[R] Extending each element in a list, or rbind()-ing arrays of different length without recycling

[Jason Shaw]

Hi,

I'm trying to take a matrix such as

     [,1] [,2] [,3] [,4] [,5]
[1,]    2    7    2    7    9
[2,]   10   10    6    8    6
[3,]    1    9    7    2    0

and generate a new matrix which contains only the unique values in each row:

     [,1] [,2] [,3] [,4] [,5]
[1,]    2    7    9   NA   NA
[2,]   10    6    8   NA   NA
[3,]    1    9    7    2    0

My problem is that I can use apply(matrix,MARGIN=1,FUN=unique) to find
the unique values, but this leaves me with a list with arrays of
different length:

> x <- apply(peaks,MARGIN=1,FUN=unique)
[[1]]
[1] 2 7 9

[[2]]
[1] 10  6  8

[[3]]
[1] 1 9 7 2 0

and using do.call("rbind",x) recycles the values of the shorter
vectors instead of filling them with NA:

> do.call("rbind",x)
     [,1] [,2] [,3] [,4] [,5]
[1,]    2    7    9    2    7
[2,]   10    6    8   10    6
[3,]    1    9    7    2    0

So, I'd like to either take every element of the list and extend it
with NAs to the length of the longest element, or rbind every element
where missing places are filled with NAs instead of recycled values.
Is this possible?  Of course, the solution is trivial using a loop,
but I'm trying to avoid this.

Thanks for any suggestions.