Re: piecewise vs which
- To: mathgroup at smc.vnet.net
- Subject: [mg60110] Re: piecewise vs which
- From: Jean-Marc Gulliet <jeanmarc.gulliet at gmail.com>
- Date: Sat, 3 Sep 2005 02:05:58 -0400 (EDT)
- Organization: The Open University, Milton Keynes, U.K.
- References: <df9437$620$1@smc.vnet.net>
- Sender: owner-wri-mathgroup at wolfram.com
Bradley Stoll wrote:
> Consider defining a function in Mathematica (v. 5.2) in two different
> ways: f[x_]=Piecewise[{{x^2,x<2},{3x,x>2}}] and
> g[x_]=Which[x<2,x^2,x>2,3x]. Notice that 2 is not in the domain of
> either function. However, if I ask for f[2], Mathematica returns 0 and if I ask
> for g[2] Mathematica (correctly) returns nothing. Is this a bug with
> Mathematica (that Mathematica returns 0 for f[2]), since 2 is not in the domain?
Hi Bradley,
Mathematica is "correct" by returning 0 for f[2] since this is the
expected behavior as documented in the documentation for *Piecewise*
"Piecewise[{{val_1, cond_1}, ...}, val] uses default value _val_ if none
of the cond_i apply. *The default for _val_ is 0.*"
http://documents.wolfram.com/mathematica/functions/Piecewise
So it may be better to define your function in the following way
In[1]:=
f[x_] = Piecewise[{{x^2, x < 2}, {3*x, x > 2}}, "Undefined"];
f[2]
Out[1]=
Undefined
Best regards,
/J.M.