Re: Do we need a When function?
- To: mathgroup at smc.vnet.net
- Subject: [mg132469] Re: Do we need a When function?
- From: David Bailey <dave at removedbailey.co.uk>
- Date: Sun, 23 Mar 2014 05:00:59 -0400 (EDT)
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- References: <20140308074231.0A4506A0B@smc.vnet.net> <lg10pf$50i$1@smc.vnet.net> <20140317062651.1B88E69F4@smc.vnet.net> <lgbk5e$12p$1@smc.vnet.net>
On 19/03/2014 08:23, Itai Seggev wrote: > David, > > Firstly, I would appreciate it if you didn't quote these tiny fragments of my > replies, which suggest I didn't already understand/address points you raise > (or that perhaps you didn't bother to read my email in its entirety). > >> On 15/03/2014 07:51, Itai Seggev wrote: >>> If[NumericQ[x], f[x]] (*are NumberQ, if want an actual number type*) >> >> The difference, which at least Szabolcs appreciates, can be seen in this >> example: >> >> In[3]:= expr = If[NumericQ[x], ToString[x]] >> >> In[4]:= expr /. x -> 2.1 >> >> (* The output here is Null *) >> >> In[7]:= SetAttributes[When, HoldAll]; >> When[cond_, val_] := val /; cond >> >> In[9]:= expr = When[NumericQ[x], ToString[x]] >> >> Out[9]= When[NumericQ[x], ToString[x]] >> >> In[10]:= expr /. x -> 2.1 >> >> Out[10]= "2.1" >> >> The effect of When is to delay its evaluation until something is true - >> which can be quite useful! > > But on the other hand you have these behaviors: > > In[59]:= ClearAll[When] > SetAttributes[When, HoldAllComplete]; > When[cond_, val_] := val /; cond > > In[62]:= expr = When[NumericQ[x], ToString[x]] > > Out[62]= When[NumericQ[x], ToString[x]] > > In[64]:= Block[{x = 2.1}, expr ] > > Out[64]= When[NumericQ[x], ToString[x]] > > In[65]:= x = 2.1; expr > > Out[65]= When[NumericQ[x], ToString[x]] > > Are these useful? > > > They are not useful - but I didn't define When with Attribute HoldAllComplete, but with HoldAll - so I don't see your point! David Bailey http://www.dbaileyconsultancy.co.uk
- References:
- Do we need a When function?
- From: David Bailey <dave@removedbailey.co.uk>
- Re: Do we need a When function?
- From: David Bailey <dave@removedbailey.co.uk>
- Do we need a When function?