programming-for-dummies

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Understanding Hash Tables
If you wanted to find the data associated with the key moron, the computer
would first calculate its hash value, which is five.
Next, it’d try to match this value with an existing value stored in the hash
table. In this case, it finds that the hash value of 5 matches up to the key
moron and the data Dick Ross, which is the data you wanted in the first
place.
Basically, a hash table works by searching through a sorted list of data (the
hash values calculated from the key) rather than the unsorted list of data.
Hash function collisions
The hash function used to create a hash table can greatly determine the efficiency
of that hash table. Ideally, the hash function should create a different
hash value for every key. Unfortunately, that’s not always possible, which
means that sometimes the hash function can create identical hash values
from different keys.
In the previous example, the hash function converted a key to a hash value
just by counting the number of characters used in the key. So if two different
keys have the same number of characters, the hash function will create the
same hash value like this:
blackbook := Dictionary new.
blackbook at: ‘moron’ put: ‘Dick Ross’.
blackbook at: ‘imbecile’ put: ‘John Adams’.
Blackbook at: ‘idiot’ put: ‘Sally Evans’.
Using the simple hash function to count the number of characters in each
key would create a table like this:
Hash table
Data
5 Dick Ross
8 John Adams
5 Sally Evans
Hash tables can’t have duplicate values because every hash value must
match with a single chunk of data. A collision occurs when a hash function
creates duplicate values from different keys. Here are two ways to prevent
collisions: