Re: Is it a bug?
- To: mathgroup at smc.vnet.net
- Subject: [mg17578] Re: [mg17548] Is it a bug?
- From: "Tomas Garza" <tgarza at mail.internet.com.mx>
- Date: Mon, 17 May 1999 02:14:29 -0400
- Sender: owner-wri-mathgroup at wolfram.com
F. Mittermayr [mitterma at linz.vai.co.at] wrote: > A = {1 + Sqrt[2] I , 1 + 2 * 3 I}; > Cases[A, _Complex] > > Out[1] = {1+6 I} > > Isn't it a strange result? It does look strange, indeed, but it so happens that although Sqrt[2] is numeric, it is not a number (in Mathematica). Cf. The Book (2.3.5): NumberQ[expr] returns False unless expr is manifestly a number (i.e., has head Complex, Integer, Rational or Real). But, you see, In[1]:= Head[Sqrt[2]] Out[1]= Power Hence it is not a number, i.e. In[2]:= NumberQ[Sqrt[2]] Out[2]= False Then 1 + Sqrt[2] I is "not" a number, let alone a complex number. But... Sqrt[2.] "is" a number, since In[3]:= Head[Sqrt[2.] ] Out[3]= Real and so In[4]:= NumberQ[1+Sqrt[2.] I] Out[4]= True Then In[5]:= Cases[A, _Complex] Out[5]= {1+1.41421 I,1+6 I} as you would have expected, where 1.41421 is to be understood as an approximation to Sqrt[2]. Good luck, Tomas Garza Mexico City