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MathGroup Archive 2007

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Re: Dot or Inner ... but not quite

  • To: mathgroup at smc.vnet.net
  • Subject: [mg79697] Re: Dot or Inner ... but not quite
  • From: Jean-Marc Gulliet <jeanmarc.gulliet at gmail.com>
  • Date: Thu, 2 Aug 2007 03:47:11 -0400 (EDT)
  • Organization: The Open University, Milton Keynes, UK
  • References: <f8n2rc$lp9$1@smc.vnet.net>

Diamond, Mark wrote:
> I have two equi-length lists, the first of number, the second of 
> (equi-length) lists of numbers , such as
> 
> l1={1,4,3};
> l2={{1,7},{1,9},{5,2}};
> 
> I want to produce 1*{1,7}+4*{1,9}+3*{5,2}
> 
> It looks so close to Inner that I thought I could use it in some form, but I 
> have ended up using
> 
> (#[[1]]*#[[2]])& /@ Transpose[{l1,l2}]
> 
> Is there a better way using one of the builtin functions with which I am 
> unfamiliar?
> 
> Cheers,
> 
> Mark Diamond 

Note that you can use *Inner*. You have to transpose the result, however.

In[1]:= l1 = {1, 4, 3};
l2 = {{1, 7}, {1, 9}, {5, 2}};

In[3]:= Transpose[Inner[Times, l1, l2, List]]

Out[3]= {{1, 7}, {4, 36}, {15, 6}}

In[4]:= Thread[l1*l2, 2]

Out[4]= {{1, 7}, {4, 36}, {15, 6}}

In[5]:= MapThread[Times, {l1, l2}]

Out[5]= {{1, 7}, {4, 36}, {15, 6}}

In[6]:= Apply[Times, Partition[Riffle[l1, l2], 2], {1}]

Out[6]= {{1, 7}, {4, 36}, {15, 6}}

Regards,
Jean-Marc


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