ArgumentMethod arguments in Ruby are interesting because of the great flexibility in how you’re allowed to supply them to methods. Ruby method arguments can loosely be broken up into two categories, required arguments and optional arguments. However, I like to break them up into three categories (I hope it will become clear by the end of this post why I do so):

  • required arguments
  • arguments with default values
  • optional arguments

Required Arguments

These are just your stock standard method arguments, e.g.:

def some_method(a, b)

To call the method above you will need to supply two arguments to the method call, e.g.:


Pretty basic stuff, nothing much to see here, moving on :).

Arguments With Default Value

In Ruby you can supply a default value for an argument. This means that if a value for the argument isn’t supplied, the default value will be used instead, e.g.:

def some_method(a, b, c=25)

You may call the method above in one of two ways:



some_method(25,"hello", 48)

In the first case you don’t supply a value for the third parameter, so it’s default value (i.e. 25) will be used in the method body. In  the second case you do supply a value, so it will be used in place of the default value. Therefore, arguments with default values are a type of optional argument.

Optional Arguments

If you want to decide at runtime how many – if any – arguments you will supply to a method, Ruby allows you to do so. You need to use a special notation when you define the method, e.g.:

def some_method(*p)

You can call the above method with any number of arguments (including none), e.g.:





some_method(25,"hello", 45, 67)

All of those will work. If no arguments are supplied, then p will be an empty array, otherwise, it will be an array that contains the values of all the arguments that were passed in.

So, far it is all pretty basic stuff. The real fun begins when you need to mix and match the three types of arguments together.

Mixing And Matching The Various Types Of Arguments

What happens when you start to mix required arguments with optional arguments? Do they have to be in any specific order, and what gets assigned to what?

The easiest case is mixing a number of required arguments with the fully optional argument (i.e. the * notation), e.g.:

def some_method(a, b, *p)

You can call the above method with two or more values. The first two values will be assigned to arguments a and b, the rest will be assigned to p as an array, pretty simple. But, what if I wanted to do the following:

def some_method(a, b, *p, q)

In this case, you can call the above method with 3 or more values. When you call the above method, the required arguments get assigned first and if there are still any values left over they get assigned as an array to the optional argument, e.g.:

some_method(25,35,45,55) - a=25, b=35, p=[45], q=55
some_method(25,35,45) - a=25, b=35, p=[], q=45
some_method(25,35,45,55,65,75) - a=25, b=35, p=[45,55,65], q=75

Notice that the required arguments get assigned the value that corresponds to their order in the argument list, while the optional argument gets all the values that are left over that correspond to it’s order in the list.

Things can get even more involved if we introduce arguments with default values:

def some_method(a, b, c=5, *p, q)

In this case you can still call the above method with three or more values. When you make a call, all required arguments must get a value assigned, if there are more values left over, then the arguments with default values will get a value assigned to them, after that if there is still something left over, the optional argument will get those values as an array, e.g.:

some_method(25,35,45) - a=25, b=35, c=5, p=[], q=45
some_method(25,35,45,55) - a=25, b=35, c=45, p=[], q=55
some_method(25,35,45,55,65) - a=25, b=35, c=45, p=[55], q=65
some_method(25,35,45,55,65,75) - a=25, b=35, c=45, p=[55,65], q=75

Once again all arguments get assigned the values that correspond to their order in the argument list. And the arguments with a default value will get something assigned to them (if possible) before the fully optional argument gets any values.

It’s all pretty cool and it might seem like you can do anything with argument lists in Ruby, but there are some things to look out for. The only real hard and fast rule to remember is when you’re mixing optional parameters and default value parameters in the argument list. In this case, all default value parameters must occur before the optional parameter in the list, e.g.:

def some_method(a, b=5, *p) - correct
def some_method(a, *p, b=5) - incorrect!!!

If your optional parameter occurs before your default one, it is a syntax error, which makes sense if you think about it. And obviously, it makes no sense to have two optional arguments in an argument list (i.e. two parameters with * notation).

Feel free to leave a comment if you know of any other interesting things or caveats when it comes to Ruby method arguments.

Image by Anders V