Thank you for the quick response and Christopher's explanation on the design background. All my tables fit in-memory. I want to explore the data interactively and relational database is does not provide me a lot of value. I was rolling my own library before I come to numpy. Then I find numpy's universal function awesome and really fit what I want to do. Now I just need to find out what to add row which is easy in Python. It is OK if it rebuild an array when I add a column, which should happen infrequently. But if adding row build a new array, this will lead to O(n^2) complexity. In anycase, I will explore the recfunctions. Thank you Wai Yip
On Sun, Dec 5, 2010 at 10:56 PM, Wai Yip Tung <tungwaiyip@yahoo.com> wrote:
I'm fairly new to numpy and I'm trying to figure out the right way to do things. Continuing on my question about using recarray as a relation. I have a recarray like this
In [339]: arr = np.array([ .....: (1, 2.2, 0.0), .....: (3, 4.5, 0.0) .....: ], .....: dtype=[ .....: ('unit',int), .....: ('price',float), .....: ('amount',float), .....: ] .....: )
In [340]: data = arr.view(recarray)
One of the most common thing I want to do is to append rows to data. I think concatenate() might be the method. But I get a problem:
In [342]: np.concatenate((data0,[1,9.0,9.0])) --------------------------------------------------------------------------- TypeError Traceback (most recent call last)
c:\Python26\Lib\site-packages\numpy\<ipython console> in <module>()
TypeError: expected a readable buffer object
The other thing I want to do is to calculate the column value. Right now it can do great thing like
In [343]: data.amount = data.unit * data.price
But sometimes it may require me to add a new column not already exist, e.g.:
In [344]: data.discount_price = data.price * 0.9
How can I add a new column? I tried column_stack. But it give a similar TypeError. I figure I need to first specify the type of the column. But I don't know how.
Check out numpy.lib.recfunctions
I often have
import numpy.lib.recfunctions as nprf
Skipper