Question

Consider that I have the following data and function returning summary statistics that I like

landlines <- data.frame(
                year=rep(c(1990,1995,2000,2005,2010),times=3),
                country=rep(c("US", "Brazil", "Asia"), each=5),
                pct =  c(0.99, 0.99, 0.98, 0.05, 0.9,
                         0.4,  0.5,  0.55, 0.5,  0.45,
                         0.7,  0.85, 0.9,  0.85, 0.75)
                )
someStats <- function(x)
{
  dp <- as.matrix(x$pct)-mean(x$pct)
  indp <- as.matrix(x$year)-mean(x$year)
  f <- lm.fit( indp,dp )$coefficients
  w <- sd(x$pct)
  m <- min(x$pct)
  results <- c(f,w,m)
  names(results) <- c("coef","sdev", "minPct")
  results
}

I can apply that function to a data subset successfully like this:

> someStats(landlines[landlines$country=="US",])
      coef      sdev    minPct 
 -0.022400  0.410938  0.050000 

or look at a breakdown by country like this:

> by(landlines, list(country=landlines$country), someStats)
country: Asia
      coef       sdev     minPct 
0.00200000 0.08215838 0.70000000 
--------------------------------------------------------------------------------------- 
country: Brazil
      coef       sdev     minPct 
0.00200000 0.05700877 0.40000000 
--------------------------------------------------------------------------------------- 
country: US
     coef      sdev    miPct 
-0.022400  0.410938  0.050000 

Trouble is, that is not the data.frame object I need for further processing, and it won’t cast as such:

> as.data.frame( by(landlines, list(country=landlines$country), someStats) )
Error in as.data.frame.default(by(landlines, list(country = landlines$country),  : 
  cannot coerce class '"by"' into a data.frame

“No problem!” I think, since the similar aggregate() function does return a data.frame:

> aggregate(landlines$pct, by=list(country=landlines$country), min)
  country    x
1    Asia 0.70
2  Brazil 0.40
3      US 0.05

Trouble is, it doesn’t work properly with arbitrary functions:

> aggregate(landlines, by=list(country=landlines$country), someStats)
Error in x$pct : $ operator is invalid for atomic vectors

What I really want to get is a data.frame object with the following columns:

• country
• coef
• sdev
• minPct

How can I do that?

Answer 1

take a look at the plyr package and in particular ddply

> ddply(landlines, .(country), someStats)
  country    coef       sdev minPct
1    Asia  0.0020 0.08215838   0.70
2  Brazil  0.0020 0.05700877   0.40
3      US -0.0224 0.41093795   0.05

Ideally your function explicitly returns a data.frame but in this case, it can be coerced to one easily and correctly.