# Hash function classification interview question

In several places on the Internet there's the interview question

Classify the Hashing Functions based on the various methods by which the key value is found.

• Direct method
• Subtraction method
• Modulo-Division method
• Digit-Extraction method
• Mid-Square method
• Folding method
• Pseudo-random method

which I find strange. I think I know a lot about hashing, but this is plain gibberish to me, could anybody explain?

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They are methods of turning a hashcode into an index of the array that contains the values. Let's say you have a hashcode of 0x12345678. A very large number and you're unlikely to have an array of this size. If you do you can just do

``````Value = array[0x12345678];
``````

And be done (direct method).

If you don't then you have a figure out a way to turn this value into one that fits the array size while trying to avoid too many collisions. The terms used are probably known by other names too, but for example you could just mask off the higher bits of the hashcode

``````Value = array[hashcode & 0xffff];
``````

Or mod the hashcode against the array size

``````Value = array[hashcode % array.size()]; // modulo division
``````

Etc etc

Edit: this link may help

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Thanks, that's it - the link seems to be the source of this ***. It still makes no sense as they're mixing many things together: 1. computing the hashCode from a key (e.g., fold and add), 2. improving (=smearing) the hash (e.g., mid-square), 3. mapping the hash to the index (e.g., modulus, "&"). –  maaartinus Dec 31 '12 at 4:28