Conditional counting in Python


not sure this was asked before, but I couldn't find an obvious answer. I'm trying to count the number of elements in a list that are equal to a certain value. The problem is that these elements are not of a built-in type. So if I have

class A:
    def __init__(self, a, b):
        self.a = a
        self.b = b

stuff = []
for i in range(1,10):
    stuff.append(A(i/2, i%2))

Now I would like a count of the list elements whose field b = 1. I came up with two solutions:

print [e.b for e in stuff].count(1)


print len([e for e in stuff if e.b == 1])

Which is the best method? Is there a better alternative? It seems that the count() method does not accept keys (at least in Python version 2.5.1.

Many thanks!

By : nicolaum


To hide reduce details, you may define a count function:

def count(condition, stuff):
    return reduce(lambda s, x: \
                  s + (1 if condition(x) else 0), stuff, 0)

Then you may use it by providing the condition for counting:

n = count(lambda i: i.b, stuff)
By : Calvin

I would prefer the second one as it's only looping over the list once.

If you use count() you're looping over the list once to get the b values, and then looping over it again to see how many of them equal 1.

A neat way may to use reduce():

reduce(lambda x,y: x + (1 if y.b == 1 else 0),list,0)

The documentation tells us that reduce() will:

Apply function of two arguments cumulatively to the items of iterable, from left to right, so as to reduce the iterable to a single value.

So we define a lambda that adds one the accumulated value only if the list item's b attribute is 1.

By : Dave Webb

This video can help you solving your question :)
By: admin