Re: Do we need a When function?

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• Subject: [mg132411] Re: Do we need a When function?
• From: Bob Hanlon <hanlonr357 at gmail.com>
• Date: Mon, 10 Mar 2014 04:40:13 -0400 (EDT)
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• References: <20140308074231.0A4506A0B@smc.vnet.net>

```You can use PatternTest ( p?test ) or Condition ( patt /; test )

Clear[f1, f2, f3, f4]

f1[x_?NumericQ] = "success";

f1 /@ {x, 1, Pi, Infinity}

{f1[x], "success", "success", f1[=E2=88=9E]}

f2[x_?(NumericQ[#] &)] = "success";

f2 /@ {x, 1, Pi, Infinity}

{f2[x], "success", "success", f2[=E2=88=9E]}

f3[x_ /; NumericQ[x]] = "success";

f3 /@ {x, 1, Pi, Infinity}

{f3[x], "success", "success", f3[=E2=88=9E]}

f4[x_] := "success" /; NumericQ[x];

f4 /@ {x, 1, Pi, Infinity}

{f4[x], "success", "success", f4[=E2=88=9E]}

The tests or conditions can be any expression that evaluates to True or
False.

Bob Hanlon

On Sat, Mar 8, 2014 at 2:42 AM, David Bailey <dave at removedbailey.co.uk>wrote:

> Dear All,
>
> Recently I wanted to create an expression that only evaluated when x was
> a number. Obviously, one way to do that is to write something like:
>
>   If[x<1000000000000,f[x]]
>
> This will stay unevaluated until x is assigned to a number - as required
> - but it is very ugly because it makes it hard to understand what is
> going on.
>
> More generally, it would be nice to write an expression that will only
> evaluate when an expression is true. I solved that by writing a function
> When:
>
> SetAttributes[When, HoldAll];
> When[cond_, val_] := val /; cond
>
> However, my point is that this construction is sufficiently useful that
> it should be built in to Mathematica, and my solution might not be
> obvious to all users.
>
> Am I missing a simpler solution?
>
> Do others agree that this is an omission in the language?
>
> David Bailey
> http://www.dbaileyconsultancy.co.uk
>

```

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