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Re: Apply a function to parts of a list
*To*: mathgroup at smc.vnet.net
*Subject*: [mg110306] Re: Apply a function to parts of a list
*From*: Bill Rowe <readnews at sbcglobal.net>
*Date*: Sun, 13 Jun 2010 04:08:13 -0400 (EDT)
On 6/12/10 at 5:32 AM, rui.rojo at gmail.com (Rui) wrote:
>I have always done very ugly code when trying to do something
>seemingly simple and basic such as applying a function to subparts
>of a list. Simple example, I want to divide by 2 all the second
>elements of a list
>{{a, 5}, {b, 6}, ...} --> {{a, 5/2}, ...}
>My ideas: Transpose -> MapAt/Map -> Transpose Build the sublist with
>Part and then rebuild it Use Map and a function like #[[All, 2]] and
>then rebuild it.
>I don't like any solution. I wanted to know if there's anything
>simple that I'm overseing
Perhaps pattern matching, i.e.,
In[17]:= {{a, 5}, {b, 6}} /. {x_, y_Integer} -> {x, y/2}
Out[17]= {{a, 5/2}, {b, 3}}
But note
In[18]:= {{a, 5}, {b, 6}} /. {x_, y_} -> {x, y/2}
Out[18]= {{a, 5}, {b/2, 3}}
This last happens since the pattern {x_,y_} matches
{{a,5},{b,6}} which is why I restricted the match in the second
position to integers. But if the list were longer, i.e.,
In[19]:= {{a, 5}, {b, 6}, {c, 7}} /. {x_, y_} -> {x, y/2}
Out[19]= {{a, 5/2}, {b, 3}, {c, 7/2}}
there is no problem with the simpler pattern
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