Groups > comp.soft-sys.math.mathematica > #16639

Re: Do we need a When function?

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From	David Bailey <dave@removedbailey.co.uk>
Newsgroups	comp.soft-sys.math.mathematica
Subject	Re: Do we need a When function?
Date	Tue, 11 Mar 2014 07:17:09 +0000 (UTC)
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On 10/03/2014 08:38, Szabolcs Horvát wrote:
> On 2014-3-8, 2:42 , David Bailey 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?
>>
>
> I see where you are coming from with this.
>
> I think this comes up most often in cases like this:
>
> http://support.wolfram.com/kb/3820
>
> When[] would make it unnecessary to define a separate function.
>
> However one might argue that in situations like this it is better
> practice to define that function anyway, if not for anything else then
> just for structuring the code better (which could avoid errors).
>
> Do you see other common use cases for it?
>
>
Well my particular use was to stop a ToString expression evaluating 
before the argument to ToString is a number.

There are obviously ways to solve this problem, just as there are ways 
to substitute for many Mathematica functions - such as Riffle - but it 
seems like the sort of function that could make code clearer to 
understand - it surprised me that it doesn't exist!

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

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Thread

Do we need a When function? David Bailey <dave@removedbailey.co.uk> - 2014-03-08 07:42 +0000
  Re: Do we need a When function? Szabolcs Horvát <szhorvat@gmail.com> - 2014-03-10 08:38 +0000
    Re: Do we need a When function? David Bailey <dave@removedbailey.co.uk> - 2014-03-11 07:17 +0000
  Re: Do we need a When function? Helen Read <readhpr@gmail.com> - 2014-03-10 08:41 +0000
  Re: Do we need a When function? Szabolcs Horvát <szhorvat@gmail.com> - 2014-04-11 06:09 +0000
    Re: Do we need a When function? David Bailey <dave@removedbailey.co.uk> - 2014-04-13 09:26 +0000

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