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RE: Can Mathematica evaluate step by step???

  • To: mathgroup at
  • Subject: [mg29387] RE: [mg29372] Can Mathematica evaluate step by step???
  • From: "David Park" <djmp at>
  • Date: Sat, 16 Jun 2001 02:47:58 -0400 (EDT)
  • Sender: owner-wri-mathgroup at


It would be interesting to know what your larger objective is. A different
Mathematica approach is probably the answer to your problem.

Once you set a value for a, you have to use rather round about methods to
keep it from being evaluated. So my first suggestion would be: don't set a
to a value if you intend to later use it as a variable. You could do
something like this (although I doubt that this is the answer you are
looking for)

a = 4;
myfunction[a_] := 2a

HoldForm @@ {myfunction[b]}
% /. b :> a
2 b
2 a

But what use would that result be for you, since it is in a HoldForm? So I
think that you must avoid setting values for anything you want to use as a
symbolic variable. It is often better to use rules, a -> 4, to substitute
values for variables, or myfunction[4] to evaluate at a specific value. In
my work I follow my own rule of never setting a single character symbol
(a,b,x,y, etc.) to a value. The chances of getting tripped up by it are just
too high.

David Park
djmp at

> From: Neri Kafkafi [mailto:NKafkafi at]
To: mathgroup at
> Assume I define:
> myfunction:=2a
> and at some later time:
> a=4
> Now if I evaluate myfunction I get 8.
> If I evaluate Hold[myfunction] I get myfunction.
> My question is, how do I get 2a ?
> In other words, I want to get the symbolic definition of
> myfunction, whether
> a is defined or not.
> In (yet) other words, I want to evaluate myfunction only one step further,
> up to 2a, but not all the way to 8.
> Is this possible in Mathematica?
> Thanks in advance,
> Neri

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