How Can I Identify Consecutive Number Groups in a Python List?

Identify Consecutive Number Groups in a List

Identifying groups of consecutive numbers in a list is a common task that arises in various programming scenarios.

Python's Built-in Tools

Fortunately, Python's built-in functions and modules provide elegant solutions for this problem. One approach leverages the groupby() function and the lambda expression:

from itertools import groupby

data = [2, 3, 4, 5, 12, 13, 14, 15, 16, 17]
for k, g in groupby(enumerate(data), lambda (i, x): i - x):
    print(map(itemgetter(1), g))

This snippet groups consecutive numbers based on the difference between their index and value, ensuring that elements within each group are consecutive.

If the desired output is a list of tuples representing the range of consecutive numbers, the following modification can be made:

ranges = []
for k, g in groupby(enumerate(data), lambda (i, x): i - x):
    group = map(itemgetter(1), g)
    ranges.append((group[0], group[-1]))

Alternate Approach

Alternatively, a custom solution can be implemented using a loop and a list comprehension:

ranges = []
for i in range(len(data)):
    if i == len(data) - 1 or data[i] + 1 != data[i + 1]:
        ranges.append((data[i], data[i]))
    else:
        start = i
        while i + 1 < len(data) and data[i] + 1 == data[i + 1]:
            i += 1
        ranges.append((data[start], data[i]))

This approach iterates through the list, identifying the starting and ending indices of each consecutive group and appending them to the ranges list.

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