ADVANCED DATA STRUCTURES PROGRAM • LEVEL 11 — STRING PATTERN MATCHING

Search with a Pattern Finite Automaton

Learn how to search with a pattern finite automaton using a clear C program.

AdvancedPattern matchingString preprocessing

PROBLEM UNDERSTANDING

Input and expected output

Sample input

No input required

Sample output

Matches: 0 9 12

COMPLETE C PROGRAM

Complete C implementation

#include <stdio.h>
#include <string.h>

int main(void)
{
    const char text[] = "AABAACAADAABAABA", pattern[] = "AABA";
    int m = (int) strlen(pattern), transition[5][2] = {{1,0},{2,0},{2,3},{4,0},{2,0}}, state = 0;
    printf("Matches:");
    for (int index = 0; text[index]; index++) {
        state = transition[state][text[index] == 'B'];
        if (state == m) printf(" %d", index - m + 1);
    }
    putchar('\n'); return 0;
}

CURRENT STEP

Select Start to walk through the important lines.

SELECTED LINE

No line selected

EXPECTED OUTPUT FOR THE SAMPLE

Matches: 0 9 12

0%Step 0 of 0

PROGRAM EXPLANATION

Algorithm and explanation

Read the required input values.
Treat the matched-prefix length as an automaton state and consume each text character once.
Display the computed result.

Treat the matched-prefix length as an automaton state and consume each text character once.

EFFICIENCY

Time and space complexity

Time complexity

O(n) after preprocessing

Auxiliary space

O(m * alphabet)

DEBUGGING CHECKLIST

Common mistakes

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Use the correct format specifier for every variable.

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Initialize variables before using their values.

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Check braces, semicolons and input order carefully.

Try it yourself

Practice: Run the program with the sample input, predict its output, and then test one boundary case of your own.