[SciPy-user] [Sparse matrix library] csr_matrix and

Dinesh B Vadhia dineshbvadhia@hotmail....
Mon Apr 28 10:57:55 CDT 2008


Okay, I see what is going on.  Uhmmm!  If I replace int8 with int16 or int32 then this will increase the size of A in memory which will defeat the purpose of the original exercise to minimize the size of A (as they are very large).  Unless there is a way to coerce the calculation A.sum(0) into a int16 then looks like I'll have to evaluate A.sum(0) in brute force fashion at the time of A's creation, save it and read it into the program at execution.

Dinesh


--------------------------------------------------------------------------------
Message: 2
Date: Mon, 28 Apr 2008 00:29:29 -0500
From: "Nathan Bell" <wnbell@gmail.com>
Subject: Re: [SciPy-user] [Sparse matrix library] csr_matrix and
column sum
To: "SciPy Users List" <scipy-user@scipy.org>
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<d05265cb0804272229q5ca927b4l7d70ff338122a230@mail.gmail.com>
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On Sun, Apr 27, 2008 at 11:57 PM, Dinesh B Vadhia
<dineshbvadhia@hotmail.com> wrote:
>
> 0 , -44 , 84 , -116 , -121 , -43 , -44 , -116 , -115 , -79 , 70 , -86 , 39 ,
> -17 , -21 , -112 , 29 , -126 , -19 , 33 , 59 , -6 , 24 , 18 , 57
>
>
> 768 , 724 , 1108 , 652 , 1927 , 2005 , 724 , 908 , 1421 , 1457 , 2118 , 1450
> , 1575 , 3055 , 2283 , 656 , 1053 , 898 , 1517 , 1569 , 1339 , 762 , 3096 ,
> 530 , 1081
>

This is due to the fact that when integer arithmetic overflows (e.g. A
+ B is too large) the result "wraps around".  The solution is to use a
data type with a greater range of values (more bits).

Replace your int8 data array with an int16 array and you will get the
expected results (albeit using one more byte per nonzero) provided
that the sums do not exceed 2^15 - 1.

To be safe, you might use int32 and not worry about ranges as much.

-- 
Nathan Bell wnbell@gmail.com
http://graphics.cs.uiuc.edu/~wnbell/


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