Why is this simple loop in R so slow?

Question

I'm a beginner with R and I have written this simple loop:

for(i in 1:12000){
    if(v$piano.tariff[i] == 2) {v$piano.tariff[i] = 0}
    else {v$piano.tariff[i] = 1}
}

Where v is a data frame and piano.tariff one of its columns. What the loop does is simply change each value of the piano.tariff column to either 1 or 0, from their initiali values of 2 and 5.

Now, the code works, but the problem is that it is ridiculously slow. It takes up to 4-5 minutes to complete! In C++ o C# such a loop would barely require a few seconds.

Why is this so slow? Is there a faster way to implement this? Or is it simply that R is slow, and that's it?

If you're newer to R a lot of these questions regarding loops and vectorization are answered in: http://www.burns-stat.com/pages/Tutor/R_inferno.pdf — Tyler Rinker, Apr 08 '13 at 16:59

score 7 · Answer 1 · edited May 23 '17 at 12:22

7

You may want to use ifelse instead which is a vectorized R function, it'll be faster

ifelse(v$piano.tariff==2, 0, 1)

Since you didn't provide a reproducible example I couldn't benchmark the performace.

edited May 23 '17 at 12:22

Community

1
1

answered Apr 08 '13 at 09:16

Jilber Urbina

58,147
10
114
138

Roman Luštrik · Accepted Answer · 2013-04-08T09:37:40.673

5

I think you could try a vectorized approach here.

EDIT Thanks to Henrik, the previous version was a bit off. I think the new approach is OK.

twos <- v$piano.tariff == 2
notwos <- v$piano.tariff != 2
v[twos, "piano.tariff"] <- 0
v[notwos, "piano.tariff"] <- 1

Fast enough for you? :)

edited Apr 08 '13 at 09:37

answered Apr 08 '13 at 09:17

Roman Luštrik

69,533
24
154
197

It's instantaneous, thanks!! Tho I modified it slightly, since I knew the other value was 5 I used a "==" in the second line, otherwise as Henrik points out, it wouldn't work properly. – Master_T Apr 08 '13 at 09:36
@Master_T see my edit. Henrik noted that my first approach was wrong. – Roman Luštrik Apr 08 '13 at 09:40

score 1 · Answer 3 · answered Apr 08 '13 at 09:20

In general you need to be careful about loops in R, but more particularly assignments inside loops like v$piano.tariff[i] = v$piano.tariff[i]+1 which cause the entire vector to be re-allocated.

Refer to Patrick Burns online book on these sorts of issues The R Inferno for a lot more detail on this.

Why is this simple loop in R so slow?

3 Answers3