On May 9, 2011, at 3:46 PM, David Winsemius wrote:
On May 9, 2011, at 9:40 AM, Pavan G wrote:
Hello All,
Let's say I have points on a x-y plane. x ranges from 0-2 and y
from 0-2.
There are points in quadrants x[0:1]---y[0:1] and in x[1:2]----
y[1:2]. I
would like to get the mean and std of the points in the x[0:1]----
y[0:1]
quadrant alone. Is there a straight forward way to do it?
I am assuming that you have yet a third vector that has values at
each of those coordinates and it is that set of categorized values
for which you want the means and std deviations: I will exemplify
the use of tapply on cut()-igorized vectors and a z-value which is
the L1 distance from the origin.
(Actually, after checking help(dist) I think the sum(x_i) is called
the Canberra distance. )
(And excuse the earlier feeble attempt at humor);
> x <- runif(2000, 0,2)
> y <- runif(2000, 0, 2)
> xc <- cut(x, c(0,1,2))
> yc <- cut(y, c(0,1,2))
> z <- x+y
> tapply(z, list(xc,yc), mean)
(0,1] (1,2]
(0,1] 1.013192 2.016095
(1,2] 1.979930 2.996229 # seems to make sense
> tapply(z, list(xc,yc), sd)
(0,1] (1,2]
(0,1] 0.4028310 0.4133113
(1,2] 0.4239014 0.3984559 # also seems sensible
--
David Winsemius, MD
West Hartford, CT
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