I am trying to run the following array method using ruby but keep getting the following error:

arr = [2,4,6,8]
i = 0
while i < arr.length do
  puts arr[i + 1] - arr[i]
  i = i + 1
end

It puts out the values on the console but also issues an error

ERROR (on RubyMine 7): C:\Ruby21\bin\ruby.exe -e $stdout.sync=true;$stderr.sync=true;load($0=ARGV.shift) C:/.../file/application.rb 2 2 2 C:/Users/inoor/RubymineProjects/file/application.rb:4:in <top (required)>': undefined method-' for nil:NilClass (NoMethodError) from -e:1:in load' from -e:1:in'

Process finished with exit code 1

Can someone please help me understand what's going on?

Solution

Whenever you see the nil:NilClass (NoMethodError) error in ruby you should look for whatever objects could have become nil. In your case, you are iterating over an array and are hitting an edge case.

Let's look at your code in detail:

# Start with this array of length 4
arr = [2,4,6,8]
# Initilize counter to 0
i = 0
# Run this loop while i < 4
while i < arr.length do
  puts arr[i + 1] - arr[i]
  # Increment by 1 each time
  i = i + 1
end

The case you are running into the nil object is at the very end. When i=3, i+1=4 and you are trying to access the index 4 element in your array. Remember, arrays in ruby (like most languages) are 0-indexed. So your array has indexes 0,1,2, and 3. If you attempt to access an index that you haven't defined a value for, ruby will return nil. So arr[4],arr[400], and arr[100000000] all return the same thing: nil.