[R] help with simple function

[Charles C. Berry]

On Tue, 27 May 2008, T.D.Rudolph wrote:

>
>
> I have a matrix of frequency counts from 0-160.
> x<-as.matrix(c(0,1,0,0,1,0,0,0,1,0,0,0,0,1))
>
> I would like to apply a function creating a new column (x[,2])containing
> values equal to:
> a) log(x[m,1]) if x[m,1] > 0; and
> b) for all x[m,1]= 0, log(next x[m,1] > 0 / count of preceding zero values
> +1)
>
> for example, x[1,2] should equal log(x[2,1]/2) = log(1/2) = -0.6931472
> whereas x[3,2] should equal log(x[5,1]/3) = log (1/3) = -1.098612
>

If you also intend that x[4,2] == x[3,2] in your example, then this seems 
what you want:

> rle.x <- rle(x[,1])
> num <- ifelse(rle.x$values == 0, c(tail(rle.x$values,-1),NA), rle.x$values )
> denom <- ifelse(rle.x$values == 0 , rle.x$lengths +1 , 1 )
> rep(log(num/denom),rle.x$lengths)
  [1] -0.6931472  0.0000000 -1.0986123 -1.0986123  0.0000000 -1.3862944 -1.3862944 -1.3862944  0.0000000 -1.6094379
[11] -1.6094379 -1.6094379 -1.6094379  0.0000000
>

See

 	?rep
 	?rle
 	?tail

HTH,

Chuck


> I will be applying this to nrow(x)=~70,000 so I would prefer to not do it by
> hand!
> Tyler
>
>
>
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Charles C. Berry                            (858) 534-2098
                                             Dept of Family/Preventive Medicine
E mailto:cberry at tajo.ucsd.edu	            UC San Diego
http://famprevmed.ucsd.edu/faculty/cberry/  La Jolla, San Diego 92093-0901