How to pick a sequence of numbers from a list?

Question

I have a startnumber and an endnumber.
From these numbers I need to pick a sequence of numbers.
The sequences is not always the same.

Example:

startnumber = 1
endnumber = 32

I need to create a list of numbers with a certain sequence
p.e.
3 numbers yes, 2 numbers no, 3 numbers yes, 2 numbers no.. etc

Expected output:

[[1-3],[6-8],[11-13],[16-18],[21-23],[26-28],[31-32]]

(at the end there are only 2 numbers remaining (31 and 32))

Is there a simple way in python to select sequences of line from a range of numbers?

I don't understand how you've defined the sequence. By 'not always the same' do you mean random? Must the number of sequences drawn each time be the same? — Bill Bell, Jan 24 '17 at 16:58
Who decides the sequence of "Take 3, drop 2, take 3, drop 2, ..."? — cadolphs, Jan 24 '17 at 16:59
@Reman `[1-3]` is the single-item list equivalent to `[-2]`, or the second item from the last if you're indexing a list. It's important to be specific. — Adam Smith, Jan 24 '17 at 17:01
@AdamSmith I think he is trying to represent the range `1, 2, 3` with `1-3` rather than the actual `1 - 3` arithmetic operation. — dabadaba, Jan 24 '17 at 17:02
@BillBell The numbers are lines in a textfile. The filtered lines are the lines I want to capture. In above case 3 lines to capture, 2 lines not to capture, 3 lines to capture, 2 lines not to capture ... till the end. — Reman, Jan 24 '17 at 17:04
@Lagerbaer, I decide this in a function. The sequence is always the same till the end of lines, but the numbers can vary. — Reman, Jan 24 '17 at 17:05
@Reman: your comment with the line capturing suggests me that you want a flat list and not a nested one. Here's how to get the flat list: — yogabonito, Jan 24 '17 at 17:07
@Reman: [i for i in range(startnumber, endnumber+1) if not i%5==0 and not i%5==4] — yogabonito, Jan 24 '17 at 17:07

Batman · Accepted Answer · 2017-01-24T17:32:16.497

4

numbers = range(1,33)
take = 3
skip = 2
seq = [list(numbers[idx:idx+take]) for idx in range(0, len(numbers),take+skip)]

edited Jan 24 '17 at 17:32

answered Jan 24 '17 at 16:59

Batman

8,571
7
41
80

that's clever. Just edit it a bit to make the `take 3, drop 2` pattern visible and customisable and hide the trick converting the 33 to 35 ☺ – Ma0 Jan 24 '17 at 17:00
Also this seems to be Python 2. In Python 3 (the one tagged) one would cast the `range` to `list`. – Ma0 Jan 24 '17 at 17:08
Thanks, this is a great solution. However it gives an output with ranges and tuples `[range(1, 4), range(6, 9), range(11, 14), range(16, 19), range(21, 24), range(26, 29), range(31, 33)]` and the 2nd value in the range is not the one I expected. – Reman Jan 24 '17 at 17:13
Sorry. I just saw the `python-3.x` tag that @Ev.Kounis pointed out above. I've edited the answer to cast the range back to a list. – Batman Jan 24 '17 at 17:33
@Batman, no problem, What would be the list comprehension if the skip and takes are inverted? First skip 2 numbers, than take 4 numbers, than skip 2 numbers than take 4 numbers ...etc – Reman Jan 24 '17 at 18:03
It would take 2, skip 3, etc. so `[[1,2],[6,7],[11,12], ...]`. – Batman Jan 24 '17 at 18:30
1

@Reman for skip first `seq =[list(numbers[idx+skip:idx+take+skip]) for idx in range(0, len(numbers)-skip,take+skip)]` – pii_ke Jan 24 '17 at 19:11

Adam Smith · Answer 2 · 2017-01-24T17:34:05.797

Extrapolating this out:

def get_data(data, filterfunc=None):
    if filterfunc is None:
        filterfunc = lambda: True  # take every line

    result = []
    sub_ = []
    for line in data:
        if filterfunc():
            sub_.append(line)
        else:
            if sub_:
                result.append(sub_)
                sub_ = []

    return result

# Example filterfunc
def example_filter(take=1, leave=1):
    """example_filter is a less-fancy version of itertools.cycle"""

    while True:
        for _ in range(take):
            yield True
        for _ in range(leave):
            yield False

# Your example
final = get_data(range(1, 33), example_filter(take=3, leave=2))

As alluded to in the docstring of example_filter, the filterfunc for get_data is really just expecting a True or False based on a call. You could change this easily to be of the signature:

def filterfunc(some_data: object) -> bool:

So that you can determine whether to take or leave based on the value (or even the index), but it currently takes no arguments and just functions as a less magic itertools.cycle (since it should return its value on call, not on iteration)

score 1 · Answer 3 · edited May 23 '17 at 11:52

from itertools import islice
def grouper(iterable, n, min_chunk=1):
    it = iter(iterable)
    while True:
       chunk = list(islice(it, n))
       if len(chunk) < min_chunk:
           return
       yield chunk

def pick_skip_seq(seq, pick, skip, skip_first=False):
    if skip_first:
        ret = [ x[skip:] for x in grouper(seq, pick+skip, skip+1) ]
    else:
        ret = [ x[:pick] for x in grouper(seq, pick+skip) ]
    return ret

pick_skip_seq(range(1,33), 3, 2) gives required list.

In pick_skip_seq(seq, pick, skip, skip_first=False), seq is sequence to pick/skip from, pick/skip are no. of elements to pick/skip, skip_first is to be set True if such behavior is desired.

grouper returns chunks of n elements, it ignores last group if it has less than min_chunk elements. It is derived from stuff given in https://stackoverflow.com/a/8991553/1921546.

Demo:

# pick 3 skip 2 
for i in range(30,35):
    print(pick_skip_seq(range(1,i), 3, 2))

# skip 3 pick 2 
for i in range(30,35):
    print(pick_skip_seq(range(1,i), 3, 2, True))

An alternative implementation of pick_skip_seq:

from itertools import chain,cycle,repeat,compress
def pick_skip_seq(seq, pick, skip, skip_first=False):
    if skip_first:
        c = cycle(chain(repeat(0, skip), repeat(1, pick)))
    else:
        c = cycle(chain(repeat(1, pick), repeat(0, skip)))
    return list(grouper(compress(seq, c), pick))

All things used are documented here: https://docs.python.org/3/library/itertools.html#itertools.compress

How to pick a sequence of numbers from a list?

3 Answers3