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MathGroup Archive 2007

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Re: Map function which adds last two numbers of a list

  • To: mathgroup at smc.vnet.net
  • Subject: [mg73567] Re: Map function which adds last two numbers of a list
  • From: Jean-Marc Gulliet <jeanmarc.gulliet at gmail.com>
  • Date: Thu, 22 Feb 2007 04:28:11 -0500 (EST)
  • Organization: The Open University, Milton Keynes, UK
  • References: <ergqrb$jg0$1@smc.vnet.net>

Christopher Pike wrote:
> Hi,
>   Consider this:
> 
> z = {{1,4,5,6},{7,8,9,1},{1,2,4,3}}
> 
> I'd like to Map a function onto z which would replace the last two items
> with their sum:
> 
> {{1,4,11},{7,8,10},{1,2,7}}
> 
> I could easily use the Table command to construct this new table, but it
> would be nicer if I new how to Map some function on z that would produce
> the same result.
> 
> Any suggestions.
> 
> Thanks, Chris Pike
> 

Hi Chris,

You could use a transformation rule as in

In[1]:=
z = {{1, 4, 5, 6}, {7, 8, 9, 1}, {1, 2, 4, 3}};

In[2]:=
z /. {u___, v_, (t_)?NumberQ} -> {u, v + t}

Out[2]=
{{1, 4, 11}, {7, 8, 10}, {1, 2, 7}}

And here is a version with a pure function mapped over the list x

In[3]:=
(Flatten[{Drop[#1, -2], Plus @@ Take[#1, -2]}] & ) /@ z

Out[3]=
{{1, 4, 11}, {7, 8, 10}, {1, 2, 7}}

Regards,
Jean-Marc


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