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Hashing tuples - remow - Dec-03-2019
I just discovered (the bad way) that:
hash((-1,0)) == hash((-2,0))I rewrote the program using:
hash(str(tuple))and got it to work.
Has someone a better idea?
Thank you!
(Using python 3.6.9)
RE: Hashing tuples - Gribouillis - Dec-03-2019
I didn't know that. Hash collisions occur. Why is it a problem for you?
RE: Hashing tuples - remow - Dec-03-2019
(Dec-03-2019, 10:36 PM)Gribouillis Wrote: I didn't know that. Hash collisions occur. Why is it a problem for you?
Well... I was trying to speed up an algorithm with an ad-hoc data structure, but it didn't work, also with small datasets. Lost some hours searching for a bug in the wrong places...
I didn't expect to find a collision so fast!
RE: Hashing tuples - Gribouillis - Dec-03-2019
remow Wrote:I didn't expect to find a collision so fast!
hash() is meant to be fast in order to implement hash-table-like algorithms. If you want robust hash functions against collisions, use the hashlib module or cryptographic modules.
RE: Hashing tuples - remow - Dec-03-2019
(Dec-03-2019, 10:51 PM)Gribouillis Wrote: hash() is meant to be fast in order to implement hash-table-like algorithms.Yes, I liked the fast part and glossed over collisions
Tomorrow I'll try to remove the
str() hack and take hash-collisions into account, hoping it won't be too slow/use too much memory with the full dataset (that I still haven't tried with the str() hack, but now I am convinced that it won't work). But now I need sleep.Thank you!
RE: Hashing tuples - scidam - Dec-04-2019
This is because
hash(-2) == hash(-1) and hash of a tuple is based on hashes of its items. So, it is expected that hash((-2,-1,-2)) will be equal to hash((-1,-1,-1)) etc.
RE: Hashing tuples - Gribouillis - Dec-04-2019
Can you explain what you want to do? Instead of the
hash() function you could perhaps rely directly on dictionaries.
RE: Hashing tuples - remow - Dec-06-2019
(Dec-04-2019, 04:55 AM)Gribouillis Wrote: Can you explain what you want to do? Instead of the hash() function you could perhaps rely directly on dictionaries.Thank you, I just did that.
At the end it was not worth the effort, I was reinventing the wheel...
Now I am waiting for the algorithm to finish.