How to Find Consecutive Numbers in a Python List

Detecting consecutive integers in a list

From the docs:

>>> from itertools import groupby
>>> from operator import itemgetter
>>> data = [ 1, 4,5,6, 10, 15,16,17,18, 22, 25,26,27,28]
>>> for k, g in groupby(enumerate(data), lambda (i, x): i-x):
...     print map(itemgetter(1), g)
...
[1]
[4, 5, 6]
[10]
[15, 16, 17, 18]
[22]
[25, 26, 27, 28]

You can adapt this fairly easily to get a printed set of ranges.

Find consecutive integers in a list

This is because you only check the next number. When you want the second number (like 9 or 3), you have to include a check for the previous number too. This will make the if a bit longer, but it'll work.

num=[8,9,4,1,2,3]

for i in range(len(num)):
    if (
        (  # check for the next number
            i + 1 != len (num) and  # don't check the end of the list
            num[i]+1==num[i+1] 
        ) or (  # check for the previous number
            i != 0 and  # don't check before the list
            num [i-1] == num [i] - 1
        )
    ): print('Con',num[i])

Also, I had to remove the -1 in your range, because I already do a manual check, and as pointed out, this prvented 3 from being shown.

Finding sequence of consecutive numbers on lists

You can greatly simplify your code by repeatedly looking for the next number in the next list.

def find( list_of_lists ):
    for element in list_of_lists[0]:
        start = element
        next = element
        for list in list_of_lists[1:]:
            next+=1
            print( "looking for {} in {}".format(next, list))
            if not next in list:
                return 'fail'
    return [x for x in range(start,next+1)]

a = [[143], [144, 210, 270], [145]]

find(a)
looking for 144 in [144, 210, 270]
looking for 145 in [145]

[143, 144, 145]

Edit: a corrected version

def find( list_of_lists ):
    num = len(list_of_lists)
    for start in list_of_lists[0]:
        try:
            print( "starting with", start )
            next = start
            last = start + num - 1
            for list in list_of_lists[1:]:
                next+=1
                print( "looking for {} in {}".format(next, list))
                if not next in list:
                    raise KeyError()
                elif next == last:
                    return [x for x in range(start,next+1)]
          except KeyError:
              pass
    return 'fail'

find(a)
starting with 1
looking for 2 in [3, 2, 6]
looking for 3 in [5, 7, 4]
starting with 3
looking for 4 in [3, 2, 6]
starting with 5
looking for 6 in [3, 2, 6]
looking for 7 in [5, 7, 4]

[5, 6, 7]

Counting consecutive numbers in a list

You could take an approach like this:

def countlist(random_list):
    retlist = []
    # Avoid IndexError for  random_list[i+1]
    for i in range(len(random_list) - 1):
        # Check if the next number is consecutive
        if random_list[i] + 1 == random_list[i+1]:
            count += 1
        else:
            # If it is not append the count and restart counting
            retlist.append(count)
            count = 1
    # Since we stopped the loop one early append the last count
    retlist.append(count)
    return retlist

Identify groups of continuous numbers in a list

more_itertools.consecutive_groups was added in version 4.0.

Demo

import more_itertools as mit


iterable = [2, 3, 4, 5, 12, 13, 14, 15, 16, 17, 20]
[list(group) for group in mit.consecutive_groups(iterable)]
# [[2, 3, 4, 5], [12, 13, 14, 15, 16, 17], [20]]

Code

Applying this tool, we make a generator function that finds ranges of consecutive numbers.

def find_ranges(iterable):
    """Yield range of consecutive numbers."""
    for group in mit.consecutive_groups(iterable):
        group = list(group)
        if len(group) == 1:
            yield group[0]
        else:
            yield group[0], group[-1]


iterable = [2, 3, 4, 5, 12, 13, 14, 15, 16, 17, 20]
list(find_ranges(iterable))
# [(2, 5), (12, 17), 20]

The source implementation emulates a classic recipe (as demonstrated by @Nadia Alramli).

Note: more_itertools is a third-party package installable via pip install more_itertools.

Identify groups of continuous numbers in a list

more_itertools.consecutive_groups was added in version 4.0.

Demo

import more_itertools as mit


iterable = [2, 3, 4, 5, 12, 13, 14, 15, 16, 17, 20]
[list(group) for group in mit.consecutive_groups(iterable)]
# [[2, 3, 4, 5], [12, 13, 14, 15, 16, 17], [20]]

Code

Applying this tool, we make a generator function that finds ranges of consecutive numbers.

def find_ranges(iterable):
    """Yield range of consecutive numbers."""
    for group in mit.consecutive_groups(iterable):
        group = list(group)
        if len(group) == 1:
            yield group[0]
        else:
            yield group[0], group[-1]


iterable = [2, 3, 4, 5, 12, 13, 14, 15, 16, 17, 20]
list(find_ranges(iterable))
# [(2, 5), (12, 17), 20]

The source implementation emulates a classic recipe (as demonstrated by @Nadia Alramli).

Note: more_itertools is a third-party package installable via pip install more_itertools.

---

Related Topics