Re: Clear variables in a list

*To*: mathgroup at smc.vnet.net*Subject*: [mg112071] Re: Clear variables in a list*From*: Leonid Shifrin <lshifr at gmail.com>*Date*: Sun, 29 Aug 2010 02:49:02 -0400 (EDT)

Chris, nothing will help if your variable list is defined as in your example (with Set, and after the variables got their definitions), since it contains already evaluated values: ?vars Global`vars vars={1,2,3} If you defined it with SetDelayed (or used Set, but before you assigned values to your variables): vars := {a, b, c} ?vars Global`vars vars:={a,b,c} Then this is possible, although I would not say it is trivial: ReleaseHold[Clear @@@ (Hold[vars] /. OwnValues[vars])] If the only purpose of your <vars> variable is to be used to clear the variables, then I suggest that you use Hold instead of List, as a container head: a = 1; b = 2; c = 3; heldvars = Hold[a, b, c]; Then clearing the variables becomes trivial, and all of the above problems are avoided: Clear @@ heldvars This is what I often use for similar purposes. As a side remark, the idiom f[Unevaluated[#]]& is flawed, because the argument gets evaluated during pure-function parameter passing stage, long before it is substituted into the body of a pure function. This is one place where the current documentation is wrong: f is *not* equivalent to f[#]&. Simple counter-example: In[17]:= Hold[Print["*"]] Out[17]= Hold[Print["*"]] In[16]:= Hold[#] &[Print["*"]] During evaluation of In[16]:= * Out[16]= Hold[Null] Returning to f[Unevaluated[#]]&: it leaks evaluation in the same way: In[13]:= ClearAll[f]; f[Unevaluated[#]] &[Print["*"]] During evaluation of In[13]:= * Out[14]= f[Unevaluated[Null]] This is what should be used instead, to achieve the desired effect: In[15]:= Function[x, f[Unevaluated[x]], HoldAll][Print["*"]] Out[15]= f[Unevaluated[Print["*"]]] Hope this helps. Regards, Leonid On Sat, Aug 28, 2010 at 3:01 PM, Chris Degnen <degnen at cwgsy.net> wrote: > Any suggestions how to clear variables in a list, eg: > > a = 1; b = 2; c = 3; vars = {a, b, c}; > > None of these work: > > Map[Clear, vars] > Map[Clear[Unevaluated[#]] &, vars] > Apply[Clear, vars, {1}] > > Only this works: Clear[a, b, c] > > >