       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)
• Delivered-to: l-mathgroup@mail-archive0.wolfram.com
• Delivered-to: l-mathgroup@wolfram.com
• Delivered-to: mathgroup-outx@smc.vnet.net
• Delivered-to: mathgroup-newsendx@smc.vnet.net
• 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:= expr = If[NumericQ[x], ToString[x]]

In:= expr /. x -> 2.1

(* The output here is Null *)

In:= SetAttributes[When, HoldAll];
When[cond_, val_] := val /; cond

In:= expr = When[NumericQ[x], ToString[x]]

Out= When[NumericQ[x], ToString[x]]

In:= expr /. x -> 2.1

Out= "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

```

• Prev by Date: Re: NIntegrate and double integral -- very slow
• Next by Date: Re: What's in an expression?
• Previous by thread: Re: Do we need a When function?
• Next by thread: Re: Do we need a When function?