object 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 a full sparse matrix from compact triplet form
fun buildSparseFromCompact(compact: Array<IntArray>, count: Int): Array<IntArray> {
val rowIdx = compact[0]
val colIdx = compact[1]
val values = compact[2]
// Determine matrix dimensions automatically
var maxRow = 0
var maxCol = 0
for (i in 0 until count) {
if (rowIdx[i] > maxRow) maxRow = rowIdx[i]
if (colIdx[i] > maxCol) maxCol = colIdx[i]
}
val rows = maxRow + 1
val cols = maxCol + 1
// Create full matrix initialized with zeros
val sparse = Array(rows) { IntArray(cols) }
// Fill matrix
for (i in 0 until count) {
sparse[rowIdx[i]][colIdx[i]] = values[i]
}
return sparse
}
// Print a matrix
fun printMatrix(mat: Array<IntArray>, title: String) {
println(title)
for (row in mat) {
println(row.joinToString(" "))
}
println()
}
@JvmStatic
fun main(args: Array<String>) {
// 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
val compact = arrayOf(
intArrayOf(0, 0, 1, 1, 1, 3, 3, 3, 4), // row indices
intArrayOf(2, 4, 2, 3, 6, 1, 2, 5, 4), // column indices
intArrayOf(3, 8, 5, 7, 1, 2, 6, 4, 9) // values
)
val count = 9
println("Compact matrix:")
for (i in 0 until 3) {
println((0 until count).joinToString(" ") { compact[i][it].toString() })
}
println()
val 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
*/
