FW: [Numpy-discussion] Questions about records

Jin-chung Hsu hsu at stsci.edu
Fri Apr 2 11:13:06 CST 2004


> I've been trying to generate a 2-dimensional numarray.records array 
> and am rather puzzled about some failures:
> 
> I tried a pair of 2x2 arrays for the buffer (one per field). I 
> thought the record would get its shape from those, but instead of a 
> 2x2 record I get a 2-length record, each of whose elements is a pair 
> of 2-length arrays.
> 
> >>>  arr1 = num.arange(4, shape=(2,2), type=num.Float64)
> >>>  arr2 = num.arange(4, shape=(2,2), type=num.Float64) + 10
> >>>  a = rec.array([arr1, arr2], names="a,b")
> >>>  a
> array(
> [(array([ 0.,  1.]), array([ 10.,  11.])),
> (array([ 2.,  3.]), array([ 12.,  13.]))],
> formats=['(2,)Float64', '(2,)Float64'],
> shape=2,
> names=['a', 'b'])
> 
> So I tried passing in 4-length arrays while specifying the shape 
> explicitly, but it failed. Is this a bug?:
> 
> >>>  arr1 = num.arange(4, type=num.Float64)
> >>>  arr2 = num.arange(4, type=num.Float64) + 10
> >>>  a = rec.array([arr1, arr2], shape=(2,2), names="a, b")
> Traceback (most recent call last):
>    File "<stdin>", line 1, in ?
>    File "/usr/local/lib/python2.3/site-packages/numarray/records.py", 
> line 384, in array
>      byteorder=byteorder, aligned=aligned)
>    File "/usr/local/lib/python2.3/site-packages/numarray/records.py", 
> line 177, in fromarrays
>      raise ValueError, "array has different lengths"
> ValueError: array has different lengths
> 
> Generating a 4-length record and reshaping it does seem to work, 
> though there seems to be a bug in __str__ which I'll report:
> 
> >>>  arr1 = num.arange(4, type=num.Float64)
> >>>  arr2 = num.arange(4, type=num.Float64) + 10
> >>>  a = rec.array([arr1, arr2], names="a, b")
> >>>  a.setshape((2,2))
> >>>  a
> Traceback (most recent call last):
>    File "<stdin>", line 1, in ?
>    File "/usr/local/lib/python2.3/site-packages/numarray/records.py", 
> line 718, in __repr__
>      outlist.append(Record.__str__(i))
> TypeError: unbound method __str__() must be called with Record 
> instance as first argument (got RecArray instance instead)
> >>>  a[1,1]
> <numarray.records.Record instance at 0x15d2ad0>
> >>>  str(a[1,1])
> '(1.0, 11.0)'
> 
> I see the str problem again if I don't specify any buffer:
> 
> >>>  import numarray as num
> >>>  import numarray.records as rec
> >>>  a = rec.array(formats="Float64,Float64", names="a,b", shape=(2,2))
> >>>  a
> Traceback (most recent call last):
>    File "<stdin>", line 1, in ?
>    File "/usr/local/lib/python2.3/site-packages/numarray/records.py", 
> line 718, in __repr__
>      outlist.append(Record.__str__(i))
> TypeError: unbound method __str__() must be called with Record 
> instance as first argument (got RecArray instance instead)
> 
> So...comments? Should I report the shape issues as a bug?
> 
> -- Russell

First of all, the records module was developed mainly having the 1-D table in 
mind.  Even though it can have higher than one dimension, it is not thoroughly 
tested, as you have found out.  However, I'd argue that in many cases that the 
need to use a 2-D (or high) table can be substituted by having an array in each 
cell(element).  In your example, instead of creating a 2x2 table with each cell 
just having one number, you may be able to use a table with just one row and 
each cell is a 2x2 array.  You can create such a record like this:

--> arr1 = num.arange(4, shape=(1,2,2), type=num.Float64)
--> arr2 = num.arange(4, shape=(1,2,2), type=num.Float64)+10
--> a = rec.array([arr1, arr2], names="a,b")

I'd be interested in your application as to why a 2x2 table is necessary.

The __str__ method in RecArray is rather primitive but usually works for 1-D 
tables.  Eventually, we'll need a C-implementation of this method for both speed 
and flexibility.

When you use a list for the buffer in the array function, it is using a 
relatively intuitive (simplistic?) approach.  It tries to figure out the (one 
dimensional) length of each item in the list and use that as the record shape.

JC Hsu





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