Merge pull request #360 from slapresta/275-unique-by

unique_by(.foo) function

builtin.c

@@ -664,6 +664,7 @@ static const char* const jq_builtins[] = {
   "def sort_by(f): _sort_by_impl(map([f]));",
   "def group_by(f): _group_by_impl(map([f]));",
   "def unique: group_by(.) | map(.[0]);",
+  "def unique_by(f): group_by(f) | map(.[0]);",
   "def max_by(f): _max_by_impl(map([f]));",
   "def min_by(f): _min_by_impl(map([f]));",
 #include "libm.h"

docs/content/3.manual/manual.yml

@@ -931,6 +931,24 @@ sections:
             input: '[1,2,5,3,5,3,1,3]'
             output: ['[1,2,3,5]']
 
+      - title: `unique_by`
+        body: |
+          
+          The `unique_by(.foo)` function takes as input an array and produces
+          an array of the same elements, in sorted order, with
+          elqements with a duplicate `.foo` field removed. Think of it as making
+          an array by taking one element out of every group produced by
+          `group_by`.
+        
+        examples:
+          - program: 'unique_by(.foo)'
+            input: '[{"foo": 1, "bar": 2}, {"foo": 1, "bar": 3}, {"foo": 4, "bar": 5}]'
+            output: ['[{"foo": 1, "bar": 2}, {"foo": 4, "bar": 5}]']
+          - program: 'unique_by(length)'
+            input: '["chunky", "bacon", "kitten", "cicada", "asparagus"]'
+            output: ['["chunky", "bacon", "asparagus"]']
+
+
       - title: `reverse`
         body: |