[GiNaC-devel] [PATCH] match: don't modify subexpression list if expression doesn't match the pattern.

[Alexei Sheplyakov]

As of now the match() method modifies the list of matched subexpressions
(its second argument) even if the expression in question does not match
the pattern. Thus, this simple program

 #include <iostream>
 #include <ginac/ginac.h>
using namespace GiNaC;

int main(int argc, char** argv)
{
	symbol x;
	ex e = pow(x, 5);
	ex pattern = pow(wild(), -1);
	lst repl;
	bool test = e.match(pattern, repl);
	std::cout << "repl = " << repl << std::endl;
}

prints

repl = {x}

Such behaviour is a bit unexpected. Sometimes it confuses even GiNaC
developers, see e.g.
http://www.ginac.de/pipermail/ginac-devel/2006-April/000942.html

Hence this patch. Now the above program prints

repl = {}

as expected.
---
 doc/tutorial/ginac.texi |    5 +----
 ginac/basic.cpp         |    8 +++++++-
 2 files changed, 8 insertions(+), 5 deletions(-)

diff --git a/doc/tutorial/ginac.texi b/doc/tutorial/ginac.texi
index 555b2d6..05fdca4 100644
--- a/doc/tutorial/ginac.texi
+++ b/doc/tutorial/ginac.texi
@@ -4420,10 +4420,7 @@ This function returns @code{true} when the expression matches the pattern
 and @code{false} if it doesn't. If used in the second form, the actual
 subexpressions matched by the wildcards get returned in the @code{repls}
 object as a list of relations of the form @samp{wildcard == expression}.
-If @code{match()} returns false, the state of @code{repls} is undefined.
-For reproducible results, the list should be empty when passed to
- at code{match()}, but it is also possible to find similarities in multiple
-expressions by passing in the result of a previous match.
+If @code{match()} returns false,  @code{repls} remains unmodified.
 
 The matching algorithm works as follows:
 
diff --git a/ginac/basic.cpp b/ginac/basic.cpp
index a3de04a..7a0633d 100644
--- a/ginac/basic.cpp
+++ b/ginac/basic.cpp
@@ -585,12 +585,18 @@ bool basic::match(const ex & pattern, lst & repl_lst) const
 		if (!match_same_type(ex_to<basic>(pattern)))
 			return false;
 
+		// Even if the expression does not match the pattern, some of
+		// its subexpressions could match it. For example, x^5*y^(-1)
+		// does not match the pattern $0^5, but its subexpression x^5
+		// does. So, save repl_lst in order to not add bogus entries.
+		lst tmp_repl = repl_lst;
 		// Otherwise the subexpressions must match one-to-one
 		for (size_t i=0; i<nops(); i++)
-			if (!op(i).match(pattern.op(i), repl_lst))
+			if (!op(i).match(pattern.op(i), tmp_repl))
 				return false;
 
 		// Looks similar enough, match found
+		repl_lst = tmp_repl;
 		return true;
 	}
 }
-- 
1.5.6


-- 
All science is either physics or stamp collecting.

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