Ruby: Equal Plus Not Equal to Plus Equal

27 January 2014 by Jess Brown

I was recency debugging an issue on an app. I had my test setup and properly failing. I was debugging and narrowing it down to the correct file. I wasn't getting an error or exception, so the specific line number wasn't known. I just knew where the general error was.

I scanned the file, only about 15 lines. I was looping through to some form fields and needed a counter like below:

- if job_skill.id.blank?
  - key = "key#{id}"
  - id =+ 1

If you paid attention to the title or have a sharp eye, you might have noticed the slight difference:

=+ != +=

I would have thought that =+ would cause some sort of exception, but it doesn't:

[1] pry(main)> i = 0
=> 0
[2] pry(main)> i =+ 1
=> 1
[3] pry(main)> i =+ 1
=> 1
[4] pry(main)> i =+ 1
=> 1
[5] pry(main)> i += 1
=> 2
[6] pry(main)> i += 1
=> 3
[7] pry(main)> i += 1
=> 4
[8] pry(main)> i += 1
=> 5
[9] pry(main)>

So, as you can see, my first two results were as expected, but then all future results overwrote the 2nd.

Nice little gotcha to watch out for!