Plotting circle

Question

I want to draw a circle like this.

The equation of the circle is : (x-4)^2+(y-2)^2=25

So i wrote down R codes using curve command. But it is not drawing circle exactly.

 curve(4-sqrt(25-(x-2)^2),xlim=c(-4,10),ylim=c(-4,10))

 curve(4+sqrt(25-(x-2)^2),add=TRUE,col="red")

and it is producing circle like following : enter image description here .

Where is/are the fault(s) in my commands? also the commands produce warning

 Warning message:
 In sqrt(25 - (x - 2)^2) : NaNs produced

I know there is a function draw.circle in R.

But i want to identify my codes fault(s).

there are far better, and simpler, ways to draw a circle centered at a specified origin and known radius. Look around a bit. — Carl Witthoft
– Carl Witthoft, Commented Jan 3, 2015 at 14:31
@CarlWitthoft: OP implies that they are doing this for pedagogical reasons. — Ben Bolker
– Ben Bolker, Commented Jan 3, 2015 at 14:35

J.R. · Accepted Answer · 2015-01-03 15:07:21Z

4

The warning message comes from the fact that you try to take sqrt() of a negative number. The circle can be drawn by increasing the data-points.:

N<-10000
curve(4-sqrt(25-(x-2)^2),xlim=c(-4,10),ylim=c(-4,10),n=N)
curve(4+sqrt(25-(x-2)^2),add=TRUE,col="red",n=N)

of course you should in general use the arguments of from,to to control the calculations of the data-points pointed out by Ben:

curve(4-sqrt(25-(x-2)^2),xlim=c(-4,10),ylim=c(-4,10),from=-3,to=7,n=N)
curve(4+sqrt(25-(x-2)^2),add=TRUE,col="red",from=-3,to=7,n=N)

also notice you are plotting (y,x) and not the usual (x,y). Judging by the comments, it seems you want:

curve(2-sqrt(25-(x-4)^2),xlim=c(-4,10),ylim=c(-4,10),from=-1,to=9,n=N)
curve(2+sqrt(25-(x-4)^2),add=TRUE,col="red",from=-1,to=9,n=N)

edited Jan 3, 2015 at 15:07

answered Jan 3, 2015 at 14:26

J.R.

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8 Comments

time Over a year ago

It is also producing warning and i didn't understood the reason why didn't xlim and ylim cover it ? Why did i need to add n and if i would use from= , to= argument inside curve instead of xlim and ylim , could i able to draw the circle parfectly ?

J.R. Over a year ago

xlim and ylim are arguments to plot, from and to is used by curve to calculate the datapoints, eg. ensure you are not trying to take sqrt() of a negative number in this case. I just increased the number of points because it was a quick fix.

time Over a year ago

But (4,7) should be a point on circumference. But it is not so. I've justified it by abline(h=7,v=4)

Ben Bolker Over a year ago

I might just delete my answer since your answer covers it. I think that once you specify from and to, however, you really don't need to increase N -- the default 101 points should be just fine. 10000 is overkill.

J.R. Over a year ago

yes xlim= will then narrow it down to almost c(from=,to=) and ylim= given by c( min(f(x)), max(f(x)) ) where f() is the function given as argument to curve.

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