CODEWITHSUNDEEP
pythonlist-comprehension
svineet
svineet

Reputation: 1889

List comprehension having wrong length

I have list seeds and leechs which return 19 on asking length using len()
And using these two lists is a list comprehension - 

sldiff = [(int(seed)-int(leech)) for seed in seeds for leech in leechs]

Each element is supposed to be the difference between the seed and leech(which are strings so have to be typecasted)
But len(sldiff) returns 361!
My questions are - Why is it happening and what should I do to get required sldiff list?

Upvotes: 1

Views: 160

Answers (3)

Michel Keijzers
Michel Keijzers

Reputation: 15367

You get the result of 19 * 19 = 361 because of the two for loops.

I'm not exactly sure what you want but probably something like:

sldiff = [int(seed[x]) - int(leech[x]) for x in xrange(len(seeds))]

Assuming that len(seeds) == len(leechs)

Upvotes: 1

zhangyangyu
zhangyangyu

Reputation: 8620

[(int(seed)-int(leech)) for seed in seeds for leech in leechs]

is similiar as:

    temp = []
    for seed in seeds:
        for leech in leechs:
            temp.append(int(seed)-int(leech))

Apparently it is 19 * 19.

I think you want

[int(x)-int(y) for x, y in zip(seeds, leechs)]

Upvotes: 2

Daniel Roseman
Daniel Roseman

Reputation: 600041

You're doing a double list comprehension - ie you're iterating through the whole of 'seeds' for each entry in 'leechs' (so 19*19, ie 361).

Seems like what you actually want to do is iterate through one list, each entry of which is a combination of the relevant entry from seeds and the one from leechs. That is what zip does:

[(int(seed) - int(leech)) for seed, leech in zip(seeds, leechs)]

Upvotes: 3

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