Most pythonic way of counting matching elements in something iterable

Question

I have an iterable of entries on which I would like to gather some simple statistics, say the count of all numbers divisible by two and the count of all numbers divisible by three. My first alternative, While only iterating through the list once and avoiding the list expansion (and keeping the split loop refactoring in mind), looks rather bloated: (alt 1) ``` r = xrange(1, 10) twos = 0 threes = 0 for v in r: if v % 2 == 0: twos+=1 if v % 3 == 0: threes+=1 print twos print threes ``` This looks rather nice, but has the drawback of expanding the expression to a list: (alt 2) ``` r = xrange(1, 10) print len([1 for v in r if v % 2 == 0]) print len([1 for v in r if v % 3 == 0]) ``` What I would really like is something like a function like this: (alt 3) ``` def count(iterable): n = 0 for i in iterable: n += 1 return n r = xrange(1, 10) print count(1 for v in r if v % 2 == 0) print count(1 for v in r if v % 3 == 0) ``` But this looks a lot like something that could be done without a functio…

Accepted Answer

Having to iterate over the list multiple times isn't elegant IMHO.

I'd probably create a function that allows doing:

```
twos, threes = countmatching(xrange(1,10),
                             lambda a: a % 2 == 0,
                             lambda a: a % 3 == 0)
```

A starting point would be something like this:

```
def countmatching(iterable, *predicates):
    v = [0] * len(predicates)
    for e in iterable:
        for i,p in enumerate(predicates):
            if p(e):
                v[i] += 1
    return tuple(v)
```

Btw, "itertools recipes" has a recipe for doing much like your alt4.

```
def quantify(seq, pred=None):
    "Count how many times the predicate is true in the sequence"
    return sum(imap(pred, seq))
```

Most pythonic way of counting matching elements in something iterable

Solution

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