[newbie] from right to left
- To: mathgroup at smc.vnet.net
- Subject: [mg48413] [newbie] from right to left
- From: János <janos.lobb at yale.edu>
- Date: Fri, 28 May 2004 00:51:00 -0400 (EDT)
- Sender: owner-wri-mathgroup at wolfram.com
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
-------------------------------------------------
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