Re: HoldForm in defined function doesn't seem to work
- To: mathgroup at smc.vnet.net
- Subject: [mg99224] Re: HoldForm in defined function doesn't seem to work
- From: David Bailey <dave at removedbailey.co.uk>
- Date: Thu, 30 Apr 2009 06:23:00 -0400 (EDT)
- References: <gt9aki$q22$1@smc.vnet.net>
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. > You need to define f to have the HoldAll attribute - otherwise its argument is evaluated long before it becomes the argument to HoldForm! SetAttributes[f,HoldAll]; If you are interested in working with held expressions, it is worth reading around the subject, and experimenting a bit. Also read up on Evaluate and Unevaluated. If you want BoxTree to display the unevaluated expression, it too should be given the HoldAll (or possibly HoldFirst) attribute. David Bailey http://www.dbaileyconsultancy.co.uk