[R] slowness when I use a list comprehension

[Laurent Rhelp]

Dear RHelp-list,

    I try to use the package comprehenr to replace a for loop by a list 
comprehension.

  I wrote the code but I certainly miss something because it is very 
slower compared to the for loops. May you please explain to me why the 
list comprehension is slower in my case.

Here is my example. I do the calculation of the square difference 
between the values of two vectors vec1 and vec2, the ratio sampling 
between vec1 and vec2 is equal to ratio_sampling. I have to use only the 
500th value of the first serie before doing the difference with the 
value of the second serie (vec2).

Thank you

Best regards

Laurent

library(tictoc)
library(comprehenr)

ratio_sampling <- 500
## size of the first serie
N1 <- 70000
## size of the second serie
N2 <- 100
## mock data
set.seed(123)
vec1 <- rnorm(N1)
vec2 <- runif(N2)


## 1. with the "for" loops

## the square differences will be stored in a vector
S_diff2 <- numeric((N1-(N2-1)*ratio_sampling))
tic()
for( j in 1:length(S_diff2)){
   sum_squares <- 0
   for( i in 1:length(vec2)){
     sum_squares = sum_squares + ((vec1[(i-1)*ratio_sampling+j] - 
vec2[i])**2)
   }
   S_diff2[j] <- sum_squares
}
toc()
## 0.22 sec elapsed
which.max(S_diff2)
## 7857

## 2. with the lists comprehension
tic()
S_diff2 <- to_vec(for( j in 1:length(S_diff2)) sum(to_vec(for( i in 
1:length(vec2)) ((vec1[(i-1)*ratio_sampling+j] - vec2[i])**2))))
toc()
## 25.09 sec elapsed
which.max(S_diff2)
## 7857