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Topic: Do we need a When function?
Replies: 5   Last Post: Apr 13, 2014 5:26 AM

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David Bailey

Posts: 714
Registered: 11/7/08
Re: Do we need a When function?
Posted: Mar 11, 2014 3:17 AM
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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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