Methods

In the context of programming, a function is a named sequence of statements that performs a computation. When you define a function, you specify the name and the sequence of statements. Later, you can “call” the function by name.

Ruby is an object-oriented programming language. Functions in Ruby are always associated with an object/class and are referred to as methods.

Method calls

We have already seen examples of method call:

>> puts 'Hello, World!'
Hello, World!
=> nil

The name of the method in above example is puts. The string passed is called the argument of the method.

It is common to say that a method “takes” an argument and “returns” a result. The result is also called the return value. The puts method always returns a special value nil.

Ruby provides methods that convert values from one type to another. The Integer method takes any value and converts it to an integer, if it can, or complains otherwise:

>> Integer('32')
=> 32
>> Integer('Hello')
ArgumentError (invalid value for Integer(): "Hello")

Integer can convert floating-point values to integers, but it doesn’t round off; it chops off the fraction part:

>> Integer(3.99999)
=> 3
>> Integer(-2.3)
=> -2

Float converts integers and strings to floating-point numbers:

>> Float(32)
=> 32.0
>> Float('3.14159')
=> 3.14159

Finally, String converts its argument to a string:

>> String(32)
=> "32"
>> String(3.14159)
=> "3.14159"

Math methods

Ruby has a Math module that provides most of the familiar mathematical functions. A module is a file that contains a collection of related methods. The Math module is available by default, later we’ll see modules which require loading.

To access one of the module methods, you have to specify the name of the module and the name of the method, separated by a dot (also known as a period). This format is called dot notation. To access one of the module variables, you have to specify module name and name of variable separated by two colons.

ratio = signal_power / noise_power
decibels = 10 * Math.log10(ratio)

radians = 0.7
height = Math.sin(radians)

The first example uses Math.log10 to compute a signal-to-noise ratio in decibels (assuming that signal_power and noise_power are defined). The Math module also provides log, which computes logarithms base e.

The second example finds the sine of radians. The name of the variable is a hint that sin and the other trigonometric methods (cos, tan, etc.) take arguments in radians. To convert from degrees to radians, divide by 180 and multiply by π:

>> degrees = 45
=> 45
>> radians = degrees / 180.0 * Math::PI
=> 0.7853981633974483
>> Math.sin(radians)
=> 0.7071067811865475

The expression Math::PI gets the variable PI from the Math module. Its value is a floating-point approximation of π, accurate to about 15 digits.

If you know trigonometry, you can check the previous result by comparing it to the square root of two divided by two:

>> Math.sqrt(2) / 2.0
=> 0.7071067811865476

Composition

So far, we have looked at the elements of a program—variables, expressions, and statements—in isolation, without talking about how to combine them.

One of the most useful features of programming languages is their ability to take small building blocks and compose them. For example, the argument of a method can be any kind of expression, including arithmetic operators:

x = Math.sin(degrees / 360.0 * 2 * Math::PI)

And even method calls:

x = Math.exp(Math.log(x+1))

Almost anywhere you can put a value, you can put an arbitrary expression, with one exception: the left side of an assignment statement has to be a variable name. Any other expression on the left side is a syntax error (we will see exceptions to this rule later).

>> minutes = hours * 60                 # right
>> hours * 60 = minutes                 # wrong!
SyntaxError: syntax error, unexpected '=', expecting end-of-input

Adding new methods

So far, we have only been using the methods that come with Ruby, but it is also possible to add new methods. A method definition specifies the name of a new method and the sequence of statements that run when the method is called.

Here is an example:

def print_lyrics()
  puts "I'm a lumberjack, and I'm okay."
  puts "I sleep all night and I work all day."
end

def is a keyword that indicates that this is a method definition. end keyword indicates end of method’s definition. The name of the method is print_lyrics. The rules for method names are similar to those for variable names: letters, numbers and underscore are legal, but the first character can’t be a number. Certain characters like ?, ! and = can appear at the end of method names, we'll discuss those in later chapters. Don’t use a keyword as the name of a method, and you should avoid having a variable and a method with the same name.

The empty parentheses after the name indicate that this method doesn’t take any arguments. In Ruby, parentheses are optional while defining and calling methods.

The first line of the method definition is called the header; the rest (excluding the end keyword) is called the body. By convention, the body is indented by two spaces. The body can contain any number of statements.

In the example above, the strings in the puts statement are enclosed in double quotes. Single quotes and double quotes do similar things; most people use single quotes except in cases like this where a single quote (which is also an apostrophe) appears in the string. Other differences between single and double quotes will be discussed later.

All quotation marks (single and double) must be “straight quotes”, usually located next to Enter on the keyboard. “Curly quotes”, like the ones in this sentence, are not legal in Ruby.

If you type a method definition in interactive mode, the interpreter displays a return value which is same as name of the method prefixed by a colon. That is a value of Symbol class and can be used as reference to the method.

>> def print_lyrics()
>>   puts "I'm a lumberjack, and I'm okay."
>>   puts "I sleep all night and I work all day."
>> end
=> :print_lyrics

The syntax for calling the new method is the same as for built-in methods:

>> print_lyrics()
I'm a lumberjack, and I'm okay.
I sleep all night and I work all day.
=> nil

Once you have defined a method, you can use it inside another method. For example, to repeat the previous refrain, we could write a method called repeat_lyrics:

def repeat_lyrics()
  print_lyrics()
  print_lyrics()
end

And then call repeat_lyrics:

>> repeat_lyrics()
I'm a lumberjack, and I'm okay.
I sleep all night and I work all day.
I'm a lumberjack, and I'm okay.
I sleep all night and I work all day.
=> nil

But that’s not really how the song goes.

Definitions and uses

Pulling together the code fragments from the previous section, the whole program looks like this:

def print_lyrics()
  puts "I'm a lumberjack, and I'm okay."
  puts "I sleep all night and I work all day."
end

def repeat_lyrics()
  print_lyrics()
  print_lyrics()
end

repeat_lyrics()

This program contains two method definitions: print_lyrics and repeat_lyrics. Method definitions get executed just like other statements. The statements inside the method do not run until the method is called, and the method definition generates no output.

As you might expect, you have to create a method before you can run it. In other words, the method definition has to run before the method gets called.

As an exercise, move the last line of this program to the top, so the method call appears before the definitions. Run the program and see what error message you get.

Now move the method call back to the bottom and move the definition of print_lyrics after the definition of repeat_lyrics. What happens when you run this program?

Flow of execution

To ensure that a method is defined before its first use, you have to know the order statements run in, which is called the flow of execution.

Execution always begins at the first statement of the program. Statements are run one at a time, in order from top to bottom.

Method definitions do not alter the flow of execution of the program, but remember that statements inside the method don’t run until the method is called.

A method call is like a detour in the flow of execution. Instead of going to the next statement, the flow jumps to the body of the method, runs the statements there, and then comes back to pick up where it left off.

That sounds simple enough, until you remember that one method can call another. While in the middle of one method, the program might have to run the statements in another method. Then, while running that new method, the program might have to run yet another method!

Fortunately, Ruby is good at keeping track of where it is, so each time a method completes, the program picks up where it left off in the method that called it. When it gets to the end of the program, it terminates.

In summary, when you read a program, you don’t always want to read from top to bottom. Sometimes it makes more sense if you follow the flow of execution.

Parameters and arguments

Some of the methods we have seen require arguments. For example, when you call Math.sin you pass a number as an argument. Some methods take more than one argument: Math.hypot takes two, the sides of a right-angled triangle.

Inside the method, the arguments are assigned to variables called parameters. Here is a definition for a method that takes an argument:

def print_twice(bruce)
  puts bruce
  puts bruce
end

This method assigns the argument to a parameter named bruce. When the method is called, it prints the value of the parameter (whatever it is) twice.

This method works with any value that can be printed.

>> print_twice('Spam')
Spam
Spam
=> nil
>> print_twice(42)
42
42
=> nil
>> print_twice(Math::PI)
3.141592653589793
3.141592653589793
=> nil

The same rules of composition that apply to built-in methods also apply to programmer-defined methods, so we can use any kind of expression as an argument for print_twice:

>> print_twice('Spam '*4)
Spam Spam Spam Spam 
Spam Spam Spam Spam 
=> nil
>> print_twice(Math.cos(Math::PI))
-1.0
-1.0
=> nil

The argument is evaluated before the method is called, so in the examples the expressions 'Spam '*4 and Math.cos(Math::PI) are only evaluated once.

You can also use a variable as an argument:

>> michael = 'Eric, the half a bee.'
=> "Eric, the half a bee."
>> print_twice(michael)
Eric, the half a bee.
Eric, the half a bee.
=> nil

The name of the variable we pass as an argument (michael) has nothing to do with the name of the parameter (bruce). It doesn’t matter what the value was called back home (in the caller); here in print_twice, we call everybody bruce.

Variables and parameters are local

When you create a variable inside a method, it is local, which means that it only exists inside the method. For example:

def cat_twice(part1, part2)
  cat = part1 + part2
  print_twice(cat)
end

This method takes two arguments, concatenates them, and prints the result twice. Here is an example that uses it:

>> line1 = 'Bing tiddle '
=> "Bing tiddle "
>> line2 = 'tiddle bang.'
=> "tiddle bang."
>> cat_twice(line1, line2)
Bing tiddle tiddle bang.
Bing tiddle tiddle bang.
=> nil

When cat_twice terminates, the variable cat is destroyed. If we try to print it, we get an exception:

>> puts cat
NameError: undefined local variable or method `cat' for main:Object

Parameters are also local. For example, outside print_twice, there is no such thing as bruce.

Stack diagrams

To keep track of which variables can be used where, it is sometimes useful to draw a stack diagram. Like state diagrams, stack diagrams show the value of each variable, but they also show the method each variable belongs to.

Each method is represented by a frame. A frame is a box with the name of a method beside it and the parameters and variables of the method inside it. The stack diagram for the previous example is shown below.

Figure 3.1: Stack diagram.

The frames are arranged in a stack that indicates which method called which, and so on. In this example, print_twice was called by cat_twice, and cat_twice was called by main, which is a special name for the topmost frame. When you create a variable outside of any method, it belongs to main.

Each parameter refers to the same value as its corresponding argument. So, part1 has the same value as line1, part2 has the same value as line2, and bruce has the same value as cat.

If an error occurs during a method call, Ruby prints the name of the method, the name of the method that called it, and the name of the method that called that, all the way back to main.

For example, if you try to access cat from within print_twice, you get a NameError:

Traceback (most recent call last):
    2: from test.rb:14:in `<main>'
    1: from test.rb:9:in `cat_twice'
test.rb:4:in `print_twice': undefined local variable
        or method `cat' for main:Object (NameError)

This list of methods is called a traceback. It tells you what program file the error occurred in, and what line, and what methods were executing at the time.

The order of the methods in the traceback is the same as the order of the frames in the stack diagram. The method that is currently running is at the bottom.

Fruitful methods and void methods

Some of the methods we have used, such as the Math methods, return results; for lack of a better name, I call them fruitful methods. Other methods, like print_twice, perform an action but don’t return a value. They are called void methods.

When you call a fruitful method, you almost always want to do something with the result; for example, you might assign it to a variable or use it as part of an expression:

x = Math.cos(radians)
golden = (Math.sqrt(5) + 1) / 2

When you call a method in interactive mode, Ruby displays the result:

>> Math.sqrt(5)
=> 2.23606797749979

But in a script, if you call a fruitful method all by itself, the return value is lost forever!

Math.sqrt(5)

This script computes the square root of 5, but since it doesn’t store or display the result, it is not very useful.

Void methods might display something on the screen or have some other effect. If you assign the result to a variable, by default you get the result of last evaluated expression of the method.

>> result = print_twice('Bing')
Bing
Bing
=> nil
>> result
=> nil

The value nil is not the same as the string 'nil'. It is a special value that has its own type:

>> nil.class
=> NilClass

The methods we have written so far are all void. It so happened that the last expression in all of them was puts method which always returns nil. We will start writing fruitful methods in a few chapters.

Why methods?

It may not be clear why it is worth the trouble to divide a program into methods. There are several reasons:

Creating a new method gives you an opportunity to name a group of statements, which makes your program easier to read and debug.
Methods can make a program smaller by eliminating repetitive code. Later, if you make a change, you only have to make it in one place.
Dividing a long program into methods allows you to debug the parts one at a time and then assemble them into a working whole.
Well-designed methods are often useful for many programs. Once you write and debug one, you can reuse it.

Debugging

One of the most important skills you will acquire is debugging. Although it can be frustrating, debugging is one of the most intellectually rich, challenging, and interesting parts of programming.

In some ways debugging is like detective work. You are confronted with clues and you have to infer the processes and events that led to the results you see.

Debugging is also like an experimental science. Once you have an idea about what is going wrong, you modify your program and try again. If your hypothesis was correct, you can predict the result of the modification, and you take a step closer to a working program. If your hypothesis was wrong, you have to come up with a new one. As Sherlock Holmes pointed out, “When you have eliminated the impossible, whatever remains, however improbable, must be the truth.” (A. Conan Doyle, The Sign of Four)

For some people, programming and debugging are the same thing. That is, programming is the process of gradually debugging a program until it does what you want. The idea is that you should start with a working program and make small modifications, debugging them as you go.

For example, Linux is an operating system that contains millions of lines of code, but it started out as a simple program Linus Torvalds used to explore the Intel 80386 chip. According to Larry Greenfield, “One of Linus’s earlier projects was a program that would switch between printing AAAA and BBBB. This later evolved to Linux.” (The Linux Users’ Guide Beta Version 1).

Glossary

method:
A named sequence of statements that performs some useful operation. Methods may or may not take arguments and may or may not produce a result.
method definition:
A statement that creates a new method, specifying its name, parameters, and the statements it contains.
header:
The first line of a method definition.
body:
The sequence of statements inside a method definition.
parameter:
A name used inside a method to refer to the value passed as an argument.
method call:
A statement that runs a method. It consists of the method name followed by an argument list in parentheses.
argument:
A value provided to a method when the method is called. This value is assigned to the corresponding parameter in the method.
local variable:
A variable defined inside a method. A local variable can only be used inside its method.
return value:
The result of a method. If a method call is used as an expression, the return value is the value of the expression.
fruitful method:
A method that returns a value.
void method:
A method that doesn’t explicitly return a value.
nil:
A special value to indicate absence of a value.
module:
A file that contains a collection of related methods and other definitions.
dot notation:
The syntax for calling a method in another module by specifying the module name followed by a dot (period) and the method name.
composition:
Using an expression as part of a larger expression, or a statement as part of a larger statement.
flow of execution:
The order statements run in.
stack diagram:
A graphical representation of a stack of methods, their variables, and the values they refer to.
frame:
A box in a stack diagram that represents a method call. It contains the local variables and parameters of the method.
traceback:
A list of the methods that are executing, printed when an exception occurs.

Exercises

Exercise 1
Write a method named right_justify that takes a string named s as a parameter and prints the string with enough leading spaces so that the last letter of the string is in column 70 of the display.

>>> right_justify('hello')
                                                                 hello

Hint: Use string concatenation and repetition. Also, Ruby provides a built-in String method called length that returns the length of a string, so the value of 'hello'.length is 5.

Exercise 2
Return value of a method definition is a symbol value you can assign to a variable or pass as an argument. For example, do_twice is a method that takes a symbol as an argument and calls it twice:

def do_twice(s)
  m = method(s)
  m.call
  m.call
end

Here’s an example that uses do_twice to call a method named print_spam twice.

def print_spam()
  puts 'spam'
end

do_twice(:print_spam)

Type this example into a script and test it.
Modify do_twice so that it takes two arguments, a method’s symbol and a value, and calls the method twice, passing the value as an argument.
Copy the definition of print_twice from earlier in this chapter to your script.
Use the modified version of do_twice to call print_twice twice, passing 'spam' as an argument.
Define a new method called do_four that takes a method’s symbol and a value and calls the method four times, passing the value as a parameter. There should be only two statements in the body of this method, not four.

Exercise 3
Note: This exercise should be done using only the statements and other features we have learned so far.

Write a method that draws a grid like the following:

+ - - - - + - - - - +
|         |         |
|         |         |
|         |         |
|         |         |
+ - - - - + - - - - +
|         |         |
|         |         |
|         |         |
|         |         |
+ - - - - + - - - - +

Hint: Use string concatenation and repetition. By defining a method that accepts two characters as arguments, you could get both types of lines.

Write a method that draws a similar grid with four rows and four columns.

Credit: This exercise is based on an exercise in Oualline, Practical C Programming, Third Edition, O’Reilly Media, 1997.

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methods.md

methods.md

Methods

Method calls

Math methods

Composition

Adding new methods

Definitions and uses

Flow of execution

Parameters and arguments

Variables and parameters are local

Stack diagrams

Fruitful methods and void methods

Why methods?

Debugging

Glossary

Exercises

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Methods

Method calls

Math methods

Composition

Adding new methods

Definitions and uses

Flow of execution

Parameters and arguments

Variables and parameters are local

Stack diagrams

Fruitful methods and void methods

Why methods?

Debugging

Glossary

Exercises