python 中最有效的计算方式在对列表中查找对

Question

I need to find if pair exists in a list of pairs. I can use a standard for loop with if statement, but is there a more computationally efficient way?

Current my data looks like:

alist = [(12970, 12980), (12970, 13000), (12970, 13012)]
pair = [12970, 13000]

for a in list:
   if a == pair:
       print("do something")

I have tried using the following but it did not return a match:

if pair in alist:
    print("do something")

Any other suggestions?

Answer 1

pair is a list of two integers.

alist has elements that are tuples .

Since these are not even of the same base type, they will not compare as equal.

Try

alist = [(12970, 12980), (12970, 13000), (12970, 13012)]
pair = (12970, 13000)    # tuple, not list
if pair in list:
    ...

If your problem requirements don't let you change the original values, then cast the type in-line:

if tuple(pair) in list:

Answer 2

If the alist is sorted you can use bisect which should have complexity O(log n).

Copying from module docs:

import bisect

def index(a, x):
    'Locate the leftmost value exactly equal to x'
    i = bisect.bisect_left(a, x)
    if i != len(a) and a[i] == x:
        return i
    raise ValueError

index(alist, (12970, 13000))

If there is no element found you will get ValueError

You could also go with converting alist to set. Then the average complexity should be O(1) on lookup