The Art of R Programming

where 1: eval(expr, envir, enclos)

where 2: eval(quote(browser()), envir = sys.frame(which))

where 3 at cities.R#13: function ()

{

if (.isMethodsDispatchOn()) {

tState <- tracingState(FALSE)

...

Okay, so the problem occurred inmind()rather thanimin()and in par-

ticular at line 13. It still could be the fault ofimin(), but for now, let’s deal

with the former.

NOTE There is another way we could have determined that the blowup occurred on line 13.
We would enter the debugger as before but probe the local variables. We could reason
that if the subscript bounds error had occurred at line 9, then the variablewminswould
not have been set, so querying it would give us an error message likeError: object
'wmins' not found.On the other hand, if the blowup occurred on line 13, evenj
would have been set.

Since the error occurred withd[i,j], let’s look at those variables:

Browse[1]> d

[,1] [,2] [,3]

[1,] 0 12 5

[2,] 12 0 8

[3,]580

Browse[1]> i

[1] 2

Browse[1]> j

[1] 12

This is indeed a problem—donly has three columns, yetj, a column

subscript, is 12.

Let’s look at the variable from which we gleanedj,wmins:

Browse[1]> wmins

[,1] [,2]

[1,] 2 1

[2,] 12 12

If you recall how the code was designed, columnkofwminsis supposed

to contain information about the minimum value in rowkofd. So here

wminsis saying that in the first row (k=1)ofd,(0,12,5), the minimum value

is 12, occurring at index 2. But it should be 5 at index 3. So, something went

wrong with this line:

wmins <- apply(dd[-n, ], 1, imin)

Debugging 297