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Re: a question about plot a list of functions.


On 6/22/05 at 1:55 AM, jiangzhou_yz at yahoo.com (Zhou Jiang) wrote:

>I defined a function as

>K=(2 r/(1-r^2))^2;
>y=1/(1+K (Sin[x])^2);
>y=y/.{r->{0.2,0.9,0.995}}

>Now the result is a list. I want to plot y vs. x in one figure and
>I did the following

>Plot[y, {x, 0, 10 Pi}];

>But Mathematica gave me an error. I changed it to

>Plot[{y[[1]], y[[2]], y[[3]]}, {x, 0, 10 Pi}];

>Mathematica gave me a correct plot. Can anyone give me some idea
>why the error message is given by Mathematica since y is a list and
>nothing is diffrent from {y[[1]], y[[2]], y[[3]]}? 

The difference is due to the way Mathematic evaluates arguments for Plot. For more detail on why this matters read the discussion of Plot in section 1.9.1 in the Mathematica Book. Meanwhile, the issue can be addressed using Evaluate. Specifically,

Plot[Evaluate@y, {x, 0, 10 Pi}];

will do what you want.
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