[R] functions of vectors : loop or vectorization

[Uwe Ligges]

On 14.07.2012 00:41, Julien Salanie wrote:
> I have a read a lot about the benefits of vectorization in R. I have a
> program that takes "almost forever" to run. A good way to see if I have
> learned something ... My problem can be summarized like this : I have a
> nonlinear function of several variables that I want to optimize over one
> letting the other describe a family of curves. In short, I wan't to optimize
> f(x,a,b) for several values of a and b.
>
> It is easily done with a loop. Here's an example :
>
> a = 1:5;
> b = 1:5;
> myfunction = function(x){y*x-(x+z)^2};
> myresults = array(dim=c(length(a),length(b)));
> for(y in a){ for(z in b) { myresults[y,z] =
> optimize(myfunction,c(-10,10),maximum=TRUE)$maximum }};
> myresults;
>
> [,1] [,2] [,3] [,4] [,5]
> [1,] -0.5 -1.5 -2.5 -3.5 -4.5
> [2,]  0.0 -1.0 -2.0 -3.0 -4.0
> [3,]  0.5 -0.5 -1.5 -2.5 -3.5
> [4,]  1.0  0.0 -1.0 -2.0 -3.0
> [5,]  1.5  0.5 -0.5 -1.5 -2.5
>
> Of course, my real life problem is a bit more complicated and runs in days
> ...
>
> I didn't find a straightforward way to do this using the apply family. I did
> a small script that works. Here it is :
>
> c = 1:5;
> d = 1:5;
> myfunction2 =
> function(c,d){optimize(function(x){c*x-(x+d)^2},c(-10,10),maximum=TRUE)$maximum};
> v.myfunction2 = Vectorize(myfunction2, c("c","d"));
> outer(c, d, v.myfunction2);
>
> all.equal(myresults,outer(c, d, v.myfunction2));
> [1] TRUE
>
> I was quite happy with my trick of separating and wrapping the functions
> until I increased the size of the two input vectors and checked for the
> processing time. I made no gain. In that case :
>
>> time.elapsed; time.elapsed2;
> Time difference of 0.08000016 secs
> Time difference of 0.07999992 secs
>
> When I changed the size of the vectors and added a logarithm here and there
> to complicate a bit, it doesn't change the problem. The two methods perform
> identically. Am I missing something ? Is there a better way to vectorize the
> problem to gain time ? How is it that my loop performs as well as "outer" ?
> Thanks in advance for your help. All the best, Julien

 From what I understand you can't without loop. Actually, the 
Vectorize() call generates a function that calls mapply() which is some 
kind of a loop. This may be a case where the transition of inner loops 
to C may be benefitial.

Best,
Uwe Ligges






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