Find _ in a list

[1234]

Hi,
I was wondering is there a way that you can find the largest number in a list that is equal or smaller than my chosen number, without going through the whole list every time I want to do it?
Is there a way that you can only include numbers that are equal or smaller than the number that I want.

[deanhystad]

Is it ok to sort the list first?

[1234]

Is it ok to sort the list first?

Yes, my list is very big, and there is thousands of numbers bigger than the ones from list 'a', so even if the list is sorted it will take a lot of steps/time to find the biggest number that is equal or smaller than the one from list 'a'.

[deanhystad]

Binary searches on sorted lists are very efficient.

[bowlofred]

Once sorted, there are tools to find the the one you want efficiently, such as bisect.

On a list of 10000000 ints, my machine took about 0.3 seconds to scan the list linearly and find the max value less than a target. It took 2 seconds to sort the list, but afterward bisect could find the highest value less than a target in 0.0001 seconds.

[1234]

Thanks,
quick question, in binary search how do you make the value that your looking for equal or smaller than 100?

[bowlofred]

import bisect

target = 100
sorted_list = [1, 2, 5, 6, 100, 102, 110, 140, 145]
pos = bisect.bisect_left(sorted_list, target) # where to insert, so the number less than is to the left.
# Beware of negative if you asked for a number less than any in the list
print(f"Greatest number less than {target} is at position {pos-1} with value {sorted_list[pos-1]}")

Output:
Greatest number less than 100 is at position 3 with value 6

[1234]

import bisect

target = 100
sorted_list = [1, 2, 5, 6, 100, 102, 110, 140, 145]
pos = bisect.bisect_left(sorted_list, target) # where to insert, so the number less than is to the left.
# Beware of negative if you asked for a number less than any in the list
print(f"Greatest number less than {target} is at position {pos-1} with value {sorted_list[pos-1]}")

Output:
Greatest number less than 100 is at position 3 with value 6

And what if my target is 100 and there is no 100 in the list, so I want to take the closest number to 100 that is smaller than 100?

[deanhystad]

Why don't you try it? You can run the posted code and change the target value.

[bowlofred]

And what if my target is 100 and there is no 100 in the list, so I want to take the closest number to 100 that is smaller than 100?

The program didn't return 100 (because 100 isn't less than 100). It returned the largest number less than 100.

You can read more about the specifics of bisect (which allow you to quickly find things in a sorted list, and to append things in a way that keeps a list sorted) in the docs.