Apply a rule to an expression only once
- To: mathgroup at smc.vnet.net
- Subject: [mg116299] Apply a rule to an expression only once
- From: Guido Walter Pettinari <coccoinomane at gmail.com>
- Date: Thu, 10 Feb 2011 05:22:37 -0500 (EST)
Hello group!
Does anybody know how to apply a rule to an expression only once?
In other words, is it possible to tell ReplaceAll to stop after the
first application of a rule?
Simple example:
expr = f a + f b;
rule = f p_ :> f [p];
ReplaceAll [ expr, rule ]
The output, as expected, is:
f [a] + f [b]
However, what I want is something like:
ReplaceAll [ expr, rule, OnlyFirst -> True ]
that gives the output
f [a] + f b.
My simple example can be solved by using ReplacePart, but alas it is
not the kind of solution I am looking for.
Thank you for your consideration!
Best regards,
Guido W. Pettinari
P.S. I found a temporary solution by using a pattern test at the time
of rule definition, i.e.
rule = f p_ /; matchFirst[] :> f [p],
with
matchFirst [] := Module [ { },
If [ TrueQ@$alreadyMatched, False, $alreadyMatched = True; True]
],
but this does not satisfy me since (i) I do not want to alter the rule
definition, and (ii) I have to update the global variable
$alreadyMatched every time I want to use the trick.