I hope I'm not missing something with my brief "comment-as-an-answer" type answer, but perhaps you're just looking for:
za.gnp@bpoint
# [1] 21
To make this less of a comment...
The str
function is usually very helpful in understanding the structure of your data. Here are the first and last few lines of str
on "za.gnp".
> str(za.gnp)
Formal class 'ur.za' [package "urca"] with 10 slots
..@ y : num [1:62] 117 120 123 130 131 ...
..@ model : chr "both"
..@ lag : int 2
..@ teststat : num -3.84
..@ cval : num [1:3] -5.57 -5.08 -4.82
..@ bpoint : int 21
..@ tstats : num [1:61] -1.29 -1.29 -1.29 -1.4 -1.41 ...
..@ res : Named num [1:59] 7.056 0.116 -5.917 -0.341 7.981 ...
.. ..- attr(*, "names")= chr [1:59] "4" "5" "6" "7" ...
..@ testreg :List of 13
<<SNIP>>
.. .. .. .. .. ..- attr(*, "names")= chr [1:7] "y" "y.l1" "trend" "y.dl1" ...
.. .. ..- attr(*, "na.action")=Class 'omit' Named int [1:3] 1 2 3
.. .. .. .. ..- attr(*, "names")= chr [1:3] "1" "2" "3"
.. ..- attr(*, "class")= chr "lm"
..@ test.name: chr "Zivot-Andrews"