Add frequencies

frequencies.md

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+---
+title: frequencies
+tags: list,intermediate
+---
+
+Returns a dictionary with the unique values of a list as keys and their frequencies as the values.
+
+Use a `for` loop to populate a dictionary, `f`, with the unique values in `lst` as keys, adding to existing keys every time the same value is encountered.
+
+```py
+from functools import reduce
+
+def frequencies(lst):
+  f = {}
+  for x in lst:
+    f[x] = f[x] + 1 if x in f else 1
+  return f
+```
+
+```py
+frequencies(['a', 'b', 'a', 'c', 'a', 'a', 'b']) # { 'a': 4, 'b': 2, 'c': 1 }
+```