[R] percent rank by an index key?

[Sundar Dorai-Raj]

t c wrote:
> What is the easiest way to calculate a percent rank “by” an index key?
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> Foe example, I have a dataset with 3 fields:
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> Year,    State,   Income ,
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> I wish to calculate the rank, by year, by state.
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> I also wish to calculate the “percent rank”, where I define percent rank as rank/n.
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> (n is the number of numeric data points within each date-state grouping.)
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> This is what I am currently doing:
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> 1.  I create a “group by” field by using the paste function to combine date and state into a field called date_state.   I then use the rank function to calculate the rank by date, by state. 
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> 2. I then add a field called “one” that I set to 1 if the value in income is numeric and to 0 if it is not.
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> 3. I then take an aggregate sum of “one”.  This gives me a count (n) for each date-state grouping.
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> 4. I next use merge to add this count to the table.
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> 5. Finally, I calculate the percent rank.
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> Pr<-rank/n
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> The merge takes quite a bit of time to process. 
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> Is there an easier/more efficient way to calculate the percent rank?
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How about using ?by:

set.seed(100)
# fake data set, replace with your own
# "Subject" is just a dummy to produce replicates
x <- expand.grid(Year = 2000:2005,
                  State = c("TX", "AL"),
                  Subject = 1:10)
x$Income <- floor(runif(NROW(x)) * 100000)

r <- by(x$Income, x[c("Year", "State")],
         function(x) {
           r <- rank(x)
           n <- length(x)
           cbind(Rank = r, PRank = r/n)
         })
x <- cbind(x, do.call("rbind", r))

HTH,

--sundar