CODEWITHSUNDEEP
python
jason
jason

Reputation: 4439

python list manipulation nesting vertically, making it look like a matrix

I have the following list. In python it looks like [15,20,25,35,-20... etc] 

15  20  25  35  -20 -15 -10 -5
10  15  20  25  -25 -20 -15 -10
5   10  15  20  -35 -25 -20 -15

I want to nest vertically like so:

[[15,10,5],[20,15,10],[25,20,15],[35,25,20],[-20,-25,-35],...etc]

So it involves some kind of a transpose operation, but counting backwards, e.g. transpose would give you [5,10,15] and not [15,10,5] for the first item in the list

What's the best way to do it? (shortest and most readable code)

If anybody also have suggestions on shortest run time would be also helpful for bigger datasets.

Upvotes: 1

Views: 100

Answers (2)

vaultah
vaultah

Reputation: 46533

from pprint import pprint
from operator import itemgetter
lst = [15, 20, 25, 35, -20, -15, -10, -5, 10, 15, 20, 25, -25,\
       -20, -15, -10, 5, 10, 15, 20, -35, -25, -20, -15]

# Python 3
l = len(lst) // 3
# Or a single slash in Python 2: l = len(lst) / 3

# Just for diversity
a = [itemgetter(*range(x, x+l))(lst) for x in range(0, len(lst), l)]

pprint(list(zip(*a)))

Output:

[(15, 10, 5),
 (20, 15, 10),
 (25, 20, 15),
 (35, 25, 20),
 (-20, -25, -35),
 (-15, -20, -25),
 (-10, -15, -20),
 (-5, -10, -15)]

Upvotes: 2

thefourtheye
thefourtheye

Reputation: 239463

You can group the elements with the list comprehension and then transpose it with zip function, like this

data = [15, 20, 25, 35, -20, -15, -10, -5, 10, 15, 20,
        25, -25, -20, -15, -10, 5, 10, 15, 20, -35, -25, -20, -15]
length = len(data) / 3
data = [data[i:i + length] for i in xrange(0, len(data), length)]

Till this point we grouped the data like this

[[15, 20, 25, 35, -20, -15, -10, -5],
 [10, 15, 20, 25, -25, -20, -15, -10],
 [5, 10, 15, 20, -35, -25, -20, -15]]

Now, we just have to transpose data, with zip

print zip(*data)

Output

[(15, 10, 5),
 (20, 15, 10),
 (25, 20, 15),
 (35, 25, 20),
 (-20, -25, -35),
 (-15, -20, -25),
 (-10, -15, -20),
 (-5, -10, -15)]

zip(*data) means we unpack all the elements of data and pass each of the elements as parameters to zip. It is equivalent to

zip(data[0], data[1], data[2])

Upvotes: 3

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