Issue with implementing the KMP algorithm

[YreiauW]

I am currently working on implementing the KMP algorithm in my program, but I am having trouble understanding how to construct the partial match table. I know that the table is used to determine the longest proper prefix of a given sub-string that is also a suffix, but I am having trouble figuring out how to implement this in my code.

For example, let's say I have the string "abcabca" and I want to find the partial match table for it. I have tried to use the following code:

string text = "abcabca";
int n = text.length();
int next[n];
next[0] = -1;
int j = -1;
for (int i = 1; i < n; i++) {
    while (j >= 0 && text[j + 1] != text[i]) {
        j = next[j];
    }
    if (text[j + 1] == text[i]) {
        j++;
    }
    next[i] = j;
}

But when I run this code, I am getting unexpected results and I am not sure where I am going wrong. Can anyone help me understand how to correctly construct the partial match table for the KMP algorithm? This article discusses using triggers to make a select statement (to check for a given condition) on the table in a few seconds or minutes. Instead of asking the database whether or not the data has been updated, the database will inform you when the data is updated. Is it the right thing to do?

[kumud123]

Here is a nice article which can help in understanding KMP algorithm KMP Algorithm Explained In Plain English – W3 Spot. It doesn't have any code implementation though.

[sheen8]

The partial match table, also known as the failure function or the "next" array, is used in the Knuth-Morris-Pratt (KMP) algorithm to efficiently search for a pattern within a text.

To construct the partial match table, you can modify your code as follows:

cpp
#include  <iostream>
#include  <string>
#include  <vector>

using namespace std;

vector<int> constructPartialMatchTable(const string& pattern) {
    int n = pattern.length();
    vector<int> next(n, 0);
    int j = 0;
    
    for (int i = 1; i < n; i++) {
        while (j > 0 && pattern[j] != pattern[i]) {
            j = next[j - 1];
        }
        
        if (pattern[j] == pattern[i]) {
            j++;
        }
        
        next[i] = j;
    }
    
    return next;
}

int main() {
    string pattern = "abcabca";
    vector<int> next = constructPartialMatchTable(pattern);
    
    cout << "Partial match table: ";
    for (int val : next) {
        cout << val << " ";
    }
    
    return 0;
}

In this code, the `constructPartialMatchTable` function takes the pattern string as input and returns the constructed partial match table as a vector of integers.

Here's how the code works:

1. It initializes the `next` array with all elements set to 0.
2. It uses two pointers, `i` and `j`, to iterate through the pattern and construct the table.
3. The outer loop starts from index 1 (not 0) because the first entry in the `next` array is always 0.
4. The inner `while` loop finds the longest proper prefix of the substring ending at `i` that is also a suffix. It does this by updating the `j` pointer using the values in the `next` array.
5. If a match is found, `j` is incremented.
6. Finally, the value of `j` is assigned to `next[i]`.

After constructing the partial match table, you can use it in the KMP algorithm to efficiently search for the pattern within a text.

Regarding your question about triggers and select statements in the context of database updates, it seems unrelated to the KMP algorithm or the code you provided. Triggers and select statements are database concepts used to automate actions or retrieve data from a database.