Searching a sorted list?

Question

What is a Pythonic way to search or manipulate sorted sequence?

Asked By: Basilevs

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Source

Answer 1

bisect is part of the standard library – is that the sort of thing you’re looking for?

Answered By: Daniel Roseman

Answer 2

It’s worth noting that there are a couple high-quality Python libraries for maintaining a sorted list which also implement fast searching: sortedcontainers and blist. Using these depends of course on how often you’re inserting/removing elements from the list and needing to search. Each of those modules provide a SortedList class which efficiently maintains the items in sort order.

From the documentation for SortedList:

L.bisect_left(value)
    Similar to the bisect module in the standard library, this returns
    an appropriate index to insert value in L. If value is already present
    in L, the insertion point will be before (to the left of) any existing
    entries.

L.bisect(value)
    Same as bisect_left.

L.bisect_right(value)
    Same as bisect_left, but if value is already present in L, the
    insertion point will be after (to the right of) any existing entries.

Both implementations use binary search to find the correct index of the given value. There’s a performance comparison page for choosing between the two modules.

Disclaimer: I am the author of the sortedcontainers module.

Answered By: GrantJ

Answer 3

Python:

import bisect

def find_in_sorted_list(elem, sorted_list):
    # https://docs.python.org/3/library/bisect.html
    'Locate the leftmost value exactly equal to x'
    i = bisect.bisect_left(sorted_list, elem)
    if i != len(sorted_list) and sorted_list[i] == elem:
        return i
    return -1

def in_sorted_list(elem, sorted_list):
    i = bisect.bisect_left(sorted_list, elem)
    return i != len(sorted_list) and sorted_list[i] == elem

L = ["aaa", "bcd", "hello", "world", "zzz"]
print(find_in_sorted_list("hello", L))  # 2
print(find_in_sorted_list("hellu", L))  # -1
print(in_sorted_list("hellu", L))       # False

Answered By: Paulo A. Ferreira

Searching a sorted list?

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