MathGroup Archive 2014

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

Search the Archive

Re: Do we need a When function?

  • To: mathgroup at
  • Subject: [mg132413] Re: Do we need a When function?
  • From: Helen Read <readhpr at>
  • Date: Mon, 10 Mar 2014 04:40:53 -0400 (EDT)
  • Delivered-to:
  • Delivered-to:
  • Delivered-to:
  • Delivered-to:
  • References: <lfehl0$5kq$>

Use NumberQ

For example:

f[x_?NumberQ] := x^3

Helen Read
University of Vermont

On 3/8/2014 2:42 AM, 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?
> David Bailey

  • Prev by Date: Re: matrix manipulation
  • Next by Date: Strange result of solve
  • Previous by thread: Re: Do we need a When function?
  • Next by thread: Re: Do we need a When function?