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 <ginac/ginac.h>
#include <iostream>
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.
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
-@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:
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;
}
}