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Re: Do we need a When function?
Posted:
Mar 11, 2014 3:17 AM


On 10/03/2014 08:38, Szabolcs Horvát wrote: > On 201438, 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



