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
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