MathGroup Archive 2014

[Date Index] [Thread Index] [Author Index]

Search the Archive

Do we need a When function?

  • To: mathgroup at
  • Subject: [mg132393] Do we need a When function?
  • From: David Bailey <dave at>
  • Date: Sat, 8 Mar 2014 02:42:31 -0500 (EST)
  • Delivered-to:
  • Delivered-to:
  • Delivered-to:
  • Delivered-to:

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:


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 

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

  • Prev by Date: Re: TreePlot
  • Next by Date: Re: Cannot move text with drawing tools
  • Previous by thread: WildCats version 0.70 released
  • Next by thread: Re: Do we need a When function?