[Numpy-discussion] question about subtraction and shape

Tony Yu tsyu80@gmail....
Thu Sep 1 16:44:49 CDT 2011


On Thu, Sep 1, 2011 at 5:33 PM, Jonas Wallin <jonas.wallin81@gmail.com>wrote:

> Hello,
>
> I implemented the following line of code:
>
> Gami[~index0].shape ----> (100,)
> sigma.shape             ----> (1,1)
> Gami[~index0]  = Gam[~index0] - sigma**2
>
> I get the error message:
> *** ValueError: array is not broadcastable to correct shape
>
> apparently
> *temp* = Gam[~index0] - sigma**2
> *temp*.shape -->  (1,100)
>
> Which seems strange to me is there any reason why *temp* gets this shape ?
>

Jonas,

Numpy changes the shape of the output array based on certain broadcasting
rules (See http://docs.scipy.org/doc/numpy/user/basics.broadcasting.html).
Broadcasting is really useful when you need it, but it can be confusing when
you don't.

Best,
-Tony
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