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MathGroup Archive 2010

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Checking variables in an expression

  • To: mathgroup at smc.vnet.net
  • Subject: [mg112461] Checking variables in an expression
  • From: Ignacio Plazeta <Ignacio.Plazeta at speednet.es>
  • Date: Thu, 16 Sep 2010 06:00:15 -0400 (EDT)

Dear Friends

I need a function myCheck[] returning the list of variable in expr not 
listed in vars.

Es.

--------------------------------------------------

expr = a + Cos[b] + c[t] - c'[t] + d ^ p - c;
vars = {a , b , c, c[t] , c'[t] , d , p};
myCheck[ expr , vars ]
{}

because only variables in vars are in expr
--------------------------------------------------

expr = a + Cos[b] + c[t] - c'[t] + d ^ p - c + w + w[t];
vars = {a , b , c, c[t] , c'[t] , d , p};
myCheck[ expr , vars ]
{w , w[t]}

because w and w[t] are in expr but not in vars
--------------------------------------------------

I have tryed the childish:

Remove[a, b, c, d, e, f, g, expr, vars]
expr = a + b + c + d + e + f + g;
vars = {a, b, c, d, e};
myRules = (Rule[#, ""]) & /@ vars
res = expr //. myRules;

but it dosn't work really fine because I don't know how to remove
integers, Rational[""/""] etc. from res

Any hints is greatly appreciated.

Warmest regards.
Ignacio


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