[Numpy-discussion] untenable matrix behavior in SVN

Alan G Isaac aisaac@american....
Sat Apr 26 10:13:12 CDT 2008

On Sat, 26 Apr 2008, Gael Varoquaux apparently wrote:
> I claim b is more important than a. IMHO, a is plain 
> wrong: you should't be indexing x with x[0][0]. 

Why?? Would you say this about a 2d array?
Why the difference?

The core argument has been that it is a **basic 
expectation** of the behavior of 2d array-like objects that 
you will be able to get, e.g., the first element with x[0][0].
(E.g., lists, tuples, arrays ... is there an exception??
It should not be broken!)

I teach with matrices and I thought you might too: if so, 
you surely have run into this expectation (which is **natural**).
In fact I truly doubt anyone has not been puzzled on first 
encounter by this::

    >>> x
    matrix([[1, 2],
            [3, 4]])
    >>> x[0]
    matrix([[1, 2]])
    >>> x[0][0]
    matrix([[1, 2]])

Another argument, which I agree with, has been that matrix 
behavior should match 2d array behavior except where there 
is a clear payoff from deviation.

What I do agree with, which I take to be implicit in your 
statement, is that if users of matrices should generally use 
multiple indexes.  But that does not answer the question 
about what we should offer when x[0] is requested.
(Or, more crucially, what you get when you iterate over
a matrix.)

Alan Isaac

PS If you have good arguments, please add them to

More information about the Numpy-discussion mailing list