[R] vectorization

[Rau, Roland]

Hi,


> -----Original Message-----
> From: r-help-bounces at stat.math.ethz.ch 
> [mailto:r-help-bounces at stat.math.ethz.ch] On Behalf Of Dimitri Joe
> Sent: Friday, June 17, 2005 7:01 PM
> To: R-Help
> Subject: [R] vectorization
> 
> Hi there,
> 
> I have a data frame (mydata) with 1 numeric variable (income) 
> and 1 factor (education). I want a new column in this data 
> with the median income for each education level. A obviously 
> inneficient way to do this is
> 
I guess the attached code (incl. simulating your data structure) is not
the most efficient way to do this, but at least (I hope so!) it does
what you wanted it to do:


####################### Beginning of Example Code

income <- runif(100)
education <- as.factor(sample(c("high", "middle", "low"),
size=length(income), replace=TRUE))
mydata <- data.frame(inc=income, edu=education)


mymedians <- tapply(X=mydata$inc, INDEX=mydata$edu, FUN=median)

mydata$medians <- ifelse(mydata$edu=="high", mymedians["high"], 0)
mydata$medians <- ifelse(mydata$edu=="middle", mymedians["middle"],
mydata$medians)
mydata$medians <- ifelse(mydata$edu=="low", mymedians["low"],
mydata$medians)

head(mydata)
mymedians

####################### End of Example Code

Maybe one can increase the speed, but I think it is sufficient for your
case of 30,000 cases as you can see from the timing on my desktop
computer here (WinXP Pro SP2, P4, 3GHz, 512MB RAM):

> time.check <- function(){
+   income <- runif(30000)
+   education <- as.factor(sample(c("high", "middle", "low"),
size=length(income), replace=TRUE))
+   mydata <- data.frame(inc=income, edu=education)
+   
+   mymedians <- tapply(X=mydata$inc, INDEX=mydata$edu, FUN=median)
+ 
+   mydata$medians <- ifelse(mydata$edu=="high", mymedians["high"], 0)
+   mydata$medians <- ifelse(mydata$edu=="middle", mymedians["middle"],
mydata$medians)
+   mydata$medians <- ifelse(mydata$edu=="low", mymedians["low"],
mydata$medians)
+   return(NULL)
+ }
> system.time(time.check())
[1] 0.36 0.02 0.38   NA   NA
>   
> version
         _              
platform i386-pc-mingw32
arch     i386           
os       mingw32        
system   i386, mingw32  
status   beta           
major    2              
minor    1.0            
year     2005           
month    04             
day      04             
language R              


Best,
Roland


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