[R] How to....

[Jim Lemon]

On Tue, 2008-07-22 at 16:31 -0400, Jia Ying Mei wrote:
> Hi everyone,
> 
> I am not new to R, but its been over a year since I've used it, so I 
> don't remember how to do some things.
> 
> I have a data set (in excel currently, but will be converted to text), 
> that has date, country and price, with a price for every country for 
> every date. I need to find the mean and median of the interval between 
> price changes for each country and the mean and median percentage change 
> for each price change within the data for a single country.
> 
> How should I go about doing this? I'd appreciate any help.

Hi Jia,
I'm not too sure what you mean by the second set of means and medians,
but this should get you started:

# first make up some data
jym.df<-data.frame(country=rep(c("au","br""cn","in","us"),each=100),
 date=rep(as.Date("02/02/2008",
  format="%d/%m/%Y"),500)+sample(-40:40,500,TRUE),
 price=rnorm(500,100,20))
# get the intervals by country - have to create a new data frame
jymint.df<-data.frame(country=rep(levels(jym.df$country),each=99),
 intervals=unlist(tapply(jym.df$date,jym.df$country,diff)))
library(prettyR)
# break down the intervals by country
brkdn(intervals~country,jymint.df,num.desc=c("mean","median"))

Jim