[R] r-ish ? how can i improve my code?

[Sean O'Riordain]

Hi Patrick, Spencer,

Thanks for that!  Both your solutions are MUCH quicker!

afaik overwork_ratio = 4/pi # what a hard way to calculate this :-)

The real problem I'm actually working on is a little more complicated 
:-) - 6 'random' variables, and 18 dependant variables (at last 
count)... currently only 4 computed terms in the 'if() c<-c+1' 
statement...  I just created the x,y area problem as a simple example of 
the style I was using.

In the past I've worked on other 'accident' related problems and one 
difficulty was in getting enough final 'z' values to be statistically 
significant as there was an enormous rejection ratio - but it was 
running on a 486 written in compiled turbo pascal and it would take days 
of work...

I'll have a go at recoding using your techniques.

Many Thanks,
Sean


Patrick Burns wrote:
> For this particular problem you can probably use
> polar coordinates.
> 
> But something similar to your code could be:
> 
> x <- runif(900)
> y <- runif(900)
> z <- sqrt(x^2 + y^2)
> okay <- z < 1
> while(any(!okay)) {
>    n.bad <- sum(!okay)
>    x[!okay] <- runif(n.bad)
>    y[!okay] <- runif(n.bad)
>    z <- sqrt(x^2 + y^2)  # restricting to !okay may or may not be useful
>    okay <- z < 1
> }
> 
> 
> 
> Patrick Burns
> 
> Burns Statistics
> patrick at burns-stat.com
> +44 (0)20 8525 0696
> http://www.burns-stat.com
> (home of S Poetry and "A Guide for the Unwilling S User")
> 
> Sean O'Riordain wrote:
> 
>> Hi Folks,
>>
>> I'm trying to learn R.  One of my intentions is to do some Monte-Carlo 
>> type modelling of road "accidents".
>>
>> Below, to simplify things, I've appended a little program which does a 
>> 'monte-carlo' type simulation.  However, it is written in a way which 
>> seems a bit un-natural in R.  Could someone help me make this a bit 
>> more R-ish please?
>>
>> Or is there a completely different approach I should be taking?
>>
>> Many thanks in advance,
>> Sean O'Riordain
>> seanpor AT acm.org
>>
>>
>> --------------------------------------------
>> n <- 900; # number of valid items required...
>>
>> x <- numeric(n);
>> y <- numeric(n);
>> z <- numeric(n);
>>
>> c <- 1;  # current 'array' pointer
>> tc <- 0; # total items actually looked at...
>>
>> while (c <= n) {
>>     x[c] = runif(1, 0, 1);
>>     y[c] = runif(1, 0, 1);
>>
>>     z[c] = sqrt(x[c]^2 + y[c]^2);
>>     if (z[c] < 1)
>>         c <- c + 1;
>>     tc <- tc + 1;
>> }
>>
>> print("'overwork' ratio");
>> print(tc/(c-1));
>> plot(x,y);
>>
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>>
>>
> 
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