as.data.frame.table() to convert by() output to a data frame
On Nov 25, 2009, at 4:11 PM, Michael Ash wrote:
Dear all, This seems to be working, but I'd like to make sure that I'm not doing anything wrong. I am using by() to construct a complicated summary statistic by several factors in my data (specifically, the 90-50 income ratio by city and race). cityrace.by <- by(microdata, list(microdata$city,microdata$race), function (x) quantile(x$income, probs=0.9) / quantile(x$income, probs=0.5) ) I would now like to use the data created by by() as a dataset with city-race as the unit of observation. However, cityrace.data <- as.data.frame(cityrace.by) does not work because "Error in as.data.frame.default(city.by) : cannot coerce class "by" into a data.frame" The following is not a documented use of as.data.frame.table(), but it seems to work. It gives the columns slightly strange names, including "Freq" for the statistic computed in by by() but otherwise, the dataframe is indexed by city and race with the 90-50 ratio as the variable cityrace.data <- as.data.frame.table(cityrace.by)
If the by-object you get happens to be a 2d array, then why not.
Tables are matrices after all.
> tt <- table(c(1,1), c(1,1))
> tt
1
1 2
> is.matrix(tt)
[1] TRUE
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David Winsemius, MD Heritage Laboratories West Hartford, CT