Re: Do we need a When function?
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- Subject: [mg132554] Re: Do we need a When function?
- From: Szabolcs HorvÃt <szhorvat at gmail.com>
- Date: Fri, 11 Apr 2014 02:09:49 -0400 (EDT)
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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? > Putting aside the question whether this should be a part of the core language, I got quite convinced that this is a pretty useful function. I am saying this one month after David's original post, so this opinion is based on some practical experience. During this month I found myself remembering and using this function on a number of occasions. (Not very often, but it does keep coming back.) Admittedly, in all cases I used it with NumericQ, so I might as well have hard-coded NumericQ into it ... SetAttributes[whenNumeric, HoldRest] whenNumeric[x_?NumericQ, val_] := val My point is that it seems that this is going to stay in my personal toolbox permanently, it is indeed a useful function. To show an example, most recently I used it while experimenting with this piecewise function: http://mathematica.stackexchange.com/q/45763/12 NIntegrate[when[NumericQ[x], First@test[x]], {x, -2, 2}] test[] is a function that returns a vector here. Its implementation details are irrelevant for my argument. Using When here was much easier than defining a separate function only to allow NIntegrate to be used on the first component of the vector. This When[] function doesn't allow me to do anything I couldn't do without it. I wouldn't miss it at all when writing a package. But it is very useful for quick-and-dirty or "improvisational" programming, i.e. what I do 90% of the time when I use Mathematica interactively. It saves me time and effort. Szabolcs P.S. One thing that bothers me slightly is that I do not yet see clearly whether Condition caching might interact badly with this function.