programming-for-dummies

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Using Indexes
4. If the value at this pth location is less than the data you’re searching
for, search the list to the right of the pth location. If the value at this
pth location is greater than the data you’re searching for, search the
list to the left of the pth location.
5. Repeat Steps 1–4.
Using Indexes
Imagine yourself trying to find a certain store in a large shopping mall. You
could wander up and down the corridors and examine each store, one by one,
which is like a sequential search. Even if you use the various sequential search
tactics, like block searching, sequential searching can still take a long time.
Instead of searching each store sequentially, here’s a faster way: Look at the
mall directory, find the store you want, and then walk straight to that store.
That’s the difference between sequential searching and indexes. An index
points you directly toward the item you want to find no matter how many
items there may be. Indexes basically act like a shortcut to searching.
Creating an index
Indexes are similar to hash tables (see Book III, Chapter 3). The main difference
is that a hash table calculates a unique value based on the total data
stored whereas an index typically stores part of the data in a separate table
that points to the rest of the data, as shown in Figure 2-7.
Indexes are most often used in databases. If you organize a list of data in
rows and columns with each column representing a field (such as Name or
Phone Number) and each row representing a record (that contains one
person’s name and phone number), an index can be as simple as a single
column that consists of the data you’re most likely to use for searching.
For example, if you have a database of names, addresses, and phone numbers,
you probably spend more time looking up someone’s phone number by
looking up his last name. So you could use the last name field as an index. At
the simplest level, an index is nothing more than an organized list of existing
data, such as a list of last names organized alphabetically (see Figure 2-7).
If you create an index based on last names and you need to search by last
name, an index can find your data faster. However, what if you want to
search by phone number or city, but your index consists only of last names?
In that case, you can create multiple indexes, one for each type of data.