How to pick consecutive numbers from a list

Question

Ive got a list which actually hold indices for another list. Hence I want to pick out the consecutive numbers from this list

index_list=[3,4,8,9,35,36,37]

from which i want the output as

[3:4], [8:9], [35:37]

---------------------MOTIVE:---------------------

I have another master list of words, which has 80 words.

  master_list=['was,'it','to,'go,'I'.........]

Thus the consecutive indices will help me pick out the required words from master_list as

master_list[3:4], master_list[8:9], master_list[35:37]

These numbers are _consecutive_, not [concurrent](https://dictionary.cambridge.org/us/dictionary/english/concurrent). Also, what's wrong with `[3:37]` as output? How do you decide when to start a new "slice"? — ForceBru, Feb 04 '19 at 16:29
@ForceBru new slice comes when there a break in constructiveness — vinita, Feb 04 '19 at 16:39
@taurus05 I didnt skip 36, I wrote [35:37], which means 35,36,37 — vinita, Feb 04 '19 at 16:40
@vinita do you expect a 2D list of starting and ending indices of consecutive elements as output? Ex:- `[[3:4],[8:9],[35:37]]` ? — taurus05, Feb 04 '19 at 16:42
@vinita, what output do you expect for input list `[1,3,5,7,9]`? Should it be `[[1, 1], [3, 3], [5, 5], [7, 7], [9, 9]]` — taurus05, Feb 04 '19 at 17:01

score 1 · Answer 1 · answered Feb 04 '19 at 16:56

There are likely lots of ways to do this. Here's one based on reduce() and islice():

from functools import reduce
from itertools import islice

qw = [3, 4, 8, 9, 12, 13, 14]

master_list = ['Thus', 'the', 'consecutive', 'indices', 'will', 'help', 'me', 'pick', 'out', 'the', 'required', 'words', 'from', 'master_list', 'as']

def divide(value, element):
    if not value[-1] or element - value[-1][-1] == 1:
        value[-1].append(element)
    else:
        value.append([element])

    return value

slices = [(array[0], array[-1]) for array in reduce(divide, qw, [[]])]

print(slices)

for sliced in slices:
    print(list(islice(master_list, *sliced)))

OUTPUT

% python3 test.py
[(3, 4), (8, 9), (12, 14)]
['indices']
['out']
['from', 'master_list']
%

Note that this treats the second number in the slice in the customary Python manner in that it's one beyond what we want. If it's really the last item of what you want then modify this element with a + 1:

(array[0], array[-1] + 1)

Thanks a lot. What if I have a list = [3, 4, 5, 16, 17, 31, 32, 33, 34] and want indices of consecutive numbers as list of list. [[0,1,2], [3,4], [5,6,7,8]] . Could you pls suggest for this case too ? — vinita, Feb 04 '19 at 17:10
@vinita, this falls out for free as it's what `reduce(divide, qw, [[]])` returns: `[[3, 4, 5], [16, 17], [31, 32, 33, 34]]` before the list comprehension cleans it up for `islice()`. — cdlane, Feb 04 '19 at 17:56

taurus05 · Answer 2 · 2019-02-04T17:36:24.710

1

Using much generalized way:

i = 0
final_list = []
flag = True
while flag:
    temp_list = [index_list[i]]
    while (i < len(index_list)-1) and (index_list[i+1] - index_list[i]==1):
        i+=1

    temp_list.append(index_list[i])
    final_list.append(temp_list)
    i+=1
    if i >= len(index_list):
        flag=False

print(final_list)

input : [3,4,8,9,35,36,37]
ouptut : [[3, 4], [8, 9], [35, 37]]

input : [1,3,5,7,9]
output : [[1, 1], [3, 3], [5, 5], [7, 7], [9, 9]]

Update:

new_list = []
for i, j in final_list:
    new_list.append(list(range(i,j+1)))
print(new_list)

input:  [3, 4, 5, 16, 17, 31, 32, 33, 34] 
output : [[3, 4, 5], [16, 17], [31, 32, 33, 34]]

edited Feb 04 '19 at 17:36

answered Feb 04 '19 at 17:05

taurus05

2,491
15
28

Thanks a lot. What if I have a list = [3, 4, 5, 16, 17, 31, 32, 33, 34] and want indices of consecutive numbers as list of list. [[0,1,2], [3,4], [5,6,7,8]] . Could you pls suggest for this case too ? – vinita Feb 04 '19 at 17:32
This is easy! I've made necessary changes @vinita. – taurus05 Feb 04 '19 at 17:36

How to pick consecutive numbers from a list

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