RE: Equivalent functions generate different plots
- To: mathgroup at smc.vnet.net
- Subject: [mg26159] RE: [mg26112] Equivalent functions generate different plots
- From: "David Park" <djmp at earthlink.net>
- Date: Thu, 30 Nov 2000 01:04:04 -0500 (EST)
- Sender: owner-wri-mathgroup at wolfram.com
Use
Plot[{f[t], g[t]} // Evaluate, {t, 0, 2.2}];
When you don't evaluate the plotting function g[t] first, then for each t
point Mathematica evaluates g[t]. When t == 1 it evaluates g[1] ->
Fit[data,{1,1,1^2}], which is not what you intended. An alternative is to
define g[t_] with a Set:
g[t_] = Fit[data, {1, t, t^2}, t]
Then
Plot[{f[t], g[t]}, {t, 0, 2.2}];
works the way you wish.
David Park
djmp at earthlink.net
http://home.earthlink.net/~djmp/
> -----Original Message-----
> From: GERMAN [mailto:gerbual at col2.telecom.com.co]
To: mathgroup at smc.vnet.net
> Hi, Group:
>
> With:
>
> In[1]:=
> data = {{0, 79.6}, {0.2, 79.2}, {0.4, 77.1}, {0.6, 73.7}, {0.8, 69.1},
> {1., 63.2}};
>
> I can to get its cuadratic regression function:
>
> In[2]:=
> Fit[data, {1, t, t^2}, t]
>
> Out[2]=
> 79.68214285714285` + 0.21249999999984936` t - 16.741071428571246` t^2
>
> Then, I can define the regression function in two different, but
> equivalent ways:
>
> First: copying and pasting the last output:
>
> In[3]:=
> f[t_] :=
> 79.68214285714285`+ 0.21249999999984936` t - 16.741071428571246 t^2
>
> Second: using directly the *Fit* function in the right hand:
>
> In[4]:=
> g[t_] := Fit[data, {1, t, t^2}, t]
>
> In this conditions:
>
> In[5]:=
> f[t] == g[t]
>
> Out[5]=
> True
>
> However:
>
> In[6]:=
> Plot[{f[t], g[t]}, {t, 0, 2.2}]
>
> Out[6]=
>
> (GRAPHICS ...!!!)
>
> genere two different plots (a straight line and a curve). The curve is
> well, but the straight line not. I DON'T UNDERSTAND. Can somebody
> explain to me this inconsistency?
>
> Thanks in advance.
>
> GERMAN
>