[R] Create subsets of data

[David Winsemius]

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. (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