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MathGroup Archive 2004

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Re: [newbie] from right to left

  • To: mathgroup at
  • Subject: [mg48424] Re: [newbie] from right to left
  • From: Mark Fisher <mark at>
  • Date: Sat, 29 May 2004 03:06:45 -0400 (EDT)
  • References: <c96i2u$ika$>
  • Sender: owner-wri-mathgroup at

Here's one way:

str = StringJoin @@ (ToString /@ {a, b,
     a, j, a, n, o, s, c, a, d, a, b, a, j, a, n, o, s, c, a, d})

First /@ (StringPosition @@ {str, "janos"})

First /@ ((StringLength[str] + 1) - (
           StringPosition @@ (StringReverse /@ {str, "janos"})))


János wrote:

> Hi,
> Let's say I have a list of letter like symbols:
> lst = {a, b, a, j, a, n, o, s, c, a, d,a, b, a, j, a, n, o, s, c, a, d}
> and have a pattern
> pat = {j, a, n, o, s}
> If I want to match by going from left to right
> Position[Partition[lst, Length[pat], 1], pat]
> gives me positions {{4}, {15}}.  So far so good.  Now the genie came 
> out from the bottle and reversed the list, so now list reads like:
> revlist = Reverse[lst]
> revlist = {d, a, c, s, o, n, a, j, a, b, a, d, a, c, s, o, n, a, j, a, 
> b, a}
> and the genie say that now I have to read it from right to left and 
> find the pattern that way.  I could now reverse the pattern
> revpat = Reverse[pat]
> revpat = {s, o, n, a, j}
> and match it up to the reversed list like:
> Position[Partition[revlst, Length[revpat], 1], revpat]
> {{4}, {15}}
> again, but that would not do me any good, because what I want is to 
> match it from right to left, so the position should be:
> {{8},{19}}
> that is, where "j" is found in revlist by reading from right to left.  
> Adding Length[pat] to the undesired result of course will give me the 
> "good" positions, but that looks somewhat "artificial".
> I can program it out procedurally, but that is not "nice" and slow.
> So, in summary, at odd occasions I read lst in western style and I can 
> find the pattern easily, but at even occasions, when my eyes are 
> reading the reversed lst from right to left, I have read it in arabic 
> style and I want to find the pattern that way too.
> Any good hint ?
> Thanks ahead
> János
> -------------------------------------------------
> clear perl code is better than unclear awk code; but NOTHING comes 
> close to unclear perl code

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