[R] python

Peter Ehlers ehlers at ucalgary.ca
Sun Nov 22 17:39:14 CET 2009



Stefan Evert wrote:
>> Sure, badly written R code does not perform as well as well written 
>> python code or C code. On the other hand badly written python code 
>> does not perform as well as well written R code.
>>
>> What happens when you try one of these :
>>
>> sum <- sum( 1:N )
> 
> R runs out of memory and crashes. :-)  I didn't tell you how big N is, 
> did I?

Really?

  N <- 1e30
  sum( 1:N )
  Error in 1:N : result would be too long a vector

  -Peter Ehlers

 >
> 
> But this is exactly the point I was trying to make (but perhaps not 
> prominently enough).  In many cases, you can vectorize at least parts of 
> your code or find a more efficient algorithm, which may be faster in R 
> than a brute-force solution in C.  But sometimes, you just cannot avoid 
> loops (let's not forget that all the forms of apply() are just loops and 
> don't give much of a speed benefit over a for-loop), function calls, 
> etc.; in this case, performance differences between interpreted 
> languages can matter.
> 
> Personally, I'd never switch from R to Perl just for speed, though.
> 
> BTW, I also tried a vectorised algorithm in R, which calculates the sum 
> above in a small number of chunks:
> 
>> N1 <- 50
>> N2 <- 1000000
>> N <- N1 * N2
>> sum <- 0
>>
>> for (i in 1:N1) {
>>         x <- as.numeric(i-1) * N2 + 1:N2
>>         sum <- sum + sum(x)
>> }
> 
> which gives
> 
> R/simple_count_vec.R              31.30 Mops/s  (50000000 ops in 1.60 s)
> 
> So an interpreted loop in Lua is still faster than this partially 
> vectorized code in R:
> 
>>> lua/simple_count.lua 65.78 Mops/s (100000000 ops in 1.52 s)
> 
> As people on the SQLite mailing list always say: there's no general 
> answer as to which language/implementation/query/... is faster and 
> better.  You just have to test the different options for your specific 
> application setting, and be prepared for one or two surprises.
> 
> Just in case this isn't obvious: If I rewrote matrix multiplication in C 
> and linked this code into R, it would run much slower than if I just 
> typed "A %*% B".
> 
> All the best,
> Stefan
> 
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