// 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
fn build_sparse_from_compact(compact: &[Vec<i32>], count: usize) -> Vec<Vec<i32>> {
let row_idx = &compact[0];
let col_idx = &compact[1];
let values = &compact[2];
// Determine matrix dimensions automatically
let mut max_row = 0;
let mut max_col = 0;
for i in 0..count {
if row_idx[i] > max_row {
max_row = row_idx[i];
}
if col_idx[i] > max_col {
max_col = col_idx[i];
}
}
let rows = (max_row + 1) as usize;
let cols = (max_col + 1) as usize;
// Create full matrix initialized with zeros
let mut sparse = vec![vec![0; cols]; rows];
// Fill matrix
for i in 0..count {
sparse[row_idx[i] as usize][col_idx[i] as usize] = values[i];
}
sparse
}
// Print a matrix
fn print_matrix(mat: &[Vec<i32>], title: &str) {
println!("{}", title);
for row in mat {
for x in row {
print!("{} ", x);
}
println!();
}
println!();
}
fn main() {
// 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 = vec![
vec![0, 0, 1, 1, 1, 3, 3, 3, 4], // row indices
vec![2, 4, 2, 3, 6, 1, 2, 5, 4], // column indices
vec![3, 8, 5, 7, 1, 2, 6, 4, 9], // values
];
let count = 9;
println!("Compact matrix:");
for i in 0..3 {
for j in 0..count {
print!("{} ", compact[i][j]);
}
println!();
}
println!();
let sparse = build_sparse_from_compact(&compact, count);
print_matrix(&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
*/
