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Asked: 2026-05-11T03:15:41+00:00

I have about 10million values that I need to put in some type of

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I have about 10million values that I need to put in some type of look up table, so I was wondering which would be more efficient a list or dict?

I know you can do something like this for both:

if something in dict_of_stuff:     pass

and

if something in list_of_stuff:     pass

My thought is the dict will be faster and more efficient.

Thanks for your help.

EDIT 1
Little more info on what I’m trying to do. Euler Problem 92. I’m making a look up table to see if a value calculated has all ready been calculated.

EDIT 2
Efficiency for look up.

EDIT 3
There are no values assosiated with the value…so would a set be better?

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1 Answer

  1. Speed

    Lookups in lists are O(n), lookups in dictionaries are amortized O(1), with regard to the number of items in the data structure. If you don’t need to associate values, use sets.

    Memory

    Both dictionaries and sets use hashing and they use much more memory than only for object storage. According to A.M. Kuchling in Beautiful Code, the implementation tries to keep the hash 2/3 full, so you might waste quite some memory.

    If you do not add new entries on the fly (which you do, based on your updated question), it might be worthwhile to sort the list and use binary search. This is O(log n), and is likely to be slower for strings, impossible for objects which do not have a natural ordering.

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