Reduce list of list to dictionary with sublist size as keys and number of occurances as value

Question

I have a list of lists and I want to count the number of times a sublist with a specific size occurs.

eg. for list [[1], [1,2], [1,2], [1,2,3]] I expect to get {1: 1, 2: 2, 3: 1}

I've tried reduce function but I have syntax error on += 1 and have no idea what is wrong.

list_of_list = [[1], [1,2], [1,2], [1,2,3]]
result = functools.reduce(lambda dict,list: dict[len(list)] += 1, list_of_list, defaultdict(lambda: 0, {}))

Answer 1

It is not a good idea to use reduce in such a complicated way when you can use collections.Counter() with map() function in a more Pythonic way:

>>> A = [[1], [1,2], [1,2], [1,2,3]]
>>> from collections import Counter
>>> 
>>> Counter(map(len,A))
Counter({2: 2, 1: 1, 3: 1})

Note that using map will perform slightly better than a generator expression because by passing a generator expression to Counter() python will get the values from generator function by itself, since using built-in function map has more performance in terms of execution time ¹ .

~$ python -m timeit --setup "A = [[1], [1,2], [1,2], [1,2,3]];from collections import Counter" "Counter(map(len,A))"
100000 loops, best of 3: 4.7 usec per loop
~$ python -m timeit --setup "A = [[1], [1,2], [1,2], [1,2,3]];from collections import Counter" "Counter(len(x) for x in A)"
100000 loops, best of 3: 4.73 usec per loop

From PEP 0289 -- Generator Expressions :

The semantics of a generator expression are equivalent to creating an anonymous generator function and calling it. For example:

 g = (x**2 for x in range(10)) print g.next() 

is equivalent to:

 def __gen(exp): for x in exp: yield x**2 g = __gen(iter(range(10))) print g.next() 

---

_{Note that since generator expressions are better in terms of memory use, if you are dealing with large data you'd better use generator expression instead of map function.}

Answer 2

You can do this using Counter as well:

list_of_list = [[1], [1,2], [1,2], [1,2,3]]
c = Counter(len(i) for i in list_of_list)

Output:

Counter({2: 2, 1: 1, 3: 1})

Answer 3

reduce is an inferior tool for this job.

Look at a collections.Counter instead. It's a dict subclass, so you should be able to use it however you were planning to use the dict.

>>> from collections import Counter
>>> L = [[1], [1, 2], [1, 2], [1, 2, 3]]
>>> Counter(len(x) for x in L)
Counter({1: 1, 2: 2, 3: 1})