The Math Forum

Search All of the Math Forum:

Views expressed in these public forums are not endorsed by NCTM or The Math Forum.

Math Forum » Discussions » Inactive » comp.soft-sys.math.mathematica

Notice: We are no longer accepting new posts, but the forums will continue to be readable.

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

Advanced Search

Back to Topic List Back to Topic List Jump to Tree View Jump to Tree View   Messages: [ Previous | Next ]
David Bailey

Posts: 714
Registered: 11/7/08
Re: Do we need a When function?
Posted: Mar 11, 2014 3:17 AM
  Click to see the message monospaced in plain text Plain Text   Click to reply to this topic Reply

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:
> 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

Point your RSS reader here for a feed of the latest messages in this topic.

[Privacy Policy] [Terms of Use]

© The Math Forum at NCTM 1994-2018. All Rights Reserved.