Level Up 7: Doesn't use magic numbers or constants, knows why

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1. Explain the problem with the use of magic numbers and constants.

The primary problem with magic numbers and constants is that we don't know their context. We don't know where they come from and their purpose. Furthermore, while the original developer may understand what they mean, in the future, he or other developers will be confused by their meaning. A secondary problem is that magic numbers and constants lead to code duplication. It is easy to continue reusing the same value throughout the code base without ensuring it is DRY.

2. Give an example of the use of 'magic numbers', and suggest a possible refactor.

Original:

class Circle
....
  def circumference(radius)
    radius * 3.1415
  end

  def area(radius)
    3.14 * radius ** 2
  end
....
end

Refactored:

class Circle
  PI = 3.1415
....
  def circumference(radius)
    radius * PI
  end

  def area(radius)
    PI * radius ** 2
  end
....
end

Another example is the use of 12 for the number of months in a year.

3. Personally, I see people get too focused on this one sometimes. Can you name an example where moving away from 'magic' values isn't actually helpful?

There can be times when the context of a magic number is super obvious and will only be used once. A good example is the robot name exercise. It contains two methods, one that generates a random number, and a second that generates a random character. The methods contain a range for possible numbers (1..10) and letters ('a'..'z'). We could move these magical ranges into constants and name them: "DIGITS" and "LETTER". But this would probably be overkill. These ranges are only used once in the entire program, and their purpose can be inferred by the methods they are located in: "generate_number" and "generate_character".