info ginac

5.4.2 Matching parts of expressions

A more general way to look for patterns in expressions is provided by the member function

bool ex::has(const ex & pattern);

This function checks whether a pattern is matched by an expression itself or by any of its subexpressions.

Again some examples in ginsh for illustration (in ginsh, has() returns ‘1’ for true and ‘0’ for false):

> has(x*sin(x+y+2*a),y);
1
> has(x*sin(x+y+2*a),x+y);
0
  (This is because in GiNaC, "x+y" is not a subexpression of "x+y+2*a" (which
   has the subexpressions "x", "y" and "2*a".)
> has(x*sin(x+y+2*a),x+y+$1);
1
  (But this is possible.)
> has(x*sin(2*(x+y)+2*a),x+y);
0
  (This fails because "2*(x+y)" automatically gets converted to "2*x+2*y" of
   which "x+y" is not a subexpression.)
> has(x+1,x^$1);
0
  (Although x^1==x and x^0==1, neither "x" nor "1" are actually of the form
   "x^something".)
> has(4*x^2-x+3,$1*x);
1
> has(4*x^2+x+3,$1*x);
0
  (Another possible pitfall. The first expression matches because the term
   "-x" has the form "(-1)*x" in GiNaC. To check whether a polynomial
   contains a linear term you should use the coeff() function instead.)

The method

bool ex::find(const ex & pattern, lst & found);

works a bit like has() but it doesn't stop upon finding the first match. Instead, it appends all found matches to the specified list. If there are multiple occurrences of the same expression, it is entered only once to the list. find() returns false if no matches were found (in ginsh, it returns an empty list):

> find(1+x+x^2+x^3,x);
{x}
> find(1+x+x^2+x^3,y);
{}
> find(1+x+x^2+x^3,x^$1);
{x^3,x^2}
  (Note the absence of "x".)
> expand((sin(x)+sin(y))*(a+b));
sin(y)*a+sin(x)*b+sin(x)*a+sin(y)*b
> find(%,sin($1));
{sin(y),sin(x)}