Greedy LCS Java Code Implementation

Question

I'm trying to make java code from this journal:

This code is about a greedy approach for computing longest common subsequences by afroza begum.

And here is my [EDITED] code:

public class LCS_Greedy {

public static void main(String[] args) {
//        String X_awal = "ABCBDABE";
//        String Y = "BDCABA";

//        String X_awal = "XMJYAUZ";
//        String Y = "MZJAWXU";
    
//        String X_awal ="AGGTAB";
//        String Y="GXTXAYB";
    
//        String X_awal = "ABCDGH";
//        String Y = "AEDFHR";
    
//        String X_awal = "AHBCDG";
//        String Y = "AEDFHR";       
    
    //greedy not always optimum
    String X_awal = "bcaaaade";
    String Y = "deaaaabc";
    String X = match(X_awal, Y);
    int[] Penanda_Y = new int[Y.length()];
    int y_length = Y.length();
    for (int k = 0; k < y_length; k++) {
        Penanda_Y[k] = 0;
    }
    int m = X.length();
    int n = Y.length();
    String L = "";
    String LSym = "";
    int R = 0;
    int i = 1;
    int[] P = new int[100];
    P[i] = posisi(X, Y, i, Penanda_Y, R);
    i = 1;
    while (i <= m) {
        if (i != m) {
            P[i + 1] = posisi(X, Y, (i + 1), Penanda_Y, R);
        }
        if (P[i + 1] == 0) {
            if (P[i] > R) {
                L = L + " " + Integer.toString(P[i]);
                LSym = LSym + " " + X.charAt(i - 1);
            }
            break;
        }

        if (P[i + 1] < R || P[i] < R) {
            R = 0;
        }
        if (P[i] > P[i + 1]) {
            if (R == 0 && i > 1) {
                L = L + " " + Integer.toString(P[i]);
                LSym = LSym + " " + X.charAt(i - 1);
                Penanda_Y[P[i] - 1] = 1;
                R = P[i];
                i = i + 1;
                if (R == Y.length() || i > X.length()) {
                    break;
                }
                P[i] = posisi(X, Y, i, Penanda_Y, R);
            } else {
                L = L + " " + Integer.toString(P[i + 1]);
                LSym = LSym + " " + X.charAt(i + 1 - 1);
                Penanda_Y[P[i + 1] - 1] = 1;
                R = P[i + 1];
                i = (i + 1) + 1;
                if (R == Y.length() || i > X.length()) {
                    break;
                }
                P[i] = posisi(X, Y, i, Penanda_Y, R);
            }

        } else {

            if (R == 0 && i > 1) {
                L = L + " " + Integer.toString(P[i + 1]);
                LSym = LSym + " " + X.charAt(i + 1 - 1);
                Penanda_Y[P[i + 1] - 1] = 1;
                R = P[i + 1];
                i = (i + 1) + 1;
                if (R == Y.length() || i > X.length()) {
                    break;
                }
                P[i] = posisi(X, Y, i, Penanda_Y, R);
            } else {
                L = L + " " + Integer.toString(P[i]);
                LSym = LSym + " " + X.charAt(i - 1);
                Penanda_Y[P[i] - 1] = 1;
                R = P[i];
                i = i + 1;
                if (R == Y.length() || i > X.length()) {
                    break;
                }
                P[i] = posisi(X, Y, i, Penanda_Y, R);
            }
        }
    }
    System.out.println("X = " + X_awal);
    System.out.println("X = " + Y);
    System.out.println("L = " + L);
    System.out.println("LSym = " + LSym);
    System.out.println("Length = " + LSym.length() / 2);
}

public static String match(String X, String Y) {
    String hasil = "";
    for (int i = 0; i < X.length(); i++) {
        for (int j = 0; j < Y.length(); j++) {
            if (X.charAt(i) == Y.charAt(j)) {
                hasil = hasil + X.charAt(i);
                break;
            }
        }
    }
    return hasil;
}

public static int posisi(String X, String Y, int i, int[] Penanda_Y, int R) {
    int n = Y.length();
    int k;
    int kr = 0;
    i = i - 1;
    for (k = 0; k < n; k++) {
        if ((X.charAt(i) == Y.charAt(k)) && Penanda_Y[k] == 0) {
            kr = k + 1;
            break;
        }
    }
    for (k = R; k < n; k++) {
        if ((X.charAt(i) == Y.charAt(k)) && Penanda_Y[k] == 0) {
            kr = k + 1;
            break;
        }
    }
    return kr;
}
}

But when I run that program, in some case it has true output, but in another case it false. What is wrong with that code? Can anybody explain to me? Thanks

Edit:

My code now can run perfectly, but it seem far away from pseudo code in that journal. Can anybody explain this? Why I cannot make code exactly from the journal? It always error when I make from pseudo code given in that journal. Thanks.

Do not try to directly translate the attached pseudocode. It's missing the preprocessing phase. — Piotr Praszmo, Jun 01 '14 at 07:34
@Banthar Thank's, I will try to make preprocessing phase and will edit my question soon — john, Jun 01 '14 at 09:18

score 0 · Answer 1 · answered May 06 '15 at 14:22

The algorithm will not lead to LCS. The greedy guess was wrong. It assumes the early match will guaranty to form LCS. Even though, the pseudocode takes the earlier matching of first two in X, which may skip a lot of common substring at the beginning of Y. The other simple test is change last 'E' to 'A' in X, the algorithm won't be able to find last matching 'A' for sure. The truth is an early match may lock down the position to forbidden longer match. In other word, we may need to skip some early match, so that we can found a longer match. Dynamic programming way is proved to be true mathematically, however, this algorithm didn't.

Greedy LCS Java Code Implementation

Edit:

1 Answers1