Tutorial: Debugging a Ruby Script
Finding out the origin of the problem
This tutorial shows how to debug a Ruby script for solving quadratic equations.
Run this script. In the console, enter the following: first quotient = 10, second quotient = 1, third quotient = 2. RubyMine reports a runtime error. It seems that we have to add a check of the discriminant value. Also, it would be useful to repeat the whole process more than once.
To avoid running into the same problem again, let's add an
if statement to check whether the discriminant is less that zero. To do that, select the statements next to discriminant calculation and then press Ctrl+Alt+T ( ).
Furthermore, let's add more checks - if the discriminant equals zero.
RubyMine creates a stub
if construct, leaving you with the task of filling it with the proper contents.
Finally, select all the statements of the script and repeat surrounding - this time, with the
While statement. You'll end up with the following code:
Let's take a closer look to see how the debugger can show your what your code is doing.
Debugging in detail
The Debug tool window shows dedicated panes for frames, variables, and watches, and the console, where all the input and output information is displayed. If you want the console to be always visible, you can drag it to one of the RubyMine window's edges.
We'll dig a little deeper into our code to find out what’s going wrong. We can use the RubyMine debugger to see exactly what’s happening in our code. To start debugging, you have to set some breakpoints first. To create breakpoints, just click in the left gutter:
If you want to see what your code does line by line, there's no need to put a breakpoint on every line, you can step through your code.
Let's see what it looks like to step through our example program: click the button, go to the Console to ask for the value, and we can see that we press our breakpoint.
We can use the stepping toolbar buttons to choose on which line we'd like to stop next.
For example, click the Step Over button () and see the blue marker moving to the next line of code:
If you want to concentrate on your own code, use the Step Into My Code button () - thus you'll avoid stepping into library classes.
RubyMine allows you to watch any variable. Just click on the toolbar of the Watches tab, and type the name of the variable you want to watch - let it be
discriminant. Note that code completion is available here:
At first, you see that discriminant equals nil - it means that the variable is not yet defined:
However, when the program execution continues to the scope that defines the variable, the watch gets the following view:
See Adding, Editing and Removing Watches section for details.
You may have noticed another RubyMine feature that makes it easy to see what your code is doing: the inline debugger. As soon as you press any breakpoint, RubyMine shows you the value of many of your variables right in the editor:
Finally, you can evaluate any expression at any time. For example, if you want to see the value of a variable, click the button.
Then in the dialog that opens, click Evaluate:
Actually, you could see the same thing with a watch. With evaluate expression you can do things that you can't do with a watch: you can change things.
For example, if you enter the desired value of the first quotient, say, 30, and then continue stepping through your script, you will get the following:
See the Evaluating Expressions section for details.
So, you've done it! Congrats! Let's repeat what you've done with the help of RubyMine:
Found out the origin of the problem
Stepped through your program
Created a watch
Evaluated an expression