Question

How to reconstruct a full sparse matrix from a compact (triplet) matrix in Java

Answer 1

import java.util.Arrays;

public class RebuildSparseMatrix {

    // A sparse matrix is a matrix in which the majority of elements are zero.

    // To rebuild a sparse matrix from a compact (triplet) representation,
    // we create a full matrix initialized with zeros, then place each
    // non‑zero element at its (row, column) position.

    // Build full sparse matrix from compact triplet form
    public static int[][] buildSparseFromCompact(int[][] compact, int count) {

        int[] rowIdx = compact[0];
        int[] colIdx = compact[1];
        int[] values = compact[2];

        // Determine matrix dimensions automatically
        int maxRow = 0;
        int maxCol = 0;

        for (int i = 0; i < count; i++) {
            if (rowIdx[i] > maxRow) maxRow = rowIdx[i];
            if (colIdx[i] > maxCol) maxCol = colIdx[i];
        }

        int rows = maxRow + 1;
        int cols = maxCol + 1;

        // Create full matrix initialized with zeros
        int[][] sparse = new int[rows][cols];

        // Fill matrix
        for (int i = 0; i < count; i++) {
            sparse[rowIdx[i]][colIdx[i]] = values[i];
        }

        return sparse;
    }

    // Print a matrix
    public static void printMatrix(int[][] mat, String title) {
        System.out.println(title);
        for (int[] row : mat) {
            for (int x : row) {
                System.out.print(x + " ");
            }
            System.out.println();
        }
        System.out.println();
    }

    public static void main(String[] args) {

        // Compact matrix:
        // 0 0 1 1 1 3 3 3 4
        // 2 4 2 3 6 1 2 5 4
        // 3 8 5 7 1 2 6 4 9

        int[][] compact = {
            {0, 0, 1, 1, 1, 3, 3, 3, 4},  // row indices
            {2, 4, 2, 3, 6, 1, 2, 5, 4},  // column indices
            {3, 8, 5, 7, 1, 2, 6, 4, 9}   // values
        };

        int count = 9;

        System.out.println("Compact matrix:");
        for (int i = 0; i < 3; i++) {
            for (int j = 0; j < count; j++) {
                System.out.print(compact[i][j] + " ");
            }
            System.out.println();
        }
        System.out.println();

        int[][] sparse = buildSparseFromCompact(compact, count);

        printMatrix(sparse, "Sparse matrix (auto-sized):");
    }
}



/*
run:

Compact matrix:
0 0 1 1 1 3 3 3 4
2 4 2 3 6 1 2 5 4
3 8 5 7 1 2 6 4 9

Sparse matrix (auto-sized):
0 0 3 0 8 0 0
0 0 5 7 0 0 1
0 0 0 0 0 0 0
0 2 6 0 0 4 0
0 0 0 0 9 0 0

*/

 

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