Python字典的求和值

Question

I wonder how to sum values using Python dictionary. I read huge file line by line and increment value for each particular key. Suppose I have the following toy file:

word1 5
word2 3
word3 1
word1 2
word2 1

The desired result I expected is:

my_dict = {'word1':7, 'word2':4, 'word3':1}

Pasted below is my current work.

my_dict = {}          
with open('test.txt') as f:
    for line in f:
        line = line.rstrip()
        line = line.split()
        word = line[0]
        frequency = line[1]
        my_dict[word] += int(frequency)

Answer 1

Use a collections.Counter() object :

from collections import Counter

my_dict = Counter()

with open('test.txt') as f:
    for line in f:
        word, freq = line.split()
        my_dict[word] += int(freq)

Note that str.rstrip() is not needed, the str.split() call with no arguments also strips the string.

Apart from defaulting non-existing keys to 0, Counter() objects have additional advantages, such as listing words ordered by frequency (including a top N), summing and subtracting.

The above code results in:

>>> my_dict
Counter({'word1': 7, 'word2': 4, 'word3': 1})
>>> for word, freq in my_dict.most_common():
...     print word, freq
... 
word1 7
word2 4
word3 1

Answer 2

You can use a defaultdict :

import collections
d = collections.defaultdict(int)
with open('text.txt') as f:
    for row in f:
        temp = row.split()
        d[temp[0]] += int(temp[1])

d is now:

defaultdict(<type 'int'>, {'word1': 7, 'word3': 1, 'word2': 4})

Answer 3

IN case someone is working with multiple columns (in my case I had the same problem but with 4 columns):

This should do the trick:

from collections import defaultdict

my_dict = defaultdict(int)

with open("input") as f:
    for line in f:
        if line.strip():
            items = line.split()
            freq = items[-1]
            lemma = tuple(items[:-1]) 

            my_dict[lemma] += int(freq)

for items, freq in my_dict.items():
    print items, freq