Initializing a dictionary with zeroes [duplicate]

Of course, just use collections.defaultdict([default_factory[, ...]]):

from collections import defaultdict fl = [0, 1, 1, 2, 1, 3, 4] seenit = defaultdict(int) for val in fl: seenit[val] += 1 print(fl) # Output defaultdict(<class 'int'>, {0: 1, 1: 3, 2: 1, 3: 1, 4: 1}) print(dict(seenit)) # Output {0: 1, 1: 3, 2: 1, 3: 1, 4: 1} 

In addition, if you don't like to import collections you can use dict.get(key[, default])

fl = [0, 1, 1, 2, 1, 3, 4] seenit = {} for val in fl: seenit[val] = seenit.get(val, 0) + 1 print(seenit) # Output {0: 1, 1: 3, 2: 1, 3: 1, 4: 1} 

Also, if you only want to solve the problem and don't mind to use exactly dictionaries you may use collection.counter([iterable-or-mapping]):

from collections import Counter fl = [0, 1, 1, 2, 1, 3, 4] seenit = Counter(f) print(seenit) # Output Counter({1: 3, 0: 1, 2: 1, 3: 1, 4: 1}) print(dict(seenit)) # Output {0: 1, 1: 3, 2: 1, 3: 1, 4: 1} 

Both collection.defaultdict and collection.Counter can be read as dictionary[key] and supports the usage of .keys(), .values(), .items(), etc. Basically they are a subclass of a common dictionary.

If you want to talk about performance I checked with timeit.timeit() the creation of the dictionary and the loop for a million of executions:

• collection.defaultdic: 2.160868141 seconds
• dict.get: 1.3540439499999999 seconds
• collection.Counter: 4.700308418999999 seconds

collection.Counter may be easier, but much slower.

You can use collections.Counter:

from collections import Counter Counter([0, 1, 1, 2, 1, 3, 4]) 

Output:

Counter({1: 3, 0: 1, 2: 1, 3: 1, 4: 1}) 

You can then address it like a dictionary:

>>> Counter({1: 3, 0: 1, 2: 1, 3: 1, 4: 1})[1] 3 >>> Counter({1: 3, 0: 1, 2: 1, 3: 1, 4: 1})[0] 1 

Using val in seenit is a bit faster than .get():

seenit = dict() for val in fl: if val in seenit : seenit[val] += 1 else: seenit[val] = 1 

For larger lists, Counter will eventually outperform all other approaches. and defaultdict is going to be faster than using .get() or val in seenit.