We have seen that the array is the best way to implement a map if the key type is integer and the range of possible keys is not too large: access to an array is O(1).

What if the key type does not fit these criteria? The idea of hashing is to use a hash function          h: KeyType &rarr integer

• Convert a key to an integer so that it can be used as an array index.

• Randomize, in the sense that two different keys usually produce different hash values, and hash values are equally likely.

• h should not be too expensive to compute.

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