The a m (
match] function takes a
vector of formulas and a rewrite-rule-style pattern, and produces
a vector of all formulas which match the pattern. The command
prompts you to enter the pattern; as for a r, you can enter
a single pattern (i.e., a formula with meta-variables), or a
vector of patterns, or a variable which contains patterns, or
you can give a blank response in which case the patterns are taken
from the top of the stack. The pattern set will be compiled once
and saved if it is stored in a variable. If there are several
patterns in the set, vector elements are kept if they match any
of the patterns.
For example, ‘match(a+b, [x, x+y, x-y, 7, x+y+z])’ will return ‘[x+y, x-y, x+y+z]’.
import mechanism is not available for pattern sets.
The a m command can also be used to extract all vector elements which satisfy any condition: The pattern ‘x :: x>0’ will select all the positive vector elements.
With the Inverse flag [
matchnot], this command extracts all
vector elements which do not match the given pattern.
There is also a function ‘matches(x, p)’ which evaluates to 1 if expression x matches pattern p, or to 0 otherwise. This is sometimes useful for including into the conditional clauses of other rewrite rules.
vmatches is just like
that if the match succeeds it returns a vector of assignments to
the meta-variables instead of the number 1. For example,
‘vmatches(f(1,2), f(a,b))’ returns ‘[a := 1, b := 2]’.
If the match fails, the function returns the number 0.