[R] sum variable as long condition is true

[Chuck Cleland]

marcg wrote:
> Hello R
> 
> For expierienced user, the following problem will be easy to solve:
> 
> a<-c(0,1,0,1,0,2,3,4,3,2)
> b<-c(3,3,3,4,4,4,7,7,7,10)
> c<-data.frame(a,b)
> 
> Data Frame c contains tow colums. I would like to sum up all values in a as long as b stays the same:
> 
> sum(a[which(b==1)])
> 
> does this, but i have to manually put in b
> 
> then i tryied st like this, but i canno't save it properly
> 
> for (i in 0:max(b)){
> 	i<-sum(a[which(b==i)])
> }
> 
> i tried to figure out, how tapply works, but neither

> with(c, tapply(a, list(b), sum))
 3  4  7 10
 1  3 10  2

> thanks alot
> 
> marc
> --
> 
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-- 
Chuck Cleland, Ph.D.
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