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From: Rui on 12 Jun 2010 05:32
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

Thanks guys

From: Bill Rowe on 13 Jun 2010 04:08
On 6/12/10 at 5:32 AM, rui.rojo(a)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


From: Carl K. Woll on 13 Jun 2010 04:09
On 6/12/2010 5:32 AM, 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
>
> Thanks guys
>
>

With:

list = {{a, 5}, {b,6}, {c,7}};

You can do:

list[[All,2]] /= 2

and list will have all second elements cut in half.

In[114]:= list

Out[114]= {{a, 5/2}, {b, 3}, {c, 7/2}}

Carl Woll
Wolfram Research

From: David Park on 13 Jun 2010 04:07
Here is one method:

mat = Transpose[{{a, b, c, d}, {5, 6, 7, 8}}]
{{a, 5}, {b, 6}, {c, 7}, {d, 8}}

MapAt[#/2 &, mat, Table[{i, 2}, {i, 4}]]
{{a, 5/2}, {b, 3}, {c, 7/2}, {d, 4}}


David Park
djmpark(a)comcast.net
http://home.comcast.net/~djmpark/


From: Rui [mailto:rui.rojo(a)gmail.com]

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

Thanks guys



From: Sjoerd C. de Vries on 13 Jun 2010 04:09
Is this simple enough:

In[41]:= list =
{{"A", 1}, {"B", 2}, {"C", 3}, {"D", 4}, {"E", 5}, {"F", 6}, {"G",
7}, {"H", 8}, {"I", 9}, {"J", 10}};

In[42]:= list /. {i_, j_} -> {i, j/2}

Out[42]= {{"A", 1/2}, {"B", 1}, {"C", 3/2}, {"D", 2}, {"E", 5/
2}, {"F", 3}, {"G", 7/2}, {"H", 4}, {"I", 9/2}, {"J", 5}}

???

Cheers -- Sjoerd

On Jun 12, 11:32 am, Rui <rui.r...(a)gmail.com> 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
>
> Thanks guys


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