Re: Newbie question about rules
- To: mathgroup at smc.vnet.net
- Subject: [mg21373] Re: Newbie question about rules
- From: "Allan Hayes" <hay at haystack.demon.co.uk>
- Date: Wed, 29 Dec 1999 23:18:46 -0500 (EST)
- References: <84dn7q$31a@smc.vnet.net>
- Sender: owner-wri-mathgroup at wolfram.com
Doug,
exp := Plot[Sin[x], {x, 0, 2 Pi}]
exp
[ display cut]
-Graphics-
The output here is indicated by -Graphics- this contains the line object of
the form Line[{{x1,y1},{x2,y2},...}] that has been constructed from Sin[x]
and then shown in the display (the display is not the output - it is a side
effect)
You can see the code in -Graphics- by looking at
InputForm[%] (*% is the previous output*)
The important thing to notice is that Sin does not appear in -Graphics-
Now,
exp /. Sin -> Cos
Evaluates as follows:
1. exp evaluates as before, displaying the sine curve and giving the same
output,
2. output/.Sin->Cos is just output since output has no Sin to be replaced.
3. even if there had been a change th eresult would only have been displayed
if Show had been used
exp /. ln : Line[_] -> {Hue[0], ln}
Changes the code to that for a red line.
But we have to tell Mathematica to show this:
Show[%]
Allan
---------------------
Allan Hayes
Mathematica Training and Consulting
Leicester UK
www.haystack.demon.co.uk
hay at haystack.demon.co.uk
Voice: +44 (0)116 271 4198
Fax: +44 (0)870 164 0565
"Doug Nelson" <dougn_NOspam at gte.net> wrote in message
news:84dn7q$31a at smc.vnet.net...
> Hello All,
>
> I just received my copy of Mathematica 4.0 and I've been reading Mastering
> Mathematica.
>
> I'm trying to apply a rule that changes "Sin" to "Cos" in an expression.
It
> seems to work fine with normal expressions, but not with a Plot function.
>
> This works:
> exp2 := Abcfunc[Sin[x]]
> exp2 /. Sin -> Cos
> out = Abcfunc[Cos[x]]
>
> However, the following gives me a sine wave plot in both cases:
>
> exp := Plot[Sin[x], {x,0,2 Pi}]
> exp
> out = < a sine wave plot >
>
> exp /. Sin -> Cos
> out = < another sine wave plot ! >
>
> Why don't I get a cosine plot?
>
> Thanks in advance,
> Doug
>