Re: List difference using patterns and substitutions.

• To: mathgroup at smc.vnet.net
• Subject: [mg71317] Re: List difference using patterns and substitutions.
• From: bghiggins at ucdavis.edu
• Date: Wed, 15 Nov 2006 06:43:02 -0500 (EST)
• References: <ejc53g\$6qf\$1@smc.vnet.net>

```Here are some ideas , though I am not sure using rules will be the most
efficient

l2 = Table[Random[Integer, {1, 10}], {20000}];

In[52]:=
res1=Partition[l2,2,1]/.{x_Integer,y_Integer}->y-x;//Timing

Out[52]=
{0.168621 Second,Null}

In[53]:=
res2=Drop[RotateLeft[l2]-l2,-1];//Timing

Out[53]=
{0.002958 Second,Null}

In[54]:=
res3=ReplaceList[l2,{___,x_,y_,___}->y-x];//Timing

Out[54]=
{10.5521 Second,Null}

In[50]:=
res3==res2==res1

Out[50]=
True

Cheers,

Brian

Nacho wrote:
> Hello.
>
> I'm trying to figure how can I build a difference list from another
> using only patterns and rule substitutions.
>
> The idea is to get from a list, another, one element shorter, where
> each value is the substraction of two consecutive elements in the
> original list, that is, from {1,2,3,5,5} get {1,1,2,0}.
>
> I've been thinking about it for a while, and I know several methods
> using a more traditional programming style (with For, for example), but
> I have no idea if it is possible to make it simple and fast with rule
> substitutions.
>
> Any idea?
>