R有效循环建议

Question

I have a dataframe running into about 500,000 rows. One of these columns contains positive integer values, say column A. let there be another column B

I now need to create a second dataframe with number of rows equal to sum(dataframe$A). this is done.

A question of performance arises when i need to fill this new data frame up with data. I am trying to create a column A2 for this second frame as follows:

A2<-vector() 
for (i in 1:nrow(dataframe)){
  A2<-c(A2,rep(dataframe$B[i],dataframe$A[i]))
}

The external loop is obviously very slow for the large number of rows being processed. Any suggestions on how to achieve this task with faster processing.

Thanks for responses

Answer 1

You simply do not need the loop at all. rep is already vectorized.

A2 <- rep(dataframe$B, dataframe$A)

Should work. As a reproducible example, here is your way using the built in mtcars dataset.

x <- vector()
for(i in 1:nrow(mtcars)) {x <- c(x, rep(mtcars$cyl[i], mtcars$gear[i]))}
> x
  [1] 6 6 6 6 6 6 6 6 4 4 4 4 6 6 6 8 8 8 6 6 6 8 8 8 4 4 4 4 4 4 4 4 6 6 6 6 6
 [38] 6 6 6 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 8
 [75] 8 8 8 8 8 8 8 8 8 8 8 4 4 4 4 4 4 4 4 4 4 4 4 4 4 8 8 8 8 8 6 6 6 6 6 8 8
[112] 8 8 8 4 4 4 4

and vectorized, it is:

x2 <- rep(mtcars$cyl, mtcars$gear)
> x2
  [1] 6 6 6 6 6 6 6 6 4 4 4 4 6 6 6 8 8 8 6 6 6 8 8 8 4 4 4 4 4 4 4 4 6 6 6 6 6
 [38] 6 6 6 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 8
 [75] 8 8 8 8 8 8 8 8 8 8 8 4 4 4 4 4 4 4 4 4 4 4 4 4 4 8 8 8 8 8 6 6 6 6 6 8 8
[112] 8 8 8 4 4 4 4

which will be orders of magnitude faster than using a loop.