Underscore on predictions

using _variableName accesses an instance variable directly, without using getter and setter methods. When you create a property (such as your predictions array property) an instance variable is created behind the scenes that has the form of _variableName. you can access that directly using _variableName, or you can use getter and setter methods that were created as well (variableName and setVariableName are the getter and setter methods which can be called using self.variableName or [self variableName] and [self setVariableName] ).

When you write your own custom getter and setter methods (you are writing a custom getter in the case of your predictions method) , you need to reference the instance variable directly so you must use _predictions. you dont want to access the instance variable using the getter/setter methods if you are inside the getter/setter method itself.