The documentation is incorrect. If you look at the matrix.rb
file from 1.9.1, you'll see this:
def []=(i, j, v)
@rows[i][j] = v
end
alias set_element []=
alias set_component []=
private :[]=, :set_element, :set_component
So the three methods are there but they are explicitly set as private.
A bit of quick experimentation indicates that a lot of the methods in the documentation are, in fact, private. There is a big block of documentation at the top of the man page that lists what are, apparently, supposed to be the available methods; that list doesn't match the list that rdoc has generated so there is some confusion.
I get the impression that instances of Matrix are meant to be immutable just like Fixnum and Number.