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doc source examples comments

1.3.0 permalink

1.2.0
1.3.0

partition-by

clojure.core

(partition-by f coll)

Applies f to each value in coll, splitting it each time f returns
a new value. Returns a lazy seq of partitions.

3 Examples top

link | changes
```
user=> (partition-by #(= 3 %) [1 2 3 4 5])
((1 2) (3) (4 5))
```
user=> (partition-by #(= 3 %) [1 2 3 4 5]) ((1 2) (3) (4 5))
link | changes
```
user=> (partition-by odd? [1 1 1 2 2 3 3])
((1 1 1) (2 2) (3 3))

user=> (partition-by even? [1 1 1 2 2 3 3])
((1 1 1) (2 2) (3 3))
```
user=> (partition-by odd? [1 1 1 2 2 3 3]) ((1 1 1) (2 2) (3 3)) user=> (partition-by even? [1 1 1 2 2 3 3]) ((1 1 1) (2 2) (3 3))
link | changes
```
;; (this is part of a solution from 4clojure.com/problem 30)
user=> (partition-by identity "Leeeeeerrroyyy")
((\L) (\e \e \e \e \e \e) (\r \r \r) (\o) (\y \y \y))
```
;; (this is part of a solution from 4clojure.com/problem 30) user=> (partition-by identity "Leeeeeerrroyyy") ((\L) (\e \e \e \e \e \e) (\r \r \r) (\o) (\y \y \y))

Source clojure/core.clj:6242 top

(defn partition-by
  "Applies f to each value in coll, splitting it each time f returns
   a new value.  Returns a lazy seq of partitions."
  {:added "1.2"
   :static true}
  [f coll]
  (lazy-seq
   (when-let [s (seq coll)]
     (let [fst (first s)
           fv (f fst)
           run (cons fst (take-while #(= fv (f %)) (next s)))]
       (cons run (partition-by f (seq (drop (count run) s))))))))

Vars in clojure.core/partition-by:

Used in 0 other vars

Comments top

1 comment(s) for partition-by.

Iceland_jack

link

It's worth mentioning that (partition-by identity …) is equivalent to the Data.List.group function in Haskell:

 
(defn group [coll]
  (partition-by identity coll))

Which proves to be an interesting idiom:

user=> (apply str 
         (for [ch (group "fffffffuuuuuuuuuuuu")] 
           (str (first ch) (count ch))))
⇒ "f7u12"

It's worth mentioning that `(partition-by identity …)` is equivalent to the `Data.List.group` function in Haskell:

 
(defn group [coll]
  (partition-by identity coll))