Count the number of elements of same value in Python [duplicate]

Question

Possible Duplicate:
How to count the frequency of the elements in a list?

I wish to count the number of elements of same value in a list and return a dict as such:

> a = map(int,[x**0.5 for x in range(20)])
> a
> [0, 1, 1, 1, 2, 2, 2, 2, 2, 3, 3, 3, 3, 3, 3, 3, 4, 4, 4, 4] 

> number_of_elements_by_value(a)
> {0:1, 1:3, 2:5, 3:7, 4:4}

I guess it is kind of a histogram?

Answer 1

This is a good way if you don't have collections.Counter available

from collections import defaultdict
d = defaultdict(int)
a = map(int, [x**0.5 for x in range(20)])
for i in a:
    d[i] += 1

print d