Why are loops slow in R?

Question

I know that loops are slow in R and that I should try to do things in a vectorised manner instead.

But, why? Why are loops slow and apply i

Answer 1

Just as a comparison (don't read too much into it!): I ran a (very) simple for loop in R and in JavaScript in Chrome and IE 8. Note that Chrome does compilation to native code, and R with the compiler package compiles to bytecode.

# In R 2.13.1, this took 500 ms
f <- function() { sum<-0.5; for(i in 1:1000000) sum<-sum+i; sum }
system.time( f() )

# And the compiled version took 130 ms
library(compiler)
g <- cmpfun(f)
system.time( g() )

@Gavin Simpson: Btw, it took 1162 ms in S-Plus...

And the "same" code as JavaScript:

// In IE8, this took 282 ms
// In Chrome 14.0, this took 4 ms
function f() {
    var sum = 0.5;
    for(i=1; i<=1000000; ++i) sum = sum + i;
    return sum;
}

var start = new Date().getTime();
f();
time = new Date().getTime() - start;