Collapsing a data.frame to a data.frame -- problems with by() and aggregate()

r

Solution

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.

Problem

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?

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