programming-for-dummies

354
Understanding Hash Tables
A
B
C
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Figure 3-7:
Hash tables
make
searching
faster by
dividing
data into
distinct
sections.
V
W
X
Y
Hash Table
Value
Mail
Versatile
Plumbing
Data
When you store data with a unique key (a key-value pair), the key is used to
help identify and retrieve the data. However, just using a key alone is no
better than storing data alone because keys are just another chunk of data.
The more data (and keys) you store, the longer it takes the computer to
search through its entire list of keys.
Converting keys with a hash function
To speed up searching, dictionaries use hash tables. Basically, a hash table
takes the keys used to identify data and then converts that key into a hash
value. This hash value gets stored into a sorted list (table), as shown in
Figure 3-8.
The exact method used to convert a key into a value is a hash function. The
converted key, or hash, now points directly to the stored data. At this point,
the computer actually stores just two chunks of data:
✦ The data itself
✦ A hash value calculated from the key
When you want to retrieve data, you give the computer the key associated
with the data that you want. The computer takes the key and uses its hash
function to convert the key to a value.