[R] problem in plot after calculating the derivative

[David Winsemius]

On Mar 8, 2011, at 12:39 AM, Hong Qin wrote:

> Here is a test code to calculate the derivative of a curve in sliding
> windows. I generated a linear example, and it should plot the  
> derivative in
> a straight line. But, it does not?!

Try adding ylim=c(0.1). You will get your straight line. I consider  
the current behavior (of not labeling the y axis and producing a  
jagged line when given a series of values whose successive differences  
are below machine tolerances to be a bug. the output from print would  
lead you do think they were equal, but if you use diff(slope you find  
that they are slightly variable.)

>
> --------------
> x = seq(0,10,by=0.1)
> y = x * 0.5 + 0.1
>
> window=15;
> half = floor( window/2)
>
> x2 = half: (length(x)-half) ;
> slope = half: (length(x)-half) ;
>
> count = 1;
> for( i in half: (length(x)-half) ) {
>   pos = (i-half):(i+half)
>   m = lm( y[pos] ~ x[pos])
>   x2[count] = x[i]
>   slope[count] = m$coeff[2];
>   count = count + 1;
> }
>
> plot( slope ~ x2, type='l' ) #does not give a straight line, why?
>
> 	[[alternative HTML version deleted]]
>
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David Winsemius, MD
West Hartford, CT