Singular Manual: rvar

  | ### 5.1.137 rvar `Syntax:` `rvar (` name `)` `rvar (` poly_expression `)` `rvar (` string_expression `)` `Type:` int `Purpose:` returns the number of the variable if the name/polynomial is a ring variable of the basering or if the string is the name of a ring variable of the basering; returns 0 if not. Hence the return value of `rvar` can also be used in a boolean context to check whether the variable exists. `Example:`
        ring r=29,(x,y,z),lp;
        rvar(x);
      ==> 1
        rvar(r);
      ==> 0
        rvar(y);
      ==> 2
        rvar(var(3));
      ==> 3
        rvar("x");
      ==> 1
See defined; ring; var; varstr.

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