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MathGroup Archive 2004

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Re: why Pi is not recognized as a positive number?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg45363] Re: why Pi is not recognized as a positive number?
  • From: "Bo Le" <bole79 at email.si>
  • Date: Sat, 3 Jan 2004 01:35:07 -0500 (EST)
  • References: <bt3e0s$plb$1@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

Hi,


Your first definition has a mistake. A typo?

One types g[x_Positive] to define g to arguments with head Positive
(g[x_Real] would be more sensible), while f[x_?Positive]  to define f to
arguments which have this property. More generally, h[x_?test] defines h to
arguments, on which the application of test returns True.

Your second definition of f is of course correct and also more common, as it
is read as 'only when x is Positive'.


Bye,

Borut
Slovenia



"Pedro L" <pedrito6 at softhome.net> wrote in message
news:bt3e0s$plb$1 at smc.vnet.net...
>   I wanted to define a function for the positive numbers. But I discovered
>   something "strange" with Mathematica.
>
>   Could you help me with this?
>
>   Thanks in advance.
>
>   Peter L  (Spain)
>
>
>
> {
>
> Cell[CellGroupData[{
> Cell[BoxData["$Version"], "Input",
>   CellLabel->"In[1]:="],
>
> Cell[BoxData["\<\"5.0 for Microsoft Windows (June 11, 2003)\"\>"], \
> "Output",
>   CellLabel->"Out[1]="]
> }, Open  ]],
>
> Cell[BoxData[
>     RowBox[{
>       RowBox[{"f", "[", "x_Positive", "]"}], ":=",
>       RowBox[{"x", "+", "1", "  "}]}]], "Input",
>   CellLabel->"In[2]:="],
>
> Cell[CellGroupData[{
>
> Cell[BoxData[
>     RowBox[{"Positive", "[", "Pi", "]"}]], "Input",
>   CellLabel->"In[3]:="],
>
> Cell[BoxData["True"], "Output",
>   CellLabel->"Out[3]="]
> }, Open  ]],
>
> Cell[CellGroupData[{
>
> Cell[BoxData[
>     RowBox[{"f", "[", "Pi", "]"}]], "Input",
>   CellLabel->"In[4]:="],
>
> Cell[BoxData[
>     RowBox[{"f", "[", "\[Pi]", "]"}]], "Output",
>   CellLabel->"Out[4]="]
> }, Open  ]],
>
> Cell[BoxData[
>     RowBox[{"Remove", "[", "f", "]"}]], "Input",
>   CellLabel->"In[5]:="],
>
> Cell[BoxData[
>     RowBox[{
>       RowBox[{"f", "[",
>         RowBox[{"x_", " ", "/;", " ",
>           RowBox[{"x", ">", "0"}]}], "]"}], ":=",
>       RowBox[{"x", "+", "1"}]}]], "Input",
>   CellLabel->"In[6]:="],
>
> Cell[CellGroupData[{
>
> Cell[BoxData[
>     RowBox[{"f", "[", "Pi", "]"}]], "Input",
>   CellLabel->"In[7]:="],
>
> Cell[BoxData[
>     RowBox[{"1", "+", "\[Pi]"}]], "Output",
>
>   CellLabel->"Out[7]="]
> }, Open  ]],
>
> Cell[BoxData[
>     RowBox[{"Remove", "[", "f", "]"}]], "Input",
>   CellLabel->"In[8]:="]
> }
>
>


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