From 267255262138af92e18203f11f49ea64d041ff0b Mon Sep 17 00:00:00 2001
From: Rohit Tanwar <mst10041967@gmail.com>
Date: Wed, 7 Feb 2018 14:03:47 +0530
Subject: [PATCH] count_by

---
 README.md            | 30 ++++++++++++++++++++++++++++++
 snippets/count_by.md | 20 ++++++++++++++++++++
 tag_database         |  3 ++-
 3 files changed, 52 insertions(+), 1 deletion(-)
 create mode 100644 snippets/count_by.md

diff --git a/README.md b/README.md
index 10db657c7..e4c0e0f5c 100644
--- a/README.md
+++ b/README.md
@@ -29,6 +29,7 @@
 <li><a href = "#shuffle"><code>shuffle</code></a></li>
 <li><a href = "#spread"><code>spread</code></a></li>
 <li><a href = "#zip"><code>zip</code></a></li>
+<li><a href = "#count_by"><code>count_by</code></a></li>
 </ul></details>
 
 ### :scroll: String
@@ -489,6 +490,35 @@ zip(['a'], [1, 2], [True, False], fill_value = '_') # [['a', 1, True], ['_', 2,
 
 <br><a href = "#table-of-contents">:arrow_up: Back to top</a>
  
+### count_by
+
+Groups the elements of a list based on the given function and returns the count of elements in each group.
+
+Use `map()` to map the values of the list using the given function. Iterate over the map and increase the the elements count each time it occurs.
+
+```py
+def count_by(arr, fn=lambda x: x):
+    key = {}
+    for el in map(fn, arr):
+        key[el] = 0 if not el in key else key[el]
+        key[el] += 1
+    return key
+ 
+ ```
+
+<details><summary>View Examples</summary>
+
+```py
+
+from math import floor
+count_by([6.1, 4.2, 6.3], floor) # {4: 1, 6: 2}
+count_by(['one', 'two', 'three'], len) # {3: 2, 5: 1}
+
+```
+</details>
+
+<br><a href = "#table-of-contents">:arrow_up: Back to top</a>
+ 
 ## :scroll: String
 
 ### count_vowels
diff --git a/snippets/count_by.md b/snippets/count_by.md
new file mode 100644
index 000000000..2b37ecd11
--- /dev/null
+++ b/snippets/count_by.md
@@ -0,0 +1,20 @@
+### count_by
+
+Groups the elements of a list based on the given function and returns the count of elements in each group.
+
+Use `map()` to map the values of the list using the given function. Iterate over the map and increase the the elements count each time it occurs.
+
+```python
+def count_by(arr, fn=lambda x: x):
+    key = {}
+    for el in map(fn, arr):
+        key[el] = 0 if not el in key else key[el]
+        key[el] += 1
+    return key
+```
+
+``` python
+from math import floor
+count_by([6.1, 4.2, 6.3], floor) # {4: 1, 6: 2}
+count_by(['one', 'two', 'three'], len) # {3: 2, 5: 1}
+```
\ No newline at end of file
diff --git a/tag_database b/tag_database
index 72a4fa20c..6e4cdc7a8 100644
--- a/tag_database
+++ b/tag_database
@@ -19,4 +19,5 @@ capitalize_every_word:string
 decapitalize:string
 palindrome:string
 is_upper_case:string
-is_lower_case:string
\ No newline at end of file
+is_lower_case:string
+count_by:list
\ No newline at end of file