undefined method `%' for nil:NilClass

I am getting "NoMethodError: undefined method `%' for nil:NilClass" for the following code block:

class Timer
    attr_accessor :seconds

    def initialize
        @seconds = 0
    end

    def time_string

        if seconds < 10
            return "00:00:0" + seconds.to_s
        elsif seconds < 60
            return "00:00:" + seconds.to_s
        elsif seconds < 540
            minutes = seconds / 60
            seconds %= 60
            #seconds = seconds - (minutes * 60)
            return "00:0" + minutes.to_s + ":0" + seconds.to_s
        end
    end

    def timer
        @timer          
    end
end

I know that 'seconds' is a Fixnum because I get NoMethod: Fixnum error when I try to #puts seconds without a #to_s. Also, the "/" operation on 'seconds' in the preceding line works fine. So why am I getting a NoMethod:nilclass error message?

Why am I even getting an error message? Shouldn't "%" work everywhere "/" does?

The following code works:

        if seconds < 10
            return "00:00:0" + seconds.to_s
        elsif seconds < 60
            return "00:00:" + seconds.to_s
        elsif seconds < 540
            minutes = @seconds / 60
            seconds = @seconds % 60
            return "00:0" + minutes.to_s + ":0" + seconds.to_s
        end

It has something to do with instance variables, and my not understanding instance variables. would love to know how the nil got in there.

Solution

It's an interaction between methods called on self, local variables, and Ruby's lack of syntactic distinction between those things.

If you change the line to this:

         self.seconds %= 60

Then it works fine.

The problem is that when Ruby sees an assignment to an unqualified name, it creates a local variable with that name rather than looking for an accessor.

Here's a simple demonstration:

irb(main):001:0> def foo=(n)
irb(main):002:1>  puts "Calling foo!"
irb(main):003:1>  @foo=n
irb(main):004:1> end    #=> nil
irb(main):005:0> foo=1    #=> 1
irb(main):006:0> @foo    #=> nil
irb(main):007:0> self.foo=2
Calling foo!
=> 2
irb(main):008:0> @foo    #=> 2