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?
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