This cheat sheet describes Ruby features. It's not a reference to the language. You
do have a reference to the language: the full text of
Programming Ruby: The Pragmatic Programmer's Guide is installed with Ruby. Select
start -> All Programs -> Ruby -> Ruby Documentation -> RubyBook Help.
Comments
In Ruby, any text on a single line that follows a # is a comment, and is ignored by the Ruby interpreter at run time.
Function Calls
Parentheses can sometimes be omitted. If you're not sure whether they're required, put them in. To be safe, put them in whenever the call is at all complicated. Even one as simple as this.
puts "hello"
puts("hello")
assert_equal(5, number)Variables
Ordinary (local) variables are created through assignment:
Now the variable
number has the value
5. Ordinary variables begin with lowercase letters. After the first character, they can contain any alphabetical or numeric character. Underscores are helpful for making them readable:
A variable's value is gotten simply by using the name of the variable. The following has the value 10:
number + this_is_my_variable # returns 10
Strings, Objects and Methods
Strings are sequences of text in quotation marks. You can use single or double quotes.
name = 'simon'
name = "simon"
Strings are objects in Ruby. This means that they have methods. (In fact everything in Ruby is an object.)
A method call looks like this:
"simon".upcase # returns "SIMON"
A method is a function for a particular type of object. In this case, the thing before the period is the object, in this case a string
simon. The method is
upcase. It capitalizes its object.
Like functions, methods can have arguments.
"bookkeeper".include?('book') # returns trueThis method is
true if its argument
book is a substring of
bookkeeper.
You can also concatenate strings using the + operator.
"dog" + "house" # returns "doghouse"
Conditionals (if)
if number == 5
puts "Success" # "Success" is a string. Strings can be surrounded with single or double quotes.
else
puts "FAILURE"
end
Put the
if,
else, and
end on separate lines as shown. You don't have to indent, but you should.
Function Definitions
def assert_equal(expected, actual)
if expected != actual
puts "FAILURE!"
end
endFunctions can return values, and those values can be assigned to variables. The return value is the last statement in the definition. Here's a simple example:
def five # note that no parentheses are required
5
end
box = five # box's value is 5
Note that we didn't need to say
five(), as is required in some languages. You can put in the parentheses if you prefer.
The value of the last statement is always the value returned by the function. Some people like to include a
return statement to make this clear, but it doesn't change how the function works. This does the same thing:
Here's a little more complicated example:
def make_positive(number)
if number < 0
-number
else
number
end
end
variable = make_positive(-5) # variable's value is 5
variable = make_positive(five) # variable's value is 5Libraries
Libraries contain functions or methods that can be used in many Ruby programs. Suppose we store the
make_positive function defined above in a file called
mathplus.rb.
To use it in another script, we must
require it:
This will cause Ruby to search its
loadpath for a file named
mathplus.rb. (It will automatically add the
.rb.) It will search the directories that normally contain Ruby libraries, as well as the current directory (typically the same directory as your script).
If your library is in a location that Ruby doesn't know about, you will need to change the loadpath:
$LOAD_PATH << 'c:/my_lib/'
Make sure you include this line
before you require libraries in it.
Arrays
This is an array with nothing in it:
This is an array with two numbers in it:
This is an array with two numbers and a string in it. You can put anything into an array.
Here's how you get something out of an array:
array = [1, 'hello', 220]
array[0] # value is 1
Here's how you get the last element out:
Here's another way to get the last element:
array.last # value is 220
Here's how you change an element:
array[0]= 'boo!' # value printed is 'boo!'
# array is now ['boo', 'hello', 220]
How long is an array?
array.length # value is 3
Here's how you tack something onto the end of an array:
array.push('fred') # array is now ['boo', 'hello', 220, 'fred']Iteration
When you do something multiple times, it is called
iteration. There are many ways to do this. The following will print
hello five times:
5.times do
puts 'hello'
end
Here's one way to print the numbers from one to 10:
for x in 1..10 do
puts x
end
And here's another:
(1..10).each do |x|
puts x
end
The part between the
do and the
end is called a
block. You can replace the
do and
end with braces:
(1..10).each { |x| puts x }The
1..10 is a range, which works like an array of the numbers from 1 to 10. The
each is a method that iterates through each element of the range. It is called an
iterator.
This prints each value of an array:
["a", "b", "c"].each { |x| puts x }What if you want to transform each element of an array? The following capitalizes each element of an array.
["hi", "there"].collect { |word| word.capitalize } # The result is ["Hi", "There"].Regular Expressions
Regular expressions are a useful feature common to many languages. They allow you to match patterns in strings.
Regular expressions are characters surrounded by
// or
%r{}. A regular expression is compared to a string like this:
Most characters in a regular expression match the same character in a string. So, these all match:
/a/ =~ 'a string'
/a/ =~ 'string me along'
This also matches:
/as/ =~ 'a string with astounding length'
Notice that the regular expression can match anywhere in the string. If you want it to match only the beginning of the string, start it with a caret:
/^as/ =~ 'alas, no match'
If you want it to match at the end, end with a dollar sign:
/no$/ =~ 'no match, alas'
If you want the regular expression to match any character in a string, use a period:
/^.s/ =~ "As if I didn't know better!"
There are a number of other special characters that let you amazing and wonderful things with strings. Ruby uses the standard syntax for regular expressions used in many scripting languages. See
Programming Ruby for more information about regular expressions.
Truth and Falsehood
If you try the examples above, you'll see that the ones that match print a number. That's the position of the first character in the match. The first expression (
/a/ =~ 'a string') returns
0. (Ruby, like most programming languages, starts counting with 0.) The second returns
10.
What happens if there's no match? Type this:
/^as/ =~ 'alas, no match'
and the result will be
nil, signifying no match. You can use these results in an
if, like this:
if /^as/ =~ some_string
puts 'the string begins with "as".'
end
In Ruby, anything but the two special values
false and
nil are considered true for purposes of an
if statement. So match results like
0 and
10 count as
true.
Blocks
A block is like a function without a name. It contains a set of parameters and one or more lines of code. Blocks are used a lot in Ruby. Iterators like
each use blocks.
Here's how to search an array for an element:
gems = ['emerald', 'pearl', 'ruby']
gems.detect { |gem| /^r/ =~ gem } # returns "ruby" The
detect method takes a block as an argument. This block returns
true if the first argument starts with an
r. (Actually it returns
0, which counts as
true.) The
detect method itself returns the first element for which its block is true.
When blocks are longer than one line, they are usually written using
do and
end. This is another way of writing the same code:
gems.detect do |gem|
/^r/ =~ gem
end
Dictionaries
A
dictionary lets you say "Give me the
value corresponding to
key." Dictionaries are also called
hashes or
associative arrays.
Here's how you create a dictionary:
Here's how you associate a value with a key:
dict['bret'] = 'texas' # looks a lot like an array, except that the key doesn't have to be a number.
Here's how you retrieve a value, given a key:
dict['bret'] # value is 'texas'.
Here's how you ask how many key/value pairs are in the dictionary:
What values does a dictionary have?
dict.values # value is the Array ['texas'].
What keys does it have?
dict.keys # value is the Array ['bret'].
Expression Substitution
Expression substitution evaluates an expression within a string:
You can do this
name = "Aidy"
puts "My name is: #{name}"
=> My name is: AidyAnd this:
puts "The sum of four times four is: #{4*4}"
=> The sum of four times four is: 16
