Re: Do we need a When function?
- To: mathgroup at smc.vnet.net
- Subject: [mg132401] Re: Do we need a When function?
- From: Itai Seggev <itais at wolfram.com>
- Date: Mon, 10 Mar 2014 04:36:53 -0400 (EDT)
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On Sat, Mar 08, 2014 at 02:42:31AM -0500, 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. Why isn't this simply If[NumericQ[x], f[x]] (*are NumberQ, if want an actual number type*) But I'm further confused by your statement that it stays unevlauate until x is assigned a number. That's not true, it will evaluate to Null unless x is numeric prior to the start of evaluation. > 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'm not sure what your use case is. F[x] will of course stay unevluated until x has some value which F knows what to do with... -- Itai Seggev Mathematica Algorithms R&D 217-398-0700
- References:
- Do we need a When function?
- From: David Bailey <dave@removedbailey.co.uk>
- Do we need a When function?