Groups > comp.soft-sys.math.mathematica > #16612

Do we need a When function?

From	David Bailey <dave@removedbailey.co.uk>
Newsgroups	comp.soft-sys.math.mathematica
Subject	Do we need a When function?
Date	2014-03-08 07:42 +0000
Message-ID	<lfehl0$5kq$1@smc.vnet.net> (permalink)
Organization	Time-Warner Telecom

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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?

David Bailey
http://www.dbaileyconsultancy.co.uk

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Thread

Do we need a When function? David Bailey <dave@removedbailey.co.uk> - 2014-03-08 07:42 +0000
  Re: Do we need a When function? Szabolcs Horvát <szhorvat@gmail.com> - 2014-03-10 08:38 +0000
    Re: Do we need a When function? David Bailey <dave@removedbailey.co.uk> - 2014-03-11 07:17 +0000
  Re: Do we need a When function? Helen Read <readhpr@gmail.com> - 2014-03-10 08:41 +0000
  Re: Do we need a When function? Szabolcs Horvát <szhorvat@gmail.com> - 2014-04-11 06:09 +0000
    Re: Do we need a When function? David Bailey <dave@removedbailey.co.uk> - 2014-04-13 09:26 +0000

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