Debugging

Using browser

The browser function can be used like a breakpoint: code execution will pause at the point it is called. Then user can then inspect variable values, execute arbitrary R code and step through the code line by line.

Once browser() is hit in the code the interactive interpreter will start. Any R code can be run as normal, and in addition the following commands are present,

Command	Meaning
c	Exit browser and continue program
f	Finish current loop or function \
n	Step Over (evaluate next statement, stepping over function calls)
s	Step Into (evaluate next statement, stepping into function calls)
where	Print stack trace
r	Invoke “resume” restart
Q	Exit browser and quit

For example we might have a script like,

toDebug <- function() {
    a = 1
    b = 2
    
    browser()
    
    for(i in 1:100) {
        a = a * b
    }
}

toDebug()

When running the above script we initially see something like,

Called from: toDebug
Browser[1]>

We could then interact with the prompt as so,

Called from: toDebug
Browser[1]> a
[1] 1
Browser[1]> b
[1] 2
Browse[1]> n
debug at #7: for (i in 1:100) {
    a = a * b
}
Browse[2]> n
debug at #8: a = a * b
Browse[2]> a
[1] 1
Browse[2]> n
debug at #8: a = a * b
Browse[2]> a
[1] 2
Browse[2]> Q

browser() can also be used as part of a functional chain, like so:

mtcars %>% group_by(cyl) %>% {browser()}

Using debug

You can set any function for debugging with debug.

debug(mean)
mean(1:3)

All subsequent calls to the function will enter debugging mode. You can disable this behavior with undebug.

undebug(mean)
mean(1:3)

If you know you only want to enter the debugging mode of a function once, consider the use of debugonce.

debugonce(mean)
mean(1:3)
mean(1:3)