How to compress consecutive numbers into range strings in Python

Convert a sorted list of consecutive integers into compact range strings like '1-3', '5-6', and '15'.

Easy Python 3.9+ Aug 9, 2026 Algorithms & data structures 15 views 0 copies

ranges compression arrays algorithm

Python code

29 lines

Python 3.9+

def compress_ranges(nums):
    """Convert a list of sorted consecutive numbers into range strings."""
    if not nums:
        return []
    
    ranges = []
    start = prev = nums[0]
    
    for num in nums[1:]:
        if num == prev + 1:
            prev = num
        else:
            if start == prev:
                ranges.append(str(start))
            else:
                ranges.append(f"{start}-{prev}")
            start = prev = num
    
    # Handle the last range
    if start == prev:
        ranges.append(str(start))
    else:
        ranges.append(f"{start}-{prev}")
    
    return ranges

if __name__ == "__main__":
    numbers = [1, 2, 3, 5, 6, 8, 10, 11, 12, 15]
    print(compress_ranges(numbers))

Output

stdout

['1-3', '5-6', '8', '10-12', '15']

How it works

The function scans the list once, tracking the start of the current run and the previous number. When a number is exactly one greater than the previous, it extends the run; otherwise it finalizes the current run (as a single number or a range) and starts a new one. After the loop, the final run is flushed. This works because the input must be sorted — the check num == prev + 1 depends on consecutive ordering. Time complexity is O(n) and space is O(n) for the output list.

Common mistakes

Forgetting to handle the last range after the loop
Assuming the input is sorted when it isn't — the code only works on ordered lists
Appending `start-prev` even when start equals prev (should be just the number)

Variations

Use a generator with `yield` to produce ranges lazily for very large lists
Use `itertools.groupby` with a key function that subtracts the index from each number to group consecutive runs

Real-world use cases

Summarizing which rows in a large dataset have errors by grouping consecutive row numbers.
Displaying available IP addresses or port ranges in a network scanning tool.
Compressing calendar dates or event IDs into human-readable ranges for log reporting.

How to compress consecutive numbers into range strings in Python

Python code

Output

How it works

Common mistakes

Variations

Real-world use cases

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