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Re: HoldForm in defined function doesn't seem to work

• To: mathgroup at smc.vnet.net
• Subject: [mg99215] Re: [mg99182] HoldForm in defined function doesn't seem to work
• From: Leonid Shifrin <lshifr at gmail.com>
• Date: Thu, 30 Apr 2009 06:21:23 -0400 (EDT)
• References: <200904291037.GAA26696@smc.vnet.net>

When you define your f, you are missing the HoldAll attribute. The
argument then gets evaluated before being passed to f. Here is what
is needed:

In[1] =
SetAttributes[f,HoldAll];
f[x_]:=HoldForm[x];

In[2] = HoldForm[123 456 789]

Out[2] =123 x 456 x 789

You could also use Ownvalues, in which case this problem is avoided
(g serves as a "function pointer"  to HoldForm):

In[3] = g = HoldForm;

In[4] = g[123 456 789]

Out[4] = 123 x 456 x 789


Regards,
Leonid


On Wed, Apr 29, 2009 at 3:37 AM, <joel.aufgang at gmail.com> wrote:

> I'm struggling to understand why this doesn't work:
>
>
> f[x_] = HoldForm[x];
> HoldForm[123 456 789] (* returns "123 x 456 x 789" *)
> f[123 456 789] (* returns"44253432" *)
>
> I don't understand why the two results are different.   Is there a
> special trick I need to do to be able to define f so that the HoldForm
> works?
>
> I'm using this to help visualize the way a notebook represents typed
> expressions, so I'm trying to do something like this:
>
> BoxTree[expr_] := TreeForm[ToBoxes[HoldForm[expr]][[1]]];
> a b c d e // BoxTree
> 123 456 789 // BoxTree (* want this to show as a RowBox like the
> previous example *)
>
>
> Any help would be greatly appreciated.  Thanks.
>
>

• References:
  • HoldForm in defined function doesn't seem to work
    • From: joel.aufgang@gmail.com