做数学列表 - python

Question

I want to create a list from an existing list.

Original have list:

mylist = ["single extra", "double double", "tripple, double, singe", "mohan point tripple decker","one","covent gardens london tw45hj", "honda"]

find out the number of words in each label in mylist:

num_words = [len(sentence.split()) for sentence in mylist]

print num_words

[2, 2, 3, 4, 1, 4, 1]

Lets pretend that mylist was a single long string for a moment,

"single extra double double tripple double singe mohan point tripple decker one covent gardens london tw45hj honda"

I want to figure out where each label starts from in that single long list.

so I know that in the original list "mylist" first index had 2 words, so it would start from 0 to 2, then the next index contained 2 words, so that would start from 3 to 5, and so on...

manually the math would be like so:

1 + 2 = 3
3 + 2 = 5
5 + 3 = 8
8 + 4 = 12
12 + 1 = 13
13 + 4 = 17
17 + 1 = 18 

I tried this:

p=0
x=1
for i, item in enumerate(num_words):
    result = num_words[p] + num_words[x]
    results = result + num_words[x]
    x += 1
    p += 1

print results

but thats failed...

I hope this makes sense.....

Thanks all

Answer 1

What you wanted to do is called running total. You can use simple loop:

>>> res, c = [], 1
>>> for x in num_words:
...     c += x
...     res.append(c)
>>> res
[3, 5, 8, 12, 13, 17, 18]

It's also could be done it one line in functional style, like this:

>>> reduce(lambda x, y: x + [x[-1] + y], num_words, [1])[1:]
[3, 5, 8, 12, 13, 17, 18]

Answer 2

On py3.x you can use itertools.accumulate :

>>> from itertools import accumulate
>>> list(accumulate([1] + lis))[1:]
[3, 5, 8, 12, 13, 17, 18]

For py2.x:

def cummutalive_sum(lis):
    total = 1
    for item in lis:
        total += item
        yield total
...         
>>> list(cummutalive_sum(lis))
[3, 5, 8, 12, 13, 17, 18]