[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 ?