programming-for-dummies

Sequential Text Search 425
In this example, the third character (T) doesn’t match, so the brute force algorithm
starts all over again by examining the second character, even though it
had previously examined that character. Because the brute force method
examines every character individually, this method is the slowest and least
efficient method for finding text.
Although the brute force method works, it can take too much time, especially
when searching through large amounts of text. To make text searching
faster and more efficient, computer scientists have developed a variety of
alternative algorithms.
The Boyer-Moore algorithm
To speed up text searching, the computer should skip any previously examined
text and jump straight to unexamined text. That’s the basis for a text
searching algorithm developed by two computer scientists (Bob Boyer and
J. Strother Moore) called the Boyer-Moore algorithm.
Like the brute force algorithm, the Boyer-Moore algorithm examines text
character by character. After the Boyer-Moore algorithm finds a partial
match, it’s smart enough to skip over previously examined characters and
start searching from the end of all examined characters, speeding up the
entire search process, as shown in Figure 3-2.
Searching for the string GAG
G
A T A G
A
G
A
T
G
G A T A G A G A
T
GA
Book IV
Chapter 3
Figure 3-2:
The Boyer-
Moore
algorithm
skips over
partially
matched
characters.
G A T A G A G A
G
GAT (Fail)
A T A G A G A
Skip previously examined text
T
T
String Searching