Re: Pattern Matching in Lists

• To: mathgroup at smc.vnet.net
• Subject: [mg35605] Re: Pattern Matching in Lists
• From: "Carl K. Woll" <carlw at u.washington.edu>
• Date: Mon, 22 Jul 2002 02:11:05 -0400 (EDT)
• References: <001301c23091\$d341ef20\$9b16989e@machine1>
• Sender: owner-wri-mathgroup at wolfram.com

```Allan,

One further small tweak.

s1[w_] := (Tr[#1] - Tr[#1 Drop[w, 1]] & )[Drop[w, -1]]

My tweak is to use BitAnd instead of multiplication:

s2[w_] := (Tr[#1] - Tr[BitAnd[#1, Drop[w, 1]]] & )[Drop[w, -1]]

Testing:

tst = Table[Random[Integer], {10^7}];

In[4]:=
s1[tst]//Timing
s2[tst]//Timing
Out[4]=
{0.641 Second, 2498910}
Out[5]=
{0.5 Second, 2498910}

So, BitAnd appears to be slightly faster.

Carl Woll
Physics Dept
U of Washington

----- Original Message -----
From: "Allan Hayes" <hay at haystack.demon.co.uk>
To: mathgroup at smc.vnet.net
Subject: [mg35605] Re: Pattern Matching in Lists

> Carl,
> Pushing your idea a bit further and avoiding one Drop and a subtraction of
> lists:
>
>     w=Table[Random[Integer],{1000000}];
>
>     Tr[Drop[w,-1](Drop[w,-1]-Drop[w,1])]//Timing
>
>         {7.85 Second,249850}
>
>     (Tr[#]-Tr[# Drop[w,1]])&[Drop[w,-1] ]//Timing
>
>         {2.75 Second,249850}
>
> --
> Allan
>
> ---------------------
> Allan Hayes
> Mathematica Training and Consulting
> Leicester UK
> www.haystack.demon.co.uk
> hay at haystack.demon.co.uk
> Voice: +44 (0)116 271 4198
> Fax: +44 (0)870 164 0565
>
>
> "Carl K. Woll" <carlw at u.washington.edu> wrote in message
> news:ahdfd2\$i3i\$1 at smc.vnet.net...
> > Anthony and newsgroup,
> >
> > I thought of another method of  solving this problem which is 5 or 6
> times
> > faster than my previous version. Here is a test case.
> >
> > tst=Table[Random[Integer],{1000000}];
> >
> > My first solution was
> >
> > In[16]:=
> > Count[Partition[tst,2,1],{1,0}]//Timing
> > Out[16]=
> > {0.704 Second, 249722}
> >
> > My second solution is
> >
> > In[17]:=
> > Tr[Drop[tst,-1](Drop[tst,-1]-Drop[tst,1])]//Timing
> > Out[17]=
> > {0.125 Second, 249722}
> >
> > Carl Woll
> > Physics Dept
> > U of Washington
> >
> > "Anthony Mendes" <amendes at zeno.ucsd.edu> wrote in message
> > news:ah5qce\$59o\$1 at smc.vnet.net...
> > >Hello,
> > >
> > > Suppose w={1,1,1,0,0,1,0,1,0,0,1,0,0}.
> > >
> > > How can I count the number of occurrences of a 1 in w immediately
> > > followed by a 0 in w?
> > >
> > > I have tried every incarnation of Count[] I can think of; for example,
> > >
> > > Count[w,{___,1,0,___}]
> > >
> > > does not seem to work.  In general, how can I count the number of
> > > occurrences of a 1 followed by a 0 in a list of 1's and 0's?  Thank
you!
> > >
> > >
> > > --
> > > Tony
> > > _____________________
> > > amendes at math.ucsd.edu
> > >
> > >
> > >
> > >
> >
> >
> >
>
>
>

```

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