Severin Pappadeux Severin Pappadeux - 1 year ago 75
R Question

Is there a difference between a^2 and a^2L?

Is there a difference between

in R?

Speed difference?


So far I see none, just wondering if ^2 is implemented as log/exp pair, but ^2L as a multiplication. What about if
is not just a vector?


No, it is NOT a duplicate, I know about difference between
. Question is, does this difference play a role for the power operator?

Answer Source

R uses an integer exponent version R_pow_di internally, but the ^ operator only calls R_pow. That being said, R_pow does special case x^2 as x*x. Therefore, the precision is identical, but the 2L version ought to be slightly slower due to long->double coercion at the C level. This is demonstrated in the following benchmark.

> lngs<-rep(2L,1e6)
> dbls<-rep(2.0,1e6)
> n<-sample(100,1e6,replace=TRUE)
> x<-rnorm(1e6)
> microbenchmark(x^lngs,x^dbls,n^lngs,n^dbls)
Unit: milliseconds
   expr       min        lq      mean    median       uq      max neval cld
 x^lngs  8.489547  9.804030 12.543227 11.719721 13.98702 19.92170   100  b 
 x^dbls  5.622067  6.724312  9.432223  7.949713 10.89252 59.15342   100 a  
 n^lngs 10.590587 13.857297 14.920559 14.200080 16.65519 19.55346   100   c
 n^dbls  8.331087  9.699143 12.414267 11.403211 14.20562 19.66389   100  b 

This is nothing to lose sleep over.

Links go to the source mirror. Note that R_ADD, R_SUB, R_MUL and R_DIV are defined as C macros to take advantage of type overloading, but R_POW is defined to inline the x^2 = x*x special case then calls R_pow (lower case). The checking for that special case again in R_pow would be duplicative except that R_pow is called internally other places.