Debugging Using a Console Debugger

pry

We’re already familiar with pry, which is another read-execute-print-loop (REPL). pry implements more features than irb (Interactive Ruby, standard REPL). With a little bit of extra effort, we can use pry as a console debugger. Let’s look into the basics of debugging with pry. Knowing how to use this tool will save us a lot of time.

If pry is not installed on our system for any reason (we can check it with which pry), we can type a simple installation command:

$ gem install pry pry-doc

This command installs two gems: pry and pry-doc. The latter is a plugin for pry (normally, plugin names for pry start with the pry- prefix) and contains documentation about native Ruby methods.

We can type pry commands once pry is executed by just typing in our terminal, for example:

$ pry
> help

Try the above command in the following terminal.

We can get help for any command from the list above by adding -h, for example:

[1] pry(main)> whereami -h
Usage: whereami [-qn] [LINES]
...

We will describe our current location. If we use binding.pry inside a method, whereami will print out the source for it.

Program to square a number

Let’s look at the Ruby program that raises a number to the second power. So, for 2 the result is 4, for 3 it is 9, for 4 it is 16, etc…This operation equals to multiplying a number by itself one time:

Can we spot anything wrong above? ...