CODEWITHSUNDEEP
pythoncountgenerator
Sait
Sait

Reputation: 19815

What is best practice to use generators for counting purposes

Let's say I have a list like:

my_list = range(10)

And I want to count how many even numbers there are in the list. Note that I am not interested with the values, I just want the count of them. So I can:

len( [0 for i in my_list if i % 2 == 0] ) # Method 1
len( [i for i in my_list if i % 2 == 0] ) # Method 2
len( [_ for i in my_list if i % 2 == 0] ) # Method 3

Is any of the above methods better than others from the speed or memory perspectives?

Actually I don't even need to construct the list, but I don't want to:

counter = 0
for item in my_list:
   if item % 2 == 0:
      counter += 1

So, which one is a good way of counting with generators?

PS: The list in my case has more memory-heavy items, that is why I want to optimize if possible.

Upvotes: 0

Views: 168

Answers (1)

Martijn Pieters
Martijn Pieters

Reputation: 1122172

Use none of the above. Use sum() and a generator expression:

sum(i % 2 == 0 for i in mylist)

In Python, the bool boolean type is a subclass of int and True has an integer value of 1, False has 0, so you can sum a series of True and False results.

The sum()-with-generator expression only has to keep one boolean in memory at a time, no intermediary list has to be produced and kept around just to calculate a length.

Alternatively, stick to filtering and sum 1 literals:

sum(1 for i in mylist if i % 2 == 0)

This results in fewer objects needing to be added up still.

Upvotes: 8

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