I see all sorts of advice about getting rid of the brackets, but as I perform my beginning ruby lessons, I want to see if I'm creating a 2-D array correctly.
Right now I'm limited to
puts my_2D_array[0]
puts my_2D_array[1] #etc
but I want to see
[ [6,6,3] , [7,4,7] , [4,7,4] ]
instead of what I'm getting, which is
663747474
What's the trick? And I'm definitely dealing with an array - not a string...
p array will give you output you've asked for:
[[6,6,3] , [7,4,7] , [4,7,4]]
Want each row on a separate line? Then use:
array.each {|e| p e}
to get:
[6,6,3]
[7,4,7]
[4,7,4]
You could add a method to the Array class:
class Array
def ppa # 'pretty-print array'
self.each {|e| p e} # or just 'each {|e| p e}'
end
end
that wold allow you to write
array.ppa
and get the same three-line output. (You could use puts and inspect instead of p.) Think you might use this often? Then put this code in a file called, say, 'array_print.rb' and add 'require array_print' to the beginning of your '.rb' code file. Each time you run your program, the statements in array_print.rb will be executed, making the Array method ppr available to you.
Let's not stop there! Suppose you also wanted nicely-formatted output for three-dimensional arrays, hashes, hashes of arrays, and so on. You could elaborate on the approach I've described above, but why reinvent the wheel? There are several excellent Ruby gems available that take care of all of this for you. One popular one is "awesome print". After having installed this gem, all you need do is add require 'awesome_print' in your code file. You can then use its ap method to format your output. (See RubyGems for instructions on how to install gems. It's easy).
To get a taste of what awesome print does, suppose that instead of the array above you wanted to display this hash:
hash = {"cat"=>["mice", "birds"], "dog"=>["master",["kids", "moms"]]}
By executing ap hash, you'd get this:
{
"cat" => [
[0] "mice",
[1] "birds"
],
"dog" => [
[0] "master",
[1] [
[0] "kids",
[1] "moms"
]
]
}
Given the example:
def method_of_doom
my_string = "I sense impending doom."
my_string.ah_ha_i_called_a_nonexistent_method
rescue NoMethodError => e:
puts "PROBLEM: " + e.to_s
rescue Exception:
puts "Uhh...there's a problem with that there method."
end
On the line where it says:
rescue NoMethodError => e:
What is the '=>' doing?
How is it different than this usage:
module FighterValues
BAMBOO_HEAD = { 'life' => 120, 'hit' => 9 }
DEATH = { 'life' => 90, 'hit' => 13 }
KOALA = { 'life' => 100, 'hit' => 10 }
CHUCK_NORRIS = { 'life' => 60000, 'hit' => 99999999 }
def chuck_fact
puts "Chuck Norris' tears can cure cancer..."
puts "Too bad he never cries."
end
end
module ConstantValues
DEATH = -5 # Pandas can live PAST DEATH.
EASY_HANDICAP = 10
MEDIUM_HANDICAP = 25
HARD_HANDICAP = 50
end
puts FighterValues::DEATH
→ {'life'=>90,'hit'=>13}
puts ConstantValues::DEATH
→ -5
The Hash Rocket is a Syntactic Token
The hash rocket is actually a syntactic token. You can find the token in the grammar defined by ext/ripper/ripper.y:
%token tASSOC /* => */
In other words, Ripper uses the hash rocket to associate things.
How tASSOC is Used
In general, this token is used in hash literals to associate a key with a value. For example:
{ :e => 'foo' }
associates the string literal foo with the symbol :e. This common usage is why people tend to think of the hash rocket as solely a hash-related construct.
On the other hand, the following associates a variable with an exception:
rescue => e
In this case, rather than associating a key with a value, Ripper is associating the variable e with the implied StandardError exception, and uses the variable to store the value of Exception#message.
Further Reading
If you understand tokenizers, lexers, and parsers, ripper.y and the various contents of ext/ripper/lib/ripper are instructive. However, on page 19 of Ruby Under a Microscope, Pat Shaughnessy warns:
Ruby doesn’t use the Lex tokenization tool, which C programmers commonly use in conjunction with a parser generator like Yacc or Bison. Instead, the Ruby core wrote the Ruby tokenization code by hand.
Just something to keep in mind when you're trying to grok Ruby's grammar at the source code level.
There are a bunch of good links on the Ruby Info page.
It depends on context.
In the context of a rescue it means:
"Assign the exception object to the variable e."
This is how it can be used as e.to_s later.
In a Hash literal it means:
A pair, represented by key=>value.
Here is a Hash literal is created from two pairs: {:name => "Fred", :age => 20}
(Ruby 1.9/2.0+ also allows {name: "Fred", age: 20} syntax, where name and age refer to Symbols.)
In a String, it is what it is:
"=>Whee!".
In this case puts FighterValues::DEATH, is equivalent to puts FighterValues::DEATH.to_s. That is, the output displayed comes from a string. Consider this: puts "{a => b}".
I started to put print-statements throughout my code. So as not to clutter up the output, I did something like:
dputs LEVEL, "string"
where LEVEL is 0 for errors, 1 for important .. 5 for verbose and is compared to DEBUG_LEVEL. Now my problem is, that in a statement like:
dputs 5, "#{big_class.inspect}"
the string is always evaluated, also if I set DEBUG_LEVEL to 1. And this evaluation can take a long time. My favourite solution would be something like:
dputs 5, '#{big_class.inspect}'
and then evaluate the string if desired. But I don't manage to get the string in a form I can evaluate. So the only think I could come up with is:
dputs( 5 ){ "#{big_class.inspect}" }
but this looks just ugly. So how can I evaluate a '#{}' string?
You could do this by having dputs use sprintf (via %). That way it can decide not to build the interpolated string unless it knows it's going to print it:
def dputs(level, format_str, *vars)
puts(format_str % vars) if level <= LEVEL
end
LEVEL = 5
name = 'Andrew'
dputs 5, 'hello %s', name
#=> hello Andrew
Or, as you suggest, you can pass a block which would defer the interpolation till the block actually runs:
def dputs(level, &string)
raise ArgumentError.new('block required') unless block_given?
puts string.call if level <= LEVEL
end
I think it's of no value whatsoever, but I just came up with:
2.3.1 :001 > s = '#{a}'
=> "\#{a}"
2.3.1 :002 > a = 1
=> 1
2.3.1 :003 > instance_eval s.inspect.gsub('\\', '')
=> "1"
2.3.1 :004 > s = 'Hello #{a} and #{a+1}!'
=> "Hello \#{a} and \#{a+1}!"
2.3.1 :005 > instance_eval s.inspect.gsub('\\', '')
=> "Hello 1 and 2!"
Don't use that in production :)
I don't think you can dodge the ugly there. The interpolation happens before the call to dputs unless you put it inside a block, which postpones it until dputs evaluates it. I don't know where dputs comes from, so I'm not sure what its semantics are, but my guess is the block would get you the lazy evaluation you want. Not pretty, but it does the job.
OK, obviously I was just too lazy. I thought there must be a more clean way to do this, Ruby being the best programming language and all ;) To evaluate a string like
a = '#{1+1} some text #{big_class.inspect}'
only when needed, I didn't find a better way than going through the string and eval all "#{}" encountered:
str = ""
"#{b}\#{}".scan( /(.*?)(#\{[^\}]*\})/ ){
str += $1
str += eval( $2[2..-2] ).to_s
}
if you're not into clarity, you can get rid of the temporary-variable str:
"#{b}\#{}".scan( /(.*?)(#\{[^\}]*\})/ ).collect{|c|
c[0] + eval( c[1][2..-2] ).to_s
}.join
The String.scan-method goes through every '#{}'-block, as there might be more than one, evaluating it (the 2..-2 cuts out the "#{" and "}") and putting it together with the rest of the string.
For the corner-case of the string not ending with a '#{}'-block, an empty block is added, just to be sure.
But well, after being some years in Ruby, this still feels clunky and C-ish. Perhaps it's time to learn a new language!
Locked. This question and its answers are locked because the question is off-topic but has historical significance. It is not currently accepting new answers or interactions.
Continuing the "Hidden features of ..." meme, let's share the lesser-known but useful features of Ruby programming language.
Try to limit this discussion with core Ruby, without any Ruby on Rails stuff.
See also:
Hidden features of C#
Hidden features of Java
Hidden features of JavaScript
Hidden features of Ruby on Rails
Hidden features of Python
(Please, just one hidden feature per answer.)
Thank you
From Ruby 1.9 Proc#=== is an alias to Proc#call, which means Proc objects can be used in case statements like so:
def multiple_of(factor)
Proc.new{|product| product.modulo(factor).zero?}
end
case number
when multiple_of(3)
puts "Multiple of 3"
when multiple_of(7)
puts "Multiple of 7"
end
Peter Cooper has a good list of Ruby tricks. Perhaps my favorite of his is allowing both single items and collections to be enumerated. (That is, treat a non-collection object as a collection containing just that object.) It looks like this:
[*items].each do |item|
# ...
end
Don't know how hidden this is, but I've found it useful when needing to make a Hash out of a one-dimensional array:
fruit = ["apple","red","banana","yellow"]
=> ["apple", "red", "banana", "yellow"]
Hash[*fruit]
=> {"apple"=>"red", "banana"=>"yellow"}
One trick I like is to use the splat (*) expander on objects other than Arrays. Here's an example on a regular expression match:
match, text, number = *"Something 981".match(/([A-z]*) ([0-9]*)/)
Other examples include:
a, b, c = *('A'..'Z')
Job = Struct.new(:name, :occupation)
tom = Job.new("Tom", "Developer")
name, occupation = *tom
Wow, no one mentioned the flip flop operator:
1.upto(100) do |i|
puts i if (i == 3)..(i == 15)
end
One of the cool things about ruby is that you can call methods and run code in places other languages would frown upon, such as in method or class definitions.
For instance, to create a class that has an unknown superclass until run time, i.e. is random, you could do the following:
class RandomSubclass < [Array, Hash, String, Fixnum, Float, TrueClass].sample
end
RandomSubclass.superclass # could output one of 6 different classes.
This uses the 1.9 Array#sample method (in 1.8.7-only, see Array#choice), and the example is pretty contrived but you can see the power here.
Another cool example is the ability to put default parameter values that are non fixed (like other languages often demand):
def do_something_at(something, at = Time.now)
# ...
end
Of course the problem with the first example is that it is evaluated at definition time, not call time. So, once a superclass has been chosen, it stays that superclass for the remainder of the program.
However, in the second example, each time you call do_something_at, the at variable will be the time that the method was called (well, very very close to it)
Another tiny feature - convert a Fixnum into any base up to 36:
>> 1234567890.to_s(2)
=> "1001001100101100000001011010010"
>> 1234567890.to_s(8)
=> "11145401322"
>> 1234567890.to_s(16)
=> "499602d2"
>> 1234567890.to_s(24)
=> "6b1230i"
>> 1234567890.to_s(36)
=> "kf12oi"
And as Huw Walters has commented, converting the other way is just as simple:
>> "kf12oi".to_i(36)
=> 1234567890
Hashes with default values! An array in this case.
parties = Hash.new {|hash, key| hash[key] = [] }
parties["Summer party"]
# => []
parties["Summer party"] << "Joe"
parties["Other party"] << "Jane"
Very useful in metaprogramming.
Another fun addition in 1.9 Proc functionality is Proc#curry which allows you to turn a Proc accepting n arguments into one accepting n-1. Here it is combined with the Proc#=== tip I mentioned above:
it_is_day_of_week = lambda{ |day_of_week, date| date.wday == day_of_week }
it_is_saturday = it_is_day_of_week.curry[6]
it_is_sunday = it_is_day_of_week.curry[0]
case Time.now
when it_is_saturday
puts "Saturday!"
when it_is_sunday
puts "Sunday!"
else
puts "Not the weekend"
end
Download Ruby 1.9 source, and issue make golf, then you can do things like this:
make golf
./goruby -e 'h'
# => Hello, world!
./goruby -e 'p St'
# => StandardError
./goruby -e 'p 1.tf'
# => 1.0
./goruby19 -e 'p Fil.exp(".")'
"/home/manveru/pkgbuilds/ruby-svn/src/trunk"
Read the golf_prelude.c for more neat things hiding away.
Boolean operators on non boolean values.
&& and ||
Both return the value of the last expression evaluated.
Which is why the ||= will update the variable with the value returned expression on the right side if the variable is undefined. This is not explicitly documented, but common knowledge.
However the &&= isn't quite so widely known about.
string &&= string + "suffix"
is equivalent to
if string
string = string + "suffix"
end
It's very handy for destructive operations that should not proceed if the variable is undefined.
The Symbol#to_proc function that Rails provides is really cool.
Instead of
Employee.collect { |emp| emp.name }
You can write:
Employee.collect(&:name)
One final one - in ruby you can use any character you want to delimit strings. Take the following code:
message = "My message"
contrived_example = "<div id=\"contrived\">#{message}</div>"
If you don't want to escape the double-quotes within the string, you can simply use a different delimiter:
contrived_example = %{<div id="contrived-example">#{message}</div>}
contrived_example = %[<div id="contrived-example">#{message}</div>]
As well as avoiding having to escape delimiters, you can use these delimiters for nicer multiline strings:
sql = %{
SELECT strings
FROM complicated_table
WHERE complicated_condition = '1'
}
Use a Range object as an infinite lazy list:
Inf = 1.0 / 0
(1..Inf).take(5) #=> [1, 2, 3, 4, 5]
More info here: http://banisterfiend.wordpress.com/2009/10/02/wtf-infinite-ranges-in-ruby/
I find using the define_method command to dynamically generate methods to be quite interesting and not as well known. For example:
((0..9).each do |n|
define_method "press_#{n}" do
#number = #number.to_i * 10 + n
end
end
The above code uses the 'define_method' command to dynamically create the methods "press1" through "press9." Rather then typing all 10 methods which essentailly contain the same code, the define method command is used to generate these methods on the fly as needed.
module_function
Module methods that are declared as module_function will create copies of themselves as private instance methods in the class that includes the Module:
module M
def not!
'not!'
end
module_function :not!
end
class C
include M
def fun
not!
end
end
M.not! # => 'not!
C.new.fun # => 'not!'
C.new.not! # => NoMethodError: private method `not!' called for #<C:0x1261a00>
If you use module_function without any arguments, then any module methods that comes after the module_function statement will automatically become module_functions themselves.
module M
module_function
def not!
'not!'
end
def yea!
'yea!'
end
end
class C
include M
def fun
not! + ' ' + yea!
end
end
M.not! # => 'not!'
M.yea! # => 'yea!'
C.new.fun # => 'not! yea!'
Short inject, like such:
Sum of range:
(1..10).inject(:+)
=> 55
Warning: this item was voted #1 Most Horrendous Hack of 2008, so use with care. Actually, avoid it like the plague, but it is most certainly Hidden Ruby.
Superators Add New Operators to Ruby
Ever want a super-secret handshake operator for some unique operation in your code? Like playing code golf? Try operators like
-~+~-
or
<---
That last one is used in the examples for reversing the order of an item.
I have nothing to do with the Superators Project beyond admiring it.
I'm late to the party, but:
You can easily take two equal-length arrays and turn them into a hash with one array supplying the keys and the other the values:
a = [:x, :y, :z]
b = [123, 456, 789]
Hash[a.zip(b)]
# => { :x => 123, :y => 456, :z => 789 }
(This works because Array#zip "zips" up the values from the two arrays:
a.zip(b) # => [[:x, 123], [:y, 456], [:z, 789]]
And Hash[] can take just such an array. I've seen people do this as well:
Hash[*a.zip(b).flatten] # unnecessary!
Which yields the same result, but the splat and flatten are wholly unnecessary--perhaps they weren't in the past?)
Auto-vivifying hashes in Ruby
def cnh # silly name "create nested hash"
Hash.new {|h,k| h[k] = Hash.new(&h.default_proc)}
end
my_hash = cnh
my_hash[1][2][3] = 4
my_hash # => { 1 => { 2 => { 3 =>4 } } }
This can just be damn handy.
Destructuring an Array
(a, b), c, d = [ [:a, :b ], :c, [:d1, :d2] ]
Where:
a #=> :a
b #=> :b
c #=> :c
d #=> [:d1, :d2]
Using this technique we can use simple assignment to get the exact values we want out of nested array of any depth.
Class.new()
Create a new class at run time. The argument can be a class to derive from, and the block is the class body. You might also want to look at const_set/const_get/const_defined? to get your new class properly registered, so that inspect prints out a name instead of a number.
Not something you need every day, but quite handy when you do.
create an array of consecutive numbers:
x = [*0..5]
sets x to [0, 1, 2, 3, 4, 5]
A lot of the magic you see in Rubyland has to do with metaprogramming, which is simply writing code that writes code for you. Ruby's attr_accessor, attr_reader, and attr_writer are all simple metaprogramming, in that they create two methods in one line, following a standard pattern. Rails does a whole lot of metaprogramming with their relationship-management methods like has_one and belongs_to.
But it's pretty simple to create your own metaprogramming tricks using class_eval to execute dynamically-written code.
The following example allows a wrapper object to forwards certain methods along to an internal object:
class Wrapper
attr_accessor :internal
def self.forwards(*methods)
methods.each do |method|
define_method method do |*arguments, &block|
internal.send method, *arguments, &block
end
end
end
forwards :to_i, :length, :split
end
w = Wrapper.new
w.internal = "12 13 14"
w.to_i # => 12
w.length # => 8
w.split('1') # => ["", "2 ", "3 ", "4"]
The method Wrapper.forwards takes symbols for the names of methods and stores them in the methods array. Then, for each of those given, we use define_method to create a new method whose job it is to send the message along, including all arguments and blocks.
A great resource for metaprogramming issues is Why the Lucky Stiff's "Seeing Metaprogramming Clearly".
use anything that responds to ===(obj) for case comparisons:
case foo
when /baz/
do_something_with_the_string_matching_baz
when 12..15
do_something_with_the_integer_between_12_and_15
when lambda { |x| x % 5 == 0 }
# only works in Ruby 1.9 or if you alias Proc#call as Proc#===
do_something_with_the_integer_that_is_a_multiple_of_5
when Bar
do_something_with_the_instance_of_Bar
when some_object
do_something_with_the_thing_that_matches_some_object
end
Module (and thus Class), Regexp, Date, and many other classes define an instance method :===(other), and can all be used.
Thanks to Farrel for the reminder of Proc#call being aliased as Proc#=== in Ruby 1.9.
The "ruby" binary (at least MRI's) supports a lot of the switches that made perl one-liners quite popular.
Significant ones:
-n Sets up an outer loop with just "gets" - which magically works with given filename or STDIN, setting each read line in $_
-p Similar to -n but with an automatic puts at the end of each loop iteration
-a Automatic call to .split on each input line, stored in $F
-i In-place edit input files
-l Automatic call to .chomp on input
-e Execute a piece of code
-c Check source code
-w With warnings
Some examples:
# Print each line with its number:
ruby -ne 'print($., ": ", $_)' < /etc/irbrc
# Print each line reversed:
ruby -lne 'puts $_.reverse' < /etc/irbrc
# Print the second column from an input CSV (dumb - no balanced quote support etc):
ruby -F, -ane 'puts $F[1]' < /etc/irbrc
# Print lines that contain "eat"
ruby -ne 'puts $_ if /eat/i' < /etc/irbrc
# Same as above:
ruby -pe 'next unless /eat/i' < /etc/irbrc
# Pass-through (like cat, but with possible line-end munging):
ruby -p -e '' < /etc/irbrc
# Uppercase all input:
ruby -p -e '$_.upcase!' < /etc/irbrc
# Same as above, but actually write to the input file, and make a backup first with extension .bak - Notice that inplace edit REQUIRES input files, not an input STDIN:
ruby -i.bak -p -e '$_.upcase!' /etc/irbrc
Feel free to google "ruby one-liners" and "perl one-liners" for tons more usable and practical examples. It essentially allows you to use ruby as a fairly powerful replacement to awk and sed.
The send() method is a general-purpose method that can be used on any Class or Object in Ruby. If not overridden, send() accepts a string and calls the name of the method whose string it is passed. For example, if the user clicks the “Clr” button, the ‘press_clear’ string will be sent to the send() method and the ‘press_clear’ method will be called. The send() method allows for a fun and dynamic way to call functions in Ruby.
%w(7 8 9 / 4 5 6 * 1 2 3 - 0 Clr = +).each do |btn|
button btn, :width => 46, :height => 46 do
method = case btn
when /[0-9]/: 'press_'+btn
when 'Clr': 'press_clear'
when '=': 'press_equals'
when '+': 'press_add'
when '-': 'press_sub'
when '*': 'press_times'
when '/': 'press_div'
end
number.send(method)
number_field.replace strong(number)
end
end
I talk more about this feature in Blogging Shoes: The Simple-Calc Application
Fool some class or module telling it has required something that it really hasn't required:
$" << "something"
This is useful for example when requiring A that in turns requires B but we don't need B in our code (and A won't use it either through our code):
For example, Backgroundrb's bdrb_test_helper requires 'test/spec', but you don't use it at all, so in your code:
$" << "test/spec"
require File.join(File.dirname(__FILE__) + "/../bdrb_test_helper")
Defining a method that accepts any number of parameters and just discards them all
def hello(*)
super
puts "hello!"
end
The above hello method only needs to puts "hello" on the screen and call super - but since the superclass hello defines parameters it has to as well - however since it doesn't actually need to use the parameters itself - it doesn't have to give them a name.
private unless Rails.env == 'test'
# e.g. a bundle of methods you want to test directly
Looks like a cool and (in some cases) nice/useful hack/feature of Ruby.