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## Re: Plotting curves defined by implicit functions

 From: kingcrimson Subject: Re: Plotting curves defined by implicit functions Date: Tue, 17 Sep 2019 07:24:33 +0200

```
> Il giorno 17 set 2019, alle ore 07:02, address@hidden ha scritto:
>
>
>
>> Il giorno 16 set 2019, alle ore 16:55, Brett Green <address@hidden> ha
>> scritto:
>>
>> Thank you! I've used contours for this before, and was wondering if there
>> was an alternative method streamlined for level curves, but that's probably
>> just a pipe dream.
>>
>> On that subject, though, I have an issue with contour. Either I'm doing
>> something mathematically inane or I'm misunderstanding how to call the
>> function. I rune the code:
>>
>> F = @(x,y) x.^2+y.^2-4;
>> x=linspace(-5,5,100);
>> y=x;
>> for k=1:100
>> for j=1:100
>> Fmat(k,j) = F(x(k),y(j));
>> end
>> end
>> contour(x,y,Fmat,0)
>>
>> and get
>>
>> warning: division by zero
>> warning: called from
>>    __contour__ at line 167 column 11
>>    contour at line 74 column 18
>>
>> I thought I asked for a contour where Fmat=0, which should be a circle of
>> radius 2. What am I missing here?
>>
>> - Brett Green
>
>
>
>>> F = @(x,y) x.^2+y.^2-4;
>>> [x, y] = meshgrid (linspace (-5,5,100));
>>> contour (x, y, F(x, y))
>
>
> have a look at meshgrid in the manual.
> c.

BTW, I think the best way to plot the curve you want in the example is

F = @(x, y) x.^2 + y.^2;
contour (x, y, F(x,y), [4 4]);

The form for the last input is not documented clearly in the manual, I

https://savannah.gnu.org/bugs/?56907

c.

```