R help - Nov 2008 - calculating an N50

Given a set of integers of different values how do I calculate the
minimum number of the largest of integers that are required, when
summed, to equal 50% of the total sum of the the set?

For example,
> length(myTable$lgth)
[1] 303403
> sum(myTable$lgth)
[1] 4735396

I know through brute force that the N50 of the following set is 42 but
I don't know how to solve this without iteration
> sum(myTable$lgth[which(myTable$lgth>=42)])
[1] 2374260

> Given a set of integers of different values how do I calculate the
> minimum number of the largest of integers that are required, when
> summed, to equal 50% of the total sum of the the set?
>
Actually I need the value of the smallest member such that the
sum of all members equal or greater to that is 50% of the total sum of the set

Try this:
> tmp <- sample( 100, 50 )
>
> tmp2 <- rev( sort(tmp) )
>
> tmp3 <- cumsum(tmp2) <= sum(tmp)/2
>
> sum(tmp3) # number needed
[1] 14
>
> tmp2[ sum(tmp3) ] # the smallest value
[1] 78
>
> sum(tmp2[tmp3]) / sum(tmp) # check
[1] 0.4894614
> sum(tmp2[ 1:(sum(tmp3)+1) ]) / sum(tmp)
[1] 0.519516

Hope this helps,


--
Gregory (Greg) L. Snow Ph.D.
Statistical Data Center
Intermountain Healthcare
greg.snow at imail.org
801.408.8111

> -----Original Message-----
> From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-
> project.org] On Behalf Of Jeremy Leipzig
> Sent: Tuesday, November 25, 2008 10:45 AM
> To: r-help at r-project.org
> Subject: Re: [R] calculating an N50
>
> > Given a set of integers of different values how do I calculate the
> > minimum number of the largest of integers that are required, when
> > summed, to equal 50% of the total sum of the the set?
> >
> Actually I need the value of the smallest member such that the
> sum of all members equal or greater to that is 50% of the total sum of
> the set
>
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