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Debugging with `println()` Guide

The First Computer Bug
[click the picture to read more]

Finding bugs in your programs is hard. It may be the most difficult part of programming. Of course, the simplest way of preventing bugs is to not have any in the first place! Planning the logic and anticipated input/output of your code before actually typing it into the interpreter is important. This has spawned the humorous adage:

Weeks of programming can save you hours of planning.

Of course, programs are written by people, and people make mistakes. Even the best programmers are not immune to buggy code. Computer scientists have developed a multitude of methods for finding errors in programs. The simplest—and the one you should concentrate on using—is called "print debugging".

Printing Variables

Consider the following code that exchanges the x and y values of a point:

size(100 , 100); strokeWeight(5); stroke(0); int x = 5; int y = 95; point(x , y); x = y; y = x; point(x , y);

The expected output is two points at opposite corners:

Screen Shot 2013-12-05 at 9.38.37 AM.png

However, the output produced is actually:

Screen Shot 2013-12-05 at 9.38.54 AM.png

Clearly the variables x and y are not what we expect them to be. So, what are they? Adding println(x); after each point in the program we change it will tell us just that (similarly for y):

size(100 , 100); strokeWeight(5); stroke(0); int x = 5; int y = 95; point(x , y); x = y; println(x); // for debugging y = x; println(y); // for debugging point(x , y);

This prints

95
95

to output—not what we expected. We've seen this error before.

Printing checkpoints

Another type of error occurs when your program crashes and you cannot easily determine where. To figure out where the problem is, you can add println() statements to your code at various points:

// ...a bunch of stuff... println("You got to point A."); // ...some more stuff... println("You got to point B."); // ...just a little bit more stuff... println("You got to point C."); // ...this is the last bit of stuff I promise...

If the output of your program states:

You got to point A.
You got to point B.

before crashing, clearly the problem lies in the area "...just a little bit more stuff...". The program execution never made it through these statements to print "You got to point C."

Debugging with println() Guide

Printing Variables

Printing checkpoints

Debugging with `println()` Guide