[R] sequential treatment of a vector for formula

[David Winsemius]

On May 26, 2010, at 10:50 PM, David Winsemius wrote:

>
> On May 26, 2010, at 6:24 PM, Frostygoat wrote:
>
>> Please pardon the simplicity of this question of biological nature.
>> I'm trying to calculate a statistic, px, the proportion of a cohort
>> that survives through the interval x:x+1.  I have the vector from
>> which the calc is to be made but I can't figure out how to tell R to
>> take the current value and divide it by the next value.
>>
>> The formula is P0=L1/LO
>>
>> The following is an example of the lifetable I'm constructing:
>>
>> example
>> =as.vector(c(8,2,1,5,6,7,7,0,8,10,13,8,11,11,11,2,7,1,5,6,8,6))
>> #prime
>> k=1
>> #Deaths#
>> deaths=numeric(k)
>> for(k in 0:max(example))
>> 	{
>> 	deaths[k]=sum(example==k)}
>> #adjust for no zero!!#
>> deaths=c(0,deaths)
>> #Alive, Kx#
>> alive=sum(deaths)-cumsum(deaths)
>> #Day, or age class,x#
>> day=seq(from=0,to=length(deaths)-1)
>> #mortality, lx#
>> lx=alive/sum(deaths)
>> #proportion surviving to next interval, px#
>> #####Here's where I'm having trouble!!###
>> p=1
>> px=numeric(p)
>> for(p in 0:length(lx))
>> 	{
>> 	px[p]=lx/lx}
>>
>> #create data frame#
>> iC=data.frame("x"=day,"Kx"=alive,"Dx"=deaths,"Lx"=lx,"Px"=px)
>> print(iC)
>
> iC$Px <- with(iC, c(1, Lx[2:nrow(iC)]/Lx[1:(nrow(iC)-1)] ) )
>

This does the same but is more compact:
iC$Px2 <- with(iC, c(1, Lx[-1]/Lx[-nrow(iC)] ) )
  iC

> -- 
> David.
>
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