Lec 19 Hashing

• You can't add custom classes to sets if they aren't hashable
• __hash__(self) -> int: from collections.abc import Hashable
• hash:
  • Map a val to an int index
• Hash table:
  • List storing elements via hashing
• Hash set:
  • Set of elements stored using hash function
• Hash function:
  • Algorithm mapping values to indexes

Example:

hash function:
i % length

set.add(11) # 11 % 10 == 1
set.add(49) # 49 % 10 == 9
set.add(24) # 24 % 10 == 4
set.add(7) # 7 % 10 == 7
set.add(17) # 7 % 10 == 7 -> makes a hash collision

• Avoidance of hash collisions:
  • Probing:
    • Moving the issue child into the next available index
  • Chaining
    • Putting a list at each index, and that list will be searched through
• Hash rules:
  • Must always return same val given the same object