Bill Allombert on Wed, 07 Aug 2013 22:49:17 +0200 |
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Re: Variable checker |
On Wed, Aug 07, 2013 at 04:25:57PM +0200, Dirk Laurie wrote: > I have written a routine that for a given expression checks whether > it contains any variables that are not in a given list. It works. > > Only problem is, I don't understand why. > > { foreign(expr,vars) = > if (type(expr)=="t_RFRAC", > return(foreign(numerator(expr)) || foreign(denominator(expr)))); > my(var=trap(,,variable(expr))); > print(expr", "var); > if(!var, return(0)); \\ expr is scalar > if(subst(vars,var,0)==vars, return(var)); \\ var is not present in vars > foreign(polcoeff(expr,0),vars) } > addhelp(foreign,"foreign(expr,vars): 0 if expr can be written in terms > of vars, otherwise an offending variable") > > ? foreign(x^2+a*x+b,[x,b]) > x^2 + a*x + b, x > b, a > %2 = a > > The value of 'expr' on the recursive call is 'b'. There's no 'a' in > it. Yet variable(expr) returns 'a'. > > I'm grateful it does. But it baffles me. subst(x^2 + a*x + b,x,0) is equal to a^0*b. This is due to variable priority: 0+a*0+b evaluate to a polynomial in "a" because "a" has higher priority than "b" (it was introduced earlier). Cheers, Bill.