[Numpy-discussion] Can I add rows and columns to recarray?

Wai Yip Tung tungwaiyip@yahoo....
Sun Dec 5 21:56:51 CST 2010


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.

Thanks,

Wai Yip



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