Adding "breakpoints" to help you debug a Ruby program

Sometimes when you are debugging a Ruby script, you want to do more than just print variables to debug. It’d be nice to pause the program at a particular moment and actually interact with all the variables that have been defined up until that point. Turns out, you can do this by adding breakpoints to your script.

a breakpoint is an intentional stopping or pausing place in a program, put in place for debugging purposes. — Wikipedia

Breakpoints can be added to your Ruby script using the pry library, which is a part of the standard Ruby library, but isn’t automatically loaded. You can load the library with require "pry".

Pry basics

Let’s say we have a program like this:

list_of_people = [
  { :name => "James", :age => 16 },
  { :name => "Yolanda", :age => 26 },
  { :name => "Mel", :age => 15 }
]

p "Enter an age and we'll tell you if we know a person who is that old:"

age_to_find = gets.chomp

list_of_people.each do |person|
  if person.fetch(:age) == age_to_find
    p "Found it!"
	end
end

Even when I enter an age that I know should be found, like 16, my program doesn’t print "Found it!" like I expect.

Using pry, I can use binding.pry to add a breakpoint before the if statement so I can interact more with the variables that were created.

binding.pry will pause the runtime of the program and open an IRB console

list_of_people = [
  { :name => "James", :age => 16 },
  { :name => "Yolanda", :age => 26 },
  { :name => "Mel", :age => 15 }
]

p "Enter an age and we'll tell you if we know a person who is that old:"

age_to_find = gets.chomp

list_of_people.each do |person|
	binding.pry
  if person.fetch(:age) == age_to_find
    p "Found it!"
	end
end

Now when you run the program it doesn’t complete—

You can use an IRB console to interact with the code that has run so far.

Now I can access variables like age_to_find and person that were defined before the breakpoint to see why the condition for my if statement never evaluates to true.

Ah, the ol’ forget-to-convert-a-String-into-an-Integer error. Gets me everytime 🤦. Now I know to convert age_to_find to an Integer.

You can type exit when you’re done to un-pause the programs execution.