       Re: Equivalent functions generate different plots

• To: mathgroup at smc.vnet.net
• Subject: [mg26148] Re: Equivalent functions generate different plots
• From: Jens-Peer Kuska <kuska at informatik.uni-leipzig.de>
• Date: Thu, 30 Nov 2000 01:03:56 -0500 (EST)
• Organization: Universitaet Leipzig
• References: <8vvoh0\$2p8@smc.vnet.net>
• Sender: owner-wri-mathgroup at wolfram.com

```Hi,

Plot[Evaluate[{f[t], g[t]}], {t, 0, 2.2}]

Plot[] has the attribute HoldAll, because you may have assigned a value
to "t" like

x = 1;
Plot[Sin[x], {x, 0, Pi}].

If Plot[] would evaluate it's arguments, the last example would give
Plot[Sin,{1,0,Pi}] and that is clearly nonsense. Your delayed
assigment
for g[t] cause Mathematica to evaluate the function inside the Plot[]
command

Hope that helps
Jens

> Then, I can define the regression function in two different, but
> equivalent ways:
>
> First: copying and pasting the last output:
>
> In:=
> f[t_] :=
>     79.68214285714285`+ 0.21249999999984936` t - 16.741071428571246 t^2
>
> Second: using directly the *Fit* function in the right hand:
>
> In:=
> g[t_] := Fit[data, {1, t, t^2}, t]
>
> In this conditions:
>
> In:=
> f[t] == g[t]
>
> Out=
> True
>
> However:
>
> In:=
> Plot[{f[t], g[t]}, {t, 0, 2.2}]
>
> Out=
>
> (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?
>

```

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