Data Structure
Networking
RDBMS
Operating System
Java
MS Excel
iOS
HTML
CSS
Android
Python
C Programming
C++
C#
MongoDB
MySQL
Javascript
PHP
Physics
Chemistry
Biology
Mathematics
English
Economics
Psychology
Social Studies
Fashion Studies
Legal Studies
- Selected Reading
- UPSC IAS Exams Notes
- Developer's Best Practices
- Questions and Answers
- Effective Resume Writing
- HR Interview Questions
- Computer Glossary
- Who is Who
Bad Character Heuristic
The bad character heuristic method is one of the approaches of Boyer Moore Algorithm. Another approach is Good Suffix Heuristic. In this method we will try to find a bad character, that means a character of the main string, which is not matching with the pattern. When the mismatch has occurred, we will shift the entire pattern until the mismatch becomes a match, otherwise, pattern moves past the bad character.
Here the time complexity is O(m/n) for best case and O(mn)for the worst case, where n is the length of the text and m is the length of the pattern.
Input and Output
Input: Main String: “ABAAABCDBBABCDDEBCABC”, Pattern “ABC” Output: Pattern found at position: 4 Pattern found at position: 10 Pattern found at position: 18
Algorithm
badCharacterHeuristic(pattern, badCharacterArray)
Input − pattern, which will be searched, the bad character array to store location
Output: Fill the bad character array for future use
Begin n := pattern length for all entries of badCharacterArray, do set all entries to -1 done for all characters of the pattern, do set last position of each character in badCharacterArray. done End
searchPattern(pattern, text)
Input − pattern, which will be searched and the main text
Output − the locations where the pattern is found
Begin patLen := length of pattern strLen := length of text. call badCharacterHeuristic(pattern, badCharacterArray) shift := 0 while shift <= (strLen - patLen), do j := patLen -1 while j >= 0 and pattern[j] = text[shift + j], do decrease j by 1 done if j < 0, then print the shift as, there is a match if shift + patLen < strLen, then shift:= shift + patLen – badCharacterArray[text[shift + patLen]] else increment shift by 1 else shift := shift + max(1, j-badCharacterArray[text[shift+j]]) done End
Example
#include<iostream>
#define MAXCHAR 256
using namespace std;
int maximum(int data1, int data2) {
if(data1 > data2)
return data1;
return data2;
}
void badCharacterHeuristic(string pattern, int badCharacter[MAXCHAR]) {
int n = pattern.size(); //find length of pattern
for(int i = 0; i<MAXCHAR; i++)
badCharacter[i] = -1; //set all character distance as -1
for(int i = 0; i < n; i++) {
badCharacter[(int)pattern[i]] = i; //set position of character in the array.
}
}
void searchPattern(string mainString, string pattern, int *array, int *index) {
int patLen = pattern.size();
int strLen = mainString.size();
int badCharacter[MAXCHAR]; //make array for bad character position
badCharacterHeuristic(pattern, badCharacter); //fill bad character array
int shift = 0;
while(shift <= (strLen - patLen)) {
int j = patLen - 1;
while(j >= 0 && pattern[j] == mainString[shift+j]) {
j--; //reduce j when pattern and main string character is matching
}
if(j < 0) {
(*index)++;
array[(*index)] = shift;
if((shift + patLen) < strLen) {
shift += patLen - badCharacter[mainString[shift + patLen]];
}else {
shift += 1;
}
}else {
shift += maximum(1, j - badCharacter[mainString[shift+j]]);
}
}
}
int main() {
string mainString = "ABAAABCDBBABCDDEBCABC";
string pattern = "ABC";
int locArray[mainString.size()];
int index = -1;
searchPattern(mainString, pattern, locArray, &index);
for(int i = 0; i <= index; i++) {
cout << "Pattern found at position: " << locArray[i]<<endl;
}
}Output
Pattern found at position: 4 Pattern found at position: 10 Pattern found at position: 18
1K+ Views
