slow computation of functions over large datasets
This takes about 2 secs for 1M rows:
n <- 1000000 exampledata <- data.frame(orderID = sample(floor(n / 5), n, replace = TRUE), itemPrice = rpois(n, 10)) require(data.table) # convert to data.table ed.dt <- data.table(exampledata) system.time(result <- ed.dt[
+ , list(total = sum(itemPrice)) + , by = orderID + ] + ) user system elapsed 1.30 0.05 1.34
str(result)
Classes ?data.table? and 'data.frame': 198708 obs. of 2 variables: $ orderID: int 1 2 3 4 5 6 8 9 10 11 ... $ total : num 49 37 72 92 50 76 34 22 65 39 ...
head(result)
orderID total [1,] 1 49 [2,] 2 37 [3,] 3 72 [4,] 4 92 [5,] 5 50 [6,] 6 76
On Wed, Aug 3, 2011 at 9:25 AM, Caroline Faisst
<caroline.faisst at gmail.com> wrote:
Hello there,
I?m computing the total value of an order from the price of the order items
using a ?for? loop and the ?ifelse? function. I do this on a large dataframe
(close to 1m lines). The computation of this function is painfully slow: in
1min only about 90 rows are calculated.
The computation time taken for a given number of rows increases with the
size of the dataset, see the example with my function below:
# small dataset: function performs well
exampledata<-data.frame(orderID=c(1,1,1,2,2,3,3,3,4),itemPrice=c(10,17,9,12,25,10,1,9,7))
exampledata[1,"orderAmount"]<-exampledata[1,"itemPrice"]
system.time(for (i in 2:length(exampledata[,1]))
{exampledata[i,"orderAmount"]<-ifelse(exampledata[i,"orderID"]==exampledata[i-1,"orderID"],exampledata[i-1,"orderAmount"]+exampledata[i,"itemPrice"],exampledata[i,"itemPrice"])})
# large dataset: the very same computational task takes much longer
exampledata2<-data.frame(orderID=c(1,1,1,2,2,3,3,3,4,5:2000000),itemPrice=c(10,17,9,12,25,10,1,9,7,25:2000020))
exampledata2[1,"orderAmount"]<-exampledata2[1,"itemPrice"]
system.time(for (i in 2:9)
{exampledata2[i,"orderAmount"]<-ifelse(exampledata2[i,"orderID"]==exampledata2[i-1,"orderID"],exampledata2[i-1,"orderAmount"]+exampledata2[i,"itemPrice"],exampledata2[i,"itemPrice"])})
Does someone know a way to increase the speed?
Thank you very much!
Caroline
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