import Foundation
// 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
func buildSparseFromCompact(_ compact: [[Int]], count: Int) -> [[Int]] {
let rowIdx = compact[0]
let colIdx = compact[1]
let values = compact[2]
// Determine matrix dimensions automatically
var maxRow = 0
var maxCol = 0
for i in 0..<count {
if rowIdx[i] > maxRow { maxRow = rowIdx[i] }
if colIdx[i] > maxCol { maxCol = colIdx[i] }
}
let rows = maxRow + 1
let cols = maxCol + 1
// Create full matrix initialized with zeros
var sparse = Array(repeating: Array(repeating: 0, count: cols), count: rows)
// Fill matrix
for i in 0..<count {
sparse[rowIdx[i]][colIdx[i]] = values[i]
}
return sparse
}
// Print a matrix
func printMatrix(_ mat: [[Int]], title: String) {
print(title)
for row in mat {
print(row.map { String($0) }.joined(separator: " "))
}
print()
}
// 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
let 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
]
let count = 9
print("Compact matrix:")
for i in 0..<3 {
print((0..<count).map { String(compact[i][$0]) }.joined(separator: " "))
}
print()
let sparse = buildSparseFromCompact(compact, count: count)
printMatrix(sparse, title: "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
*/
