Re: working with lists

• To: mathgroup at smc.vnet.net
• Subject: [mg113208] Re: working with lists
• From: Ray Koopman <koopman at sfu.ca>
• Date: Tue, 19 Oct 2010 05:53:41 -0400 (EDT)
• References: <i9h596\$128\$1@smc.vnet.net>

```On Oct 18, 2:50 am, Sam Takoy <sam.ta... at yahoo.com> wrote:
> Hi,
>
> I'm not very good at working with lists. May I ask for someone to
> work out an example which has several elements of what I need to do.
>
> What's the best way to write a function f[list] that goes through
> each element of the lest, doubles each element divisible by three
> and reduces each of the following elements by 1. That is
>
> f[{ 1 2 3 5 7}] is { 1 2 6 4 12 }
>
>
> Sam

Here are two (of what I'm sure will be many) examples. The first
requires you to recognize that Decrement has a higher precedence
than Plus. The second avoids adding 'a' twice on each pass and
is a tiny bit faster but is more convoluted, with one function
nested inside another.

In[1]:=
f1[x_] := Block[{a = 0}, If[Mod[#+a,3]==0, 2(#+a--), #+a]& /@ x ];
f2[x_] := Block[{a = 0}, If[Mod[#,3]==0, a-- ; 2#, #]& [#+a]& /@ x ];
f1[{1,2,3,5,7}]
f2[{1,2,3,5,7}]
f1[Range@20]
f2[Range@20]

Out[3]= {1,2,6,4,12}
Out[4]= {1,2,6,4,12}
Out[5]= {1,2,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6}
Out[6]= {1,2,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6}

```

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