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Re: Pattern problem: How to count from a long list of numbers all occurrences of 2 numbers next to each others?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg87411] Re: [mg87393] Pattern problem: How to count from a long list of numbers all occurrences of 2 numbers next to each others?
  • From: "W Craig Carter" <ccarter at mit.edu>
  • Date: Thu, 10 Apr 2008 02:10:41 -0400 (EDT)
  • References: <200804090956.FAA25006@smc.vnet.net>

Hello Nasser,
This is not the most elegant solution, but it works:

Count[Thread[List[x, RotateLeft[x]]], {1, 3}]

which would be convenient to write as a function

CountMySeq[x_List, a_Integer, b_Integer] := Count[Thread[List[x,
RotateLeft[x]]], {a,b}]

To see what is going on here:
x = {1, 3, 3, 3, 2, 3, 3, 1, 3, 3}

RotateLeft[x] is {3, 3, 3, 2, 3, 3, 1, 3, 3, 1}

Thread[List[x,RotateLeft[x]]] is {{1,3}, {3,3}, etc]


On Wed, Apr 9, 2008 at 5:56 AM, Nasser Abbasi <nma at 12000.org> wrote:

> Hello;
>
> I think using Pattern is my weakest point in Mathematica.
>
> I have this list, say this: (it is all a list of integers, no real
> numbers).
>
> x = {1, 3, 3, 3, 2, 3, 3, 1, 3, 3}
>
> And I want to count how many say a 3 followed immediately by 3. So in the
> above list, there will be 4 such occurrences. And if I want to count how
> many 1 followed by a 3, there will be 2 such cases, etc...
>
> I tried Count[] but I do not know how to set the pattern for "3 followed
> by
> a comma followed by 3" or just "3 followed immediately by 3".
>
> I tried few things, such as the following
>
> In[68]:= Count[x, {3, 3}_]
> Out[68]= 0
>
> Also tried Cases, but again, I am not to good with Patterns, so not sure
> how
> to set this up at this moment.
>
> Any ideas will be appreciated.
>
> Nasser
> I really need to sit down and study Patterns in Mathematica really well
> one
> day :)
>
>
>


-- 
W. Craig Carter


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