Sequence Generation with Number applied to string

Question

I have tried the Sequence Generator like Lambda, List comprehension and others but it seems that I am not able to get what I really want. My final goal is to print sequence of words from a string like string[1:3]

What I am looking for :

a = [0,13,26,39]
b = [12,25,38,51]

str = 'If you are done with the file, move to the command area across from the file name in the RL screen and type'

read = str.split()

read[0:12]
['If', 'you', 'are', 'done', 'with', 'the', 'file,', 'move', 'to', 'the', 'command', 'area']
read[13:25]
['from', 'the', 'file', 'name', 'in', 'the', 'RL', 'screen', 'and', 'type']

Answer 1

Use zip:

>>> a = [0,13,26,39]
>>> b = [12,25,38,51]
>>> strs = 'If you are done with the file, move to the command area across from the file name in the RL screen and type'
>>> spl = strs.split()
>>> for x,y in zip(a,b):
...     print spl[x:y]
...     
['If', 'you', 'are', 'done', 'with', 'the', 'file,', 'move', 'to', 'the', 'command', 'area']
['from', 'the', 'file', 'name', 'in', 'the', 'RL', 'screen', 'and', 'type']
[]
[]

zip returns list of tuples, where each tuple contains items on the same index from the iterables passed to it:

>>> zip(a,b)
[(0, 12), (13, 25), (26, 38), (39, 51)]

Use itertools.izip if you want memory efficient solution, as it returns an iterator.

You can use str.join if you want to create a string from that sliced list:

for x,y in zip(a,b):
    print " ".join(spl[x:y])
...     
If you are done with the file, move to the command area
from the file name in the RL screen and type

Update: Creating a and b:

>>> n = 5
>>> a = range(0, 13*n, 13)
>>> b = [ x + 12 for x in a]
>>> a
[0, 13, 26, 39, 52]
>>> b
[12, 25, 38, 51, 64]

Answer 2

Do you mean:

>>> [read[i:j] for i, j in zip(a,b)]
[['If', 'you', 'are', 'done', 'with', 'the', 'file,', 'move', 'to', 'the', 
'command',    'area'], ['from', 'the', 'file', 'name', 'in', 'the', 'RL',
'screen', 'and', 'type'], [], []]

or

>>> ' '.join[read[i:j] for i, j in zip(a,b)][0])
'If you are done with the file, move to the command area'

>>> ' '.join[read[i:j] for i, j in zip(a,b)][1])
'from the file name in the RL screen and type'