Week 3, lecture 8 notes

week-03/lecture-08-debugging-part-3.md

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+Debugging - Part 3
+==================
+
+traceback()
+-----------
+
+	> mean(x)
+	Error in mean(x) : object 'x' not found
+	> traceback()
+	1: mean(x)
+	>
+
+Example:
+
+	> lm(y - x)
+	Error in eval(expr, envir, enclos) : object 'y' not found
+	> traceback()
+	7: eval(expr, envir, enclos)
+	6: eval(prevars, data, env)
+	5: model.frame.default(formula = y - x, drop.unused.levels = TRUE)
+	4: model.frame(formula = y - x, drop.unused.levels = TRUE)
+	3: eval(expr, envir, enclos)
+	2: eval(mf, parent.frame())
+	1: lm(y - x)
+
+
+debug()
+-------
+
+	> debug(lm)
+	> lm(y - x)
+	debugging in: lm(y - x)
+	debug: {
+	    ret.x <- x
+	    ret.y <- y
+	    cl <- match.call()
+	…
+	    if (!qr) 
+	        z$qr <- NULL
+	    z
+	}
+	Browse[2]> n
+	debug: ret.x <- x
+	Browse[2]> n
+	debug: ret.y <- y
+	Browse[2]> n
+	debug: cl <- match.call()
+	Browse[2]> n
+	debug: mf <- match.call(expand.dots = FALSE)
+	Browse[2]> n
+	debug: m <- match(c("formula", "data", "subset", "weights", "na.action", 
+	    "offset"), names(mf), 0L)
+
+
+recover()
+---------
+
+	> options(error = recover)
+	> read.csv('nosuchfile')
+	Error in file(file, "rt") : cannot open the connection
+	In addition: Warning message:
+	In file(file, "rt") :
+	  cannot open file 'nosuchfile': No such file or directory
+
+	Enter a frame number, or 0 to exit   
+
+	1: read.csv("nosuchfile")
+	2: read.table(file = file, header = header, sep = sep, quote = quote, dec = dec, fil
+	3: file(file, "rt")
+
+	Selection: 
+
+
+Summary
+-------
+
+* There are three main indications of a problem/condition: message, warning, error
+	* Only an error is fatal
+* When analyzing a function with a problem, make sure you can reproduce the problem. clearly state your expectations and how the output differs from your expectation
+* Interactive debugging tools traceback, debug, browser, trace, and recover can be used to find problematic code in functions
+* Debugging tools are not a substitute for thinking!