# 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
def build_sparse_from_compact(compact, count)
row_idx = compact[0]
col_idx = compact[1]
values = compact[2]
# Determine matrix dimensions automatically
max_row = 0
max_col = 0
count.times do |i|
max_row = row_idx[i] if row_idx[i] > max_row
max_col = col_idx[i] if col_idx[i] > max_col
end
rows = max_row + 1
cols = max_col + 1
# Create full matrix initialized with zeros
sparse = Array.new(rows) { Array.new(cols, 0) }
# Fill matrix
count.times do |i|
sparse[row_idx[i]][col_idx[i]] = values[i]
end
sparse
end
# Print a matrix
def print_matrix(mat, title)
puts title
mat.each do |row|
puts row.join(" ")
end
puts
end
# 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
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
]
count = 9
puts "Compact matrix:"
3.times do |i|
puts compact[i][0...count].join(" ")
end
puts
sparse = build_sparse_from_compact(compact, count)
print_matrix(sparse, "Sparse matrix (auto-sized):")
=begin
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
=end
