Sudoku starting point in ruby

sudoku.rb

grid = [
    [nil,3,8,     nil,nil,nil,  7,9,1      ],
    [2,nil,nil,   nil,nil,nil,  4,nil,6    ],
    [nil,nil,nil, nil,6  ,9  ,  nil,8,nil  ],

    [nil,nil,nil, 9,1,nil,      nil,5,nil  ],
    [9,nil,nil,   nil,nil,nil,  nil,nil,7  ],
    [nil,5,nil,   nil,8,4,      nil,nil,nil],

    [nil,6,nil,   8,9,nil,      nil,nil,nil],
    [7,nil,9,     nil,nil,nil,  nil,nil,8  ],
    [3,8,1,       nil,nil,nil,  5,2,nil    ]
]

def rows_are_valid? grid
    (0..8).each do |row_index|
        row = grid[row_index]
        return false if row.reject(&:nil?).uniq != row.reject(&:nil?)
    end
    true
end

def cols_are_valid? grid
    (0..8).each do |col_index|
        col = grid.map { |row| row[col_index] }
        return false if col.reject(&:nil?).uniq != col.reject(&:nil?)
    end
    true
end

def squares_are_valid? grid
    (0..2).each do |x_index|
       (0..2).each do |y_index|
          x_start = x_index * 3
          y_start = y_index * 3
          square = grid[y_start][x_start..x_start+2] + grid[y_start + 1][x_start..x_start+2] + grid[y_start + 2][x_start..x_start+2]
          return false if square.reject(&:nil?).uniq != square.reject(&:nil?)
       end
    end
    true
end

def is_valid? grid
    rows_are_valid? grid and cols_are_valid? grid and squares_are_valid? grid
end

puts is_valid? grid