[R] How to compute within-group mean and sd?

[Rui Barradas]

Hello,


Reeyarn_李智洋_10928113 wrote
> 
> Hi, I want to run something like
>   SELECT firm_id, count(*), mean(value), sd(value)
>   FROM table
>   GROUP BY firm_id;
> 
> But I have to write a for loop like
>   for ( id in unique(table$firm_id ) {
>     print(paste(  id, mean(table[firm_id == id, "value"])  ))
>   }
> 
> Is there any way to do it easier? Thanks :)
> 
> 
> Best,
> Reeyarn Lee
> 
> 	[[alternative HTML version deleted]]
> 
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> 

You can use sql with package 'sqldf'.

library(sqldf) # It needs package 'tcltk'
?sqldf

tbl <- data.frame(firm_id=rep(c(1,2), 10), value=rnorm(20))

sqldf("SELECT firm_id, count(*), avg(value), stdev(value)
  FROM tbl
  GROUP BY firm_id;")

I've changed the name of your data.frame because 'table' is an R function.

Hope this helps,

Rui Barradas


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