[Numpy-discussion] Matlab -> NumPy translation and indexing
Mon Mar 5 08:24:15 CST 2007
On 3/5/2007 2:13 PM, David Koch wrote:
> - Am I correct in assuming that all arrays have to be initialized to
> their final number of elements in NumPy (using empty/zero for instance)?
You can also create an array from a Python list, data in a file, another
array or a memory mapping. In these cases you don't need to know the
size of the array in advance.
But once created you cannot append elements to a NumPy array as you can
with a Python list. NumPy arrays never call realloc due to the
complication from view arrays.
> - Given a matrix R, is there an equvialent to the Matlab operation
> R(:,j) =  (which removes column j and "shrinks" the matrix?
Matlab does not have view arrays and no regard for memory consumption or
fragmentation, so it can make a copy behind the scenes. In NumPy you
have to be more explicit about what is done.
idx = arange(0,a.shape)
idx = idx[idx != j]
> - For (basic ?) indexing of an ndarray - is there any reason to prefer
> A[i,j] over A[i][j]?
The latter involves two function calls and creation of an intermediate
object (a view array).
> - Is it "pythonic" to initialize vectors to 2 dimensions so that
> vec.shape == (len(vec), 1) instead of vec.shape == (len(vec),)?
I have no idea.
More information about the Numpy-discussion