Laurent,
Thank you for introducing me to a package I did not know existed as I use
features like list comprehension in python all the time and could see using it
in R now that I know it is available.
As to why you see your example as slow, I see you used a fairly complex and
nested expression and wonder if it was a better way to go. As you are dealing
with an interpreter doing delayed evaluation, I can imagine reasons it can be
slow. But note the package comprehenr may not be designed to be more efficient
than loops or of the more built-in functional methods that can be faster. The
package is there perhaps more as a compatibility helper that allows you to
write closer to the python style and perhaps re-shapes what you wrote into a
set of instructions in more native R.
Just for comparison, in python, things like comprehensions for list or
dictionaries or tuples often are syntactic sugar and the interpreter may simply
rewrite them more like the first program you typed and evaluates that. The
comprehensions are more designed for users who can think another way and write
things more compactly as one-liners. Depending on implementations, they may be
faster or slower on some examples.
I am not saying there is nothing else that is slowing it down for you. I am
suggesting that using the feature as currently implemented may not be an
advantage except in your thought process. It may be it could be improved, such
as by replacing more functionality out of R and into compiled languages as has
been done for many packages.
Avi
-----Original Message-----
From: R-help <r-help-boun...@r-project.org> On Behalf Of Laurent Rhelp
Sent: Sunday, June 16, 2024 11:28 AM
To: r-help@r-project.org
Subject: [R] slowness when I use a list comprehension
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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