[R] double vs. list

[Liaw, Andy]

> From: David Andel
> 
> Hi all
> 
> I've got trouble with getting a "list" object and not being able to 
> change that:
> 
> Lets say x is a simple matrix and I want to remove all 
> elements <3 to get
> 3
> 5 7
> 
>  > x <- matrix(c(1,2,3,5,1,7), nrow=2, ncol=3, byrow=T)
>  > x
>       [,1] [,2] [,3]
> [1,]    1    2    3
> [2,]    5    1    7
>  > typeof(x)
> [1] "double"
>  > y <- sapply(c(1,2), function(i) which(x[i,]<3))
>  > y
> [[1]]
> [1] 1 2
> 
> [[2]]
> [1] 2
> 
>  > typeof(y)
> [1] "list"
> 
> Of course I could remove these elements right away and produce a "x" 
> without elements <3, but also of type "list".
> But because rule will be more complicated than this I need to 
> know the 
> elements to remove explicitely first.
> 
> Now when trying to remove elements 1 and 2 from x[1,], I get an error:
>  > y[1]
> [[1]]
> [1] 1 2
> 
>  > x[1,][-y[1]]
> Error in -y[1] : Invalid argument to unary operator
> 
> But this works:
> 
>  > x[1,][-c(1,2)]
> [1] 3
> 
> What can I do differently?

Use double brackets to access a list component.  Single bracket gives you
back a list with one component.

I.e., try:

x[1,][-y[[1]]]

And BTW, isn't this really what you want?

> apply(x, 1, function(x) x[x>=3])
[[1]]
[1] 3

[[2]]
[1] 5 7

HTH,
Andy

> 
> Thanks,
> David
> 
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