Add sum_by snippet

snippets/max_by.md

@@ -5,7 +5,7 @@ tags: math,list,function,beginner
 
 Returns the maximum value of a list, after mapping each element to a value using the provided function.
 
-use `map()` with `fn` to map each element to a value using the provided function, convert to a `list` and use `max()` to return the maximum value.
+Use `map()` with `fn` to map each element to a value using the provided function, convert to a `list` and use `max()` to return the maximum value.
 
 ```py
 def max_by(lst, fn):

snippets/min_by.md

@@ -5,7 +5,7 @@ tags: math,list,function,beginner
 
 Returns the minimum value of a list, after mapping each element to a value using the provided function.
 
-use `map()` with `fn` to map each element to a value using the provided function, convert to a `list` and use `min()` to return the minimum value.
+Use `map()` with `fn` to map each element to a value using the provided function, convert to a `list` and use `min()` to return the minimum value.
 
 ```py
 def min_by(lst, fn):

snippets/sum_by.md

@@ -0,0 +1,17 @@
+---
+title: sum_by
+tags: math,list,function,beginner
+---
+
+Returns the sum of a list, after mapping each element to a value using the provided function.
+
+Use `map()` with `fn` to map each element to a value using the provided function, convert to a `list` and use `sum()` to return the sum of the values.
+
+```py
+def sum_by(lst, fn):
+  return sum(list(map(fn,lst)))
+```
+
+```py
+sum_by([{ 'n': 4 }, { 'n': 2 }, { 'n': 8 }, { 'n': 6 }], lambda v : v['n']) # 20
+```