I guess I am trying to show you two things at once.

First, z and zz are identical except I added a call to which() for z. So z tells us 'which of the rows have fewer than 10 items that are greater than zero?', and zz gives us a boolean vector with TRUE if there are fewer than 10 items that are greater than zero, and FALSE otherwise. Since there are no rows that fulfill this criterion, zz is a vector of all FALSE, and z is an integer(0) because there are no TRUE values (e.g., when you say 'which of these rows fulfill this criterion, you get an integer(0) because none of them do so).

When you do something like

dat[-z,]

and z is a vector of numbers, say (1,3,4,5,7) you are really doing

dat[c(-1,-3,-4,-5,-7),]

As an example:

> z <- c(1,3,4,5,7)
> dat<- matrix(1:10, ncol=10, nrow = 10)
> dat[-z,]
     [,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8] [,9] [,10]
[1,]    2    2    2    2    2    2    2    2    2     2
[2,]    6    6    6    6    6    6    6    6    6     6
[3,]    8    8    8    8    8    8    8    8    8     8
[4,]    9    9    9    9    9    9    9    9    9     9
[5,]   10   10   10   10   10   10   10   10   10    10
> dat[c(-1,-3,-4,-5,-7),]
     [,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8] [,9] [,10]
[1,]    2    2    2    2    2    2    2    2    2     2
[2,]    6    6    6    6    6    6    6    6    6     6
[3,]    8    8    8    8    8    8    8    8    8     8
[4,]    9    9    9    9    9    9    9    9    9     9
[5,]   10   10   10   10   10   10   10   10   10    10

But if z is integer(0), then you return NOTHING because the negation of integer(0) is integer(0)! So you need to use zz, which is the boolean representation of what you are trying to do. It is usually safer to use a boolean vector rather than a vector of rows for this reason.

Rather than belabor the point further, as this is really just basic R stuff, I highly recommend you read 'An Introduction to R', which you can find at r-project.org. If you are having problems with basic subsetting of things, then you need to get the basics down first.

The second point I am trying to make is this; you already know that your filtering criterion will not filter out any rows. But then you go ahead and use a subsetting function when you know it should not filter anything out. So you are doing something that you expect will do nothing. I suppose in a pedantic sense it is useful to see if a filtering criterion that isn't supposed to filter will not do so, but from a 'trying to get things done' perspective it isn't really a useful exercise.