Counting the amount of occurrences in a list of tuples

问题

I am fairly new to python, but I haven't been able to find a solution to my problem anywhere.

I want to count the occurrences of a string inside a list of tuples.

Here is the list of tuples:

list1 = [
         ('12392', 'some string', 'some other string'),
         ('12392', 'some new string', 'some other string'),
         ('7862', None, 'some other string')
        ]

I've tried this but it just prints 0

for entry in list1:
    print list1.count(entry[0])

As the same ID occurs twice in the list, this should return:

2
1

I also tried to increment a counter for each occurrence of the same ID but couldn't quite grasp how to write it.

*EDIT: Using Eumiro's awesome answer. I just realized that I didn't explain the whole problem. I actually need the total amount of entries which has a value more than 1. But if I try doing:

for name, value in list1:

    if value > 1:
        print value

I get this error:

ValueError: Too many values to unpack

回答1:

Maybe collections.Counter could solve your problem:

from collections import Counter
Counter(elem[0] for elem in list1)

returns

Counter({'12392': 2, '7862': 1})

It is fast since it iterates over your list just once. You iterate over entries and then try to get a count of these entries within your list. That cannot be done with .count, but might be done as follows:

for entry in list1:
    print sum(1 for elem in list1 if elem[0] == entry[0])

But seriously, have a look at collections.Counter.

EDIT: I actually need the total amount of entries which has a value more than 1.

You can still use the Counter:

c = Counter(elem[0] for elem in list1)
sum(v for k, v in c.iteritems() if v > 1)

returns 2, i.e. the sum of counts that are higher than 1.

回答2:

list1.count(entry[0]) will not work because it looks at each of the three tuples in list1, eg. ('12392', 'some string', 'some other string') and checks if they are equal to '12392' for example, which is obviously not the case.

@eurmiro's answer shows you how to do it with Counter (which is the best way!) but here is a poor man's version to illustrate how Counter works using a dictionary and the dict.get(k, [,d]) method which will attempt to get a key (k), but if it doesn't exist it returns the default value instead (d):

>>> list1 = [
         ('12392', 'some string', 'some other string'),
         ('12392', 'some new string', 'some other string'),
         ('7862', None, 'some other string')
]
>>> d = {}
>>> for x, y, z in list1:
        d[x] = d.get(x, 0) + 1


>>> d
{'12392': 2, '7862': 1}

回答3:

I needed some extra functionality that Counter didn't have. I have a list of tuples that the first element is the key and the second element is the amount to add. @jamylak solution was a great adaptation for this!

>>> list = [(0,5), (3,2), (2,1), (0,2), (3,4)]

>>> d = {}
>>> for x, y in list1:
    d[x] = d.get(x, 0) + y

>>> d
{0: 7, 2: 1, 3: 6}

来源：https://stackoverflow.com/questions/16013485/counting-the-amount-of-occurrences-in-a-list-of-tuples

标签

python

list

tuples

counting