, enable warning: (set! *warn-on-reflection* true) ;; We'll def a double-array but won't type-hint the var: (def my-array (double-array [10.0 20.0 30.0 40.0 50.0 60.0])) ;; and try to amap over it without using `doubles` or type hinting amap's args: (amap my-array i _ (unchecked-inc ^double (aget my-array i))) ;; Reflection warning… call to static method alength on clojure.lang.RT can't be resolved (argument types: unknown). ;; Reflection warning… call to static method aclone on clojure.lang.RT can't be resolved (argument types: unknown). ;; Reflection warning… call to static method aget on clojure.lang.RT can't be resolved (argument types: unknown, int). ;; Reflection warning… call to static method aset on clojure.lang.RT can't be resolved (argument types: unknown, int, double). ;; => [11.0, 21.0, 31.0, 41.0, 51.0, 61.0] ;; We can use `doubles` to avoid reflection: (amap (doubles my-array) i _ (unchecked-inc ^double (aget (doubles my-array) i))) ;; => [11.0, 21.0, 31.0, 41.0, 51.0, 61.0] ;; Just as we can type hint in place: (amap ^doubles my-array i _ (unchecked-inc ^double (aget ^doubles my-array i))) ;; => [11.0, 21.0, 31.0, 41.0, 51.0, 61.0] ;; Or type hint the var: (def ^"[D" my-array (double-array [10 20 30 40 50 60])) (amap my-array i _ (unchecked-inc ^double (aget my-array i))) ;; => [11.0, 21.0, 31.0, 41.0, 51.0, 61.0]

Note by shockbob

Anybody know what this is used for? All I could find is that you can cast an existing double-array to another double-array???

user=> (doubles [1 2 3 4 5])
java.lang.ClassCastException: clojure.lang.PersistentVector cannot be cast to [D (NO_SOURCE_FILE:0)
user=> (doubles (int-array [2 3 2]))
java.lang.ClassCastException: [I cannot be cast to [D (NO_SOURCE_FILE:0)
user=> (doubles (float-array [2 3 2]))
java.lang.ClassCastException: [F cannot be cast to [D (NO_SOURCE_FILE:0)
user=> (doubles (double-array [2 3 2]))
#
user=> (type (double-array [2 3 2]))
[D
user=> (type (doubles (double-array [2 3 2])))
[D
user=>

See also

• clojure.core/double-array (1)

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