group-by function koan

This commit is contained in:
Takayuki Goto 2013-09-16 13:42:18 -05:00 committed by Colin Jones
parent f35e258e7f
commit d6b1b57942
2 changed files with 43 additions and 1 deletions

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@ -209,4 +209,14 @@
5
:hello
(6 :these :are)
]}]]
]}]
["21_group_by" {"__" [odd?
{5 ["hello" "world"] 3 ["foo" "bar"]}
{1 [{:name "Bob" :id 1}
{:last-name "Smith" :id 1}]
2 [{:name "Mike" :id 2}]}
nil
{:nauty-list [{:name "Jimmy" :bad true}
{:name "Joe" :bad true}]
:nice-list [{:name "Jack" :bad false}]}]}]
]

32
src/koans/21_group_by.clj Normal file
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@ -0,0 +1,32 @@
(defn get-odds-and-evens [coll]
(let [{odds true evens false} (group-by __ coll)]
[odds evens]))
(meditations
"To split a collection categorized by keys use the group-by function."
(= __ (group-by count ["hello" "world" "foo" "bar"]))
"Sometimes you want to filter a sequence but also keep the predicate."
(= (get-odds-and-evens '(1 2 3 4 5))
[[1 3 5] [2 4]])
"You can also group by a primary key"
(= __
(group-by :id [{:id 1 :name "Bob"}
{:id 2 :name "Mike"}
{:id 1 :last-name "Smith"} ]))
"but be careful when you group by non primary key"
(= {"Bob" [{:name "Bob" :id 1}], "Mike" [{:name "Mike" :id 2}], __ [{:last-name "Smith" :id 1}]}
(group-by :name [{:id 1 :name "Bob"}
{:id 2 :name "Mike"}
{:id 1 :last-name "Smith"}]))
"The true power of group-by is using a custom function"
(= __
(group-by #(if (:bad %) :nauty-list :nice-list)
[{:name "Jimmy" :bad true}
{:name "Jack" :bad false}
{:name "Joe" :bad true}])))