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Re: Do we need a When function?

• To: mathgroup at smc.vnet.net
• Subject: [mg132440] Re: Do we need a When function?
• From: David Bailey <dave at removedbailey.co.uk>
• Date: Mon, 17 Mar 2014 02:26:51 -0400 (EDT)
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• References: <20140308074231.0A4506A0B@smc.vnet.net> <lg10pf$50i$1@smc.vnet.net>

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!

David Bailey
http://www.dbaileyconsultancy.co.uk

• Follow-Ups:
  • Re: Do we need a When function?
    • From: Itai Seggev <itais@wolfram.com>

• References:
  • Do we need a When function?
    • From: David Bailey <dave@removedbailey.co.uk>