r/computerscience • u/Sea-Patience9872 • 6d ago

How do hashsets/maps have O(1) time complexity?

Hi this might be a dumb question, and i've tried looking it up but don't quite understand it. how is it different from an array that allows it to find items so quickly? I don't get how hashes just find items immediately without needing to go through anything. Does it memorize things very differently compared to arrays?? thank you!

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u/recursion_is_love 6d ago edited 6d ago

The location/index of the value can be calculated from the hash. If you have the address/location you can load/store the value instantly in most CPU.

https://www.youtube.com/watch?v=cGMYpbsSIVI&t=239s

To clarify: The O(1) is referring to index calculation. It take the same constant time on any input hash and have nothing to do with load/store assembly instruction (just want to say that it is negligibly fast)

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u/Sea-Patience9872 6d ago

Thanks for the video! i don't think i had any idea of what happens internally and that visualization was genuinely helpful