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Mystery: Why is the as.character () function in data.table faster if I add and subtract another variable?

I noticed something very peculiar when converting dates to character classes for large datasets. As an example, I created a data set with layouts as follows:

DT = data.table(x=rep("2007-1-1", 1e9), y = rep(1,1e9))
DT[,x] <- as.Date(DT[,x])

Now I would like to convert the column x from date format dates to a character.

DT[,x.character:= as.character(x)]

This takes a little time for large datasets, and I noticed that the time required for the conversion decreases dramatically if we did the following:

DT[,x.character:= as.character(x+y-y)]

All I did here was add y and subtract y, so I really get the same results. From a logical point of view, it seems that I am making the computer work more. However, is there a reason why this method will lead to a faster start than direct conversion?

10000 system.time() :

DT = data.table(x=rep(as.Date("2007-1-1"), 1e5), y = rep(1,1e5))

system.time(DT[,x.character:= as.character(x)]) 
> user  system elapsed 
1.89    0.12    2.03 

system.time(DT[,x.character:= as.character(x+y-y)]) 
> user  system elapsed 
0.635   0.008   0.643 

system.time(DT[,x.character.sub:= as.character(x+y-y+y-y)]) 
> user  system elapsed 
0.347   0.004   0.351

, , , y-y . , ?

!

+4
1

, as.character R, . .

> library(data.table)
data.table 1.9.3  For help type: help("data.table")
> DT = data.table(x=rep(as.Date("2007-1-1"), 1e5), y = rep(1,1e5))
> system.time(DT[,x.character := as.character(x)]) 
   user  system elapsed 
  0.572   0.012   0.584 
> system.time(DT[,x.character := as.character(x)]) 
   user  system elapsed 
  0.389   0.008   0.397 
> system.time(DT[,x.character := as.character(x)]) 
   user  system elapsed 
  0.332   0.004   0.337

, data.table. :

> x <- rep(as.Date("2007-1-1"), 1e5)
> system.time(as.character(x)) 
   user  system elapsed 
  0.529   0.008   0.537 
> system.time(as.character(x)) 
   user  system elapsed 
  0.312   0.012   0.324 
> system.time(as.character(x)) 
   user  system elapsed 
  0.327   0.008   0.335
+13

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