[R] Count of rows while looping through data

[Phil Spector]

Jeanna -
     I can't imagine how you could solve this problem with a 
loop, but here's one way to solve it using R:

First, I'll create a data frame with a family variable:

> x = data.frame(family = rep(1:20,sample(2:5,20,replace=TRUE)))

Next, I'll number each family member within each family:

> x$seq = ave(x$family,x$family,FUN=seq)

Now I'll choose a random number within each family:

> x$use = ave(x$family,x$family,FUN=function(x)sample(1:length(x),1))

Finally, I'll select the family member whose sequence number 
matches the random number:

> answer = subset(x,seq == use)

Hope this helps.  Take a look at the help page for the ave function
to understand how it works.
 					- Phil Spector
 					 Statistical Computing Facility
 					 Department of Statistics
 					 UC Berkeley
 					 spector at stat.berkeley.edu


On Tue, 24 May 2011, Jeanna wrote:

> I have a data table with one column that indicates families, and subsequent
> columns with other characteristics.  I want to randomize one member of the
> family to a separate table.  My approach is to count the number of members,
> set up a random number generator, and assign the family member based on
> where they fall within the random number spectrum.
>
> Is there a way to count the number of family members as I loop through the
> whole table?
>
> Something like this:
> for (j in 1:15){
> 	if (x$family[j] == x$family[j+1]){
>        count = count +1
> (which doesn't work)
>
> as I do the larger:
> for (i in 2:nrow(x.tab)){
>
>
> --
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