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Re: Scalars Instead of Lists with One Element
*To*: mathgroup at smc.vnet.net
*Subject*: [mg83565] Re: Scalars Instead of Lists with One Element
*From*: Szabolcs Horvát <szhorvat at gmail.com>
*Date*: Fri, 23 Nov 2007 05:29:10 -0500 (EST)
*References*: <fhu75c$72j$1@smc.vnet.net> <4742F294.5050606@gmail.com> <fi3jfv$b34$1@smc.vnet.net>
Gregory Lypny wrote:
> Thanks everyone for your insights,
>
> I've found the problem. Say you want to sum a list whose elements are
> 13 and 9. Mathematica will return a list with one element, {22},
> rather than 22 if the original list is specified as a 2x1 column vector.
>
> x = {{13}, {9}}; y = Total@x >>> returns {22}
>
> This also happens if you write x as I have above, or you use
> Mathematica's Insert menu to create a more visually appealing column
> vector,
Tip: Use CTRL+Enter and CTRL+, instead of the insert menu.
> but it does not happen if you define x as a 2x1 array using
> the Array command.
Yes, it does:
In[1]:= Array[# &, {2, 1}]
Out[1]= {{1}, {2}}
In[2]:= Total[%]
Out[2]= {3}
>
> I think it will happen with any matrix calculation whose result should
> otherwise be a scalar. If we now let y be the row vector {1, 1} then
>
> y.x >>> returns {22}
>
This is because Mathematica works with tensors of arbitrary dimensions,
not just matrices or vectors. So in Mathematica it does not make sense
to speak about column or row vectors---all vectors (1D arrays) are
treated in the same way. {{1},{2}} is a 2 by 1 matrix and {{1, 2}} is a
1 by 2 matrix, not vectors. (Of course all this is just a question of
naming conventions. I am just trying to explain why Mathematica works
this way.)
> The upshot of this is that any table that is created from calculations
> that make use of column vectors or matrix math will likely have a
> depth greater than 3, and you won't be able to cut and paste directly
> into a word processor or spreadsheet.
When you multiply a vector with a matrix, you get a vector. If you
multiply a vector with a vector, you get a scalar:
In[4]:= {x, y}.{{a, b}, {c, d}}
Out[4]= {a x + c y, b x + d y}
In[5]:= {{a, b}, {c, d}}.{x, y}
Out[5]= {a x + b y, c x + d y}
In[6]:= {x, y}.{a, b}
Out[6]= a x + b y
If the distinction between column and row vectors is important, just
work with matrices, and you'll always get matrices as the result (no
scalars):
In[7]:= {{x, y}}.{{a}, {b}}
Out[7]= {{a x + b y}}
In[8]:= {{a}, {b}}.{{x, y}}
Out[8]= {{a x, a y}, {b x, b y}}
If it is not important to differentiate between row and column vectors,
then use 1D arrays to represent vectors. The Outer product can be
calculated like this:
In[9]:= Outer[Times, {a, b}, {x, y}]
Out[9]= {{a x, a y}, {b x, b y}}
>
> I'm going to have a look at some of the work-arounds that have been
> suggested in this thread and my related thread "Copy and Pasting
> Tables into Spreadsheet".
>
>
> Gregory
>
--
Szabolcs
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