The Art of R Programming

5 mind <- function(d) {

6 n <- nrow(d)

7 add a column to identify row number for apply()

8 dd <- cbind(d,1:n)

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

10 wmins will be 2xn, 1st row being indices and 2nd being values

11 i <- which.min(wmins[1,])

12 j <- wmins[2,i]

13 return(c(d[i,j],i,j))

14 }

15

16 finds the location, value of the minimum in a row x

17 imin <- function(x) {

18 n <- length(x)

19 i <- x[n]

20 j <- which.min(x[(i+1):(n-1)])

21 return(c(j,x[j]))

22 }

Let’s use R’s debugging tools to find and fix the problems.

We’ll run it first on a small test case:

> source("cities.R")

> m <- rbind(c(0,12,5),c(12,0,8),c(5,8,0))

>m

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

[1,] 0 12 5

[2,] 12 0 8

[3,]580

> mind(m)

Error in mind(m) : subscript out of bounds

Not an auspicious start! Unfortunately, the error message doesn’t tell us

where the code blew up. But the debugger will give us that information:

> options(error=recover)

> mind(m)

Error in mind(m) : subscript out of bounds

Enter a frame number, or 0 to exit

1: mind(m)

Selection: 1

Called from: eval(expr, envir, enclos)

Browse[1]> where

296 Chapter 13