python - Searching a sorted list?

31
votes

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

3
pythonsearchsorting
Sequence of what? Also, what kind of search (binary, etc.)? - Alex Bliskovsky
I believe the question is trying to be "canonical" or "generic" and so the meaning of "sequence" may be using the Python documentation definition of a sequence (i.e. python 2.x "There are seven sequence types: strings, Unicode strings, lists, tuples, bytearrays, buffers, and xrange objects.") - Trevor Boyd Smith

3 Answers

28
votes

bisect is part of the standard library - is that the sort of thing you're looking for?

21
votes

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.

6
votes

Python:

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