Re: HoldAll/HoldRest
- To: mathgroup at smc.vnet.net
- Subject: [mg118519] Re: HoldAll/HoldRest
- From: Heike Gramberg <heike.gramberg at gmail.com>
- Date: Sun, 1 May 2011 06:20:25 -0400 (EDT)
When you evaluate valueQ[x], x gets evaluated before it's plugged into the function definition, so you're effectively evaluating If[ValueQ[5],5,False] which returns False. To get the right behaviour, you could use Unevaluated, e.g. valueQ[Unevaluated[x]] You could also add the Attribute HoldAll to valueQ SetAttributes[valueQ,HoldAll]; valueQ[x] out: 5 Heike. On 29 Apr 2011, at 12:33, Scot T. Martin wrote: > Some kind of issue going on here between the interactions of HoldAll of ValueQ[...] and HoldRest of If[...]. Anyone know the solution so that Out[3] results in True? > > > In[1]:= valueQ = If[ValueQ[#], #, False] & > > > > Out[1]= If[ValueQ[#1], #1, False] & > > > > In[2]:= x = 5 > > > > Out[2]= 5 > > > > In[3]:= valueQ[5] > > > > Out[3]= False