ADVANCED DATA STRUCTURES PROGRAM • LEVEL 11 — STRING PATTERN MATCHING

Search with the Z Algorithm

Learn how to search with the z algorithm using a clear C program.

AdvancedPattern matchingString preprocessing

PROBLEM UNDERSTANDING

Input and expected output

Sample input

No input required

Sample output

Matches: 0 2 4

COMPLETE C PROGRAM

Complete C implementation

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

int main(void)
{
    const char combined[] = "ABA$ABABABA";
    int length = (int) strlen(combined), z[30] = {0}, left = 0, right = 0;
    for (int index = 1; index < length; index++) {
        if (index <= right) z[index] = right - index + 1 < z[index - left] ? right - index + 1 : z[index - left];
        while (index + z[index] < length && combined[z[index]] == combined[index + z[index]]) z[index]++;
        if (index + z[index] - 1 > right) { left = index; right = index + z[index] - 1; }
    }
    printf("Matches:");
    for (int index = 4; index < length; index++) if (z[index] == 3) printf(" %d", index - 4);
    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 2 4

0%Step 0 of 0

PROGRAM EXPLANATION

Algorithm and explanation

Read the required input values.
Build Z values for pattern, separator and text; a full pattern-length value marks a match.
Display the computed result.

Build Z values for pattern, separator and text; a full pattern-length value marks a match.

EFFICIENCY

Time and space complexity

Time complexity

O(n + m)

Auxiliary space

O(pattern length)

DEBUGGING CHECKLIST

Common mistakes

Check this

Use the correct format specifier for every variable.

Check this

Initialize variables before using their values.

Check this

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.