Different parse priorities when argument is Hash - ruby

Two cases:
ruby-1.9.2-p180 > puts {}.class
=> NilClass
and
ruby-1.9.2-p180 > puts "a".class
String
=> nil
It looks like puts {}.class is equivalent to (puts {}).class and puts "a".class is equivalent to puts ("a".class). Why is it so?

It's treating {} as a block, not as a hash. RubyInside says so, and also shows how you can prove it using Ripper.

Related

Ruby: methods as array elements - how do they work?

This probably isn't something you should try at home, but for some reason or another I tried to create an array of methods in Ruby.
I started by defining two methods.
irb(main):001:0> def test1
irb(main):002:1> puts "test!"
irb(main):003:1> end
=> nil
irb(main):004:0> def test2
irb(main):005:1> puts "test2!"
irb(main):006:1> end
=> nil
The weird thing happens when you try to put it into an actual array. It seems to run both methods.
irb(main):007:0> array = [test1, test2]
test!
test2!
=> [nil, nil]
And afterwards, the array is empty.
irb(main):008:0> puts array
=> nil
Can someone explain to me why it runs the methods? Other than that the whole excercise is seriously in need of an exorcist?
What you're storing in your array is the result of calling your methods, not the methods themselves.
def test1
puts "foo!"
end
def test2
puts "bar!"
end
You can store references to the actual methods like this:
> arr = [method(:test1), method(:test2)]
# => [#<Method: Object#test1>, #<Method: Object#test2>]
Later, you can call the referenced methods like this:
> arr.each {|m| m.call }
foo!
bar!
#alestanis explained the reason well. If you were trying to store the methods, then you can do what Lars Haugseth says or you could do the folllowing:
test1 = Proc.new { puts "test!" }
test2 = Proc.new { puts "test2!" }
a = [test1, test2]
This may make your code much more readable.
Here is an irb run.
1.9.3p194 :009 > test1 = Proc.new { puts "test!" }
=> #<Proc:0x00000002798a90#(irb):9>
1.9.3p194 :010 > test2 = Proc.new { puts "test2!" }
=> #<Proc:0x00000002792988#(irb):10>
1.9.3p194 :011 > a = [test1, test2]
=> [#<Proc:0x00000002798a90#(irb):9>, #<Proc:0x00000002792988#(irb):10>]
Your array never contains anything else than two nil values. I tricks you by putting the strings when evaluating. But the return value of each function still is nil.
Your code runs the two methods because you're actually calling the methods when you say "test1" and "test2" - parentheses are optional for ruby method calls.
Since both of your methods just contain a "puts", which returns nil, your resulting array is just an array of two nils.
If you had a square method and wanted to create an array with the square values of 2 and 4, you would write
array = [square(2), square(4)]
Here you are doing exactly the same thing, except that your test methods don't return anything and that's why your final array seems empty (actually, it contains [nil, nil]).
Here's my two-pennies worth. Building on the solutions already posted, this is an example of a working example. What might be handy for some here is that it includes method arguments and the use of self (which refers to the instance of the PromotionalRules class when it is instantiated) and the array of symbols, which is neat - I got that from the Ruby docs on the #send method here. Hope this helps someone!
class PromotionalRules
PROMOTIONS = [:lavender_heart_promotion, :ten_percent_discount]
def apply_promotions total, basket
#total = total
if PROMOTIONS.count > 0
PROMOTIONS.each { |promotion| #total = self.send promotion, #total, basket }
end
#total.round(2)
end
def lavender_heart_promotion total, basket
if two_or_more_lavender_hearts? basket
basket.map { |item| total -= 0.75 if item == 001 }
end
total
end
def two_or_more_lavender_hearts? basket
n = 0
basket.each do |item|
n += 1 if item == 001
end
n >= 2
end
def ten_percent_discount total, *arg
if total > 60.00
total = total - total/10
end
total
end
end
Thanks to everyone for their help. I love the open-source nature of coding - threads just get better and better as people iterate over each other's solutions!

Can't manage to check if a Ruby Hash contains a value

I have a ruby hash which looks like
{"10.1.1.6"=>"nick", "127.0.0.1"=>"nick1"}
But I can't manage to check if a certain string is already in the Hash. I tried has_value?, getting array of values using values then using include? to check if it contains it, but always returns false, when I know that it exists. For example, I try to add "172.16.10.252"=>"nick" to the hash and I do:
class SomeClass
def initialize(*args)
super(*args)
#nicks = Hash.new
end
def serve(io)
loop do
line = io.readline
ip = io.peeraddr[3]
begin
if /NICK (.*)/ =~ line
nick = $1
if #nicks.has_value?(nick) # it fails here
puts "New nick #{$1}"
#nicks[ip] = nick.gsub("\r", "")
io.puts "Your new nick is #{nick}"
else
message = {:ERROR => "100", :INFO=>"#{nick}"}.to_json
io.puts message
end
end
rescue Exception => e
puts "Exception! #{e}-#{e.backtrace}"
end
end
end
end
On irb it works fine, but on my script it doesn't
1.9.3p125 :001 > h = {"10.1.1.6"=>"nick", "127.0.0.1"=>"nick1"}
=> {"10.1.1.6"=>"nick", "127.0.0.1"=>"nick1"}
1.9.3p125 :002 > h.has_value?('nick')
=> true
1.9.3p125 :003 > if h.has_value?('nick')
1.9.3p125 :004?> puts "yes"
1.9.3p125 :005?> else
1.9.3p125 :006 > puts "no"
1.9.3p125 :007?> end
yes
=> nil
1.9.3p125 :008 >
What I'm doing wrong?
I'm not sure if you're using "$1" the way you intend to.
In your code at this line:
if /NICK (.*)/ =~ line
nick = $1
if #nicks.has_value?(nick) # it fails here
puts "New nick #{$1}"
if line is "NICK says a bunch of things", $1 will be "says a bunch of things". So you're not really looking for the value 'nick' in your hash, but for 'says a bunch of things'.
You should check how your regex is working, I wouldn't say anything is wrong with a hash.

Accessing elements of nested hashes in ruby [duplicate]

This question already has answers here:
Ruby Style: How to check whether a nested hash element exists
(16 answers)
How to avoid NoMethodError for nil elements when accessing nested hashes? [duplicate]
(4 answers)
Closed 7 years ago.
I'm working a little utility written in ruby that makes extensive use of nested hashes. Currently, I'm checking access to nested hash elements as follows:
structure = { :a => { :b => 'foo' }}
# I want structure[:a][:b]
value = nil
if structure.has_key?(:a) && structure[:a].has_key?(:b) then
value = structure[:a][:b]
end
Is there a better way to do this? I'd like to be able to say:
value = structure[:a][:b]
And get nil if :a is not a key in structure, etc.
Traditionally, you really had to do something like this:
structure[:a] && structure[:a][:b]
However, Ruby 2.3 added a method Hash#dig that makes this way more graceful:
structure.dig :a, :b # nil if it misses anywhere along the way
There is a gem called ruby_dig that will back-patch this for you.
Hash and Array have a method called dig.
value = structure.dig(:a, :b)
It returns nil if the key is missing at any level.
If you are using a version of Ruby older than 2.3, you can install a gem such as ruby_dig or hash_dig_and_collect, or implement this functionality yourself:
module RubyDig
def dig(key, *rest)
if value = (self[key] rescue nil)
if rest.empty?
value
elsif value.respond_to?(:dig)
value.dig(*rest)
end
end
end
end
if RUBY_VERSION < '2.3'
Array.send(:include, RubyDig)
Hash.send(:include, RubyDig)
end
The way I usually do this these days is:
h = Hash.new { |h,k| h[k] = {} }
This will give you a hash that creates a new hash as the entry for a missing key, but returns nil for the second level of key:
h['foo'] -> {}
h['foo']['bar'] -> nil
You can nest this to add multiple layers that can be addressed this way:
h = Hash.new { |h, k| h[k] = Hash.new { |hh, kk| hh[kk] = {} } }
h['bar'] -> {}
h['tar']['zar'] -> {}
h['scar']['far']['mar'] -> nil
You can also chain indefinitely by using the default_proc method:
h = Hash.new { |h, k| h[k] = Hash.new(&h.default_proc) }
h['bar'] -> {}
h['tar']['star']['par'] -> {}
The above code creates a hash whose default proc creates a new Hash with the same default proc. So, a hash created as a default value when a lookup for an unseen key occurs will have the same default behavior.
EDIT: More details
Ruby hashes allow you to control how default values are created when a lookup occurs for a new key. When specified, this behavior is encapsulated as a Proc object and is reachable via the default_proc and default_proc= methods. The default proc can also be specified by passing a block to Hash.new.
Let's break this code down a little. This is not idiomatic ruby, but it's easier to break it out into multiple lines:
1. recursive_hash = Hash.new do |h, k|
2. h[k] = Hash.new(&h.default_proc)
3. end
Line 1 declares a variable recursive_hash to be a new Hash and begins a block to be recursive_hash's default_proc. The block is passed two objects: h, which is the Hash instance the key lookup is being performed on, and k, the key being looked up.
Line 2 sets the default value in the hash to a new Hash instance. The default behavior for this hash is supplied by passing a Proc created from the default_proc of the hash the lookup is occurring in; ie, the default proc the block itself is defining.
Here's an example from an IRB session:
irb(main):011:0> recursive_hash = Hash.new do |h,k|
irb(main):012:1* h[k] = Hash.new(&h.default_proc)
irb(main):013:1> end
=> {}
irb(main):014:0> recursive_hash[:foo]
=> {}
irb(main):015:0> recursive_hash
=> {:foo=>{}}
When the hash at recursive_hash[:foo] was created, its default_proc was supplied by recursive_hash's default_proc. This has two effects:
The default behavior for recursive_hash[:foo] is the same as recursive_hash.
The default behavior for hashes created by recursive_hash[:foo]'s default_proc will be the same as recursive_hash.
So, continuing in IRB, we get the following:
irb(main):016:0> recursive_hash[:foo][:bar]
=> {}
irb(main):017:0> recursive_hash
=> {:foo=>{:bar=>{}}}
irb(main):018:0> recursive_hash[:foo][:bar][:zap]
=> {}
irb(main):019:0> recursive_hash
=> {:foo=>{:bar=>{:zap=>{}}}}
I made rubygem for this. Try vine.
Install:
gem install vine
Usage:
hash.access("a.b.c")
I think one of the most readable solutions is using Hashie:
require 'hashie'
myhash = Hashie::Mash.new({foo: {bar: "blah" }})
myhash.foo.bar
=> "blah"
myhash.foo?
=> true
# use "underscore dot" for multi-level testing
myhash.foo_.bar?
=> true
myhash.foo_.huh_.what?
=> false
value = structure[:a][:b] rescue nil
Solution 1
I suggested this in my question before:
class NilClass; def to_hash; {} end end
Hash#to_hash is already defined, and returns self. Then you can do:
value = structure[:a].to_hash[:b]
The to_hash ensures that you get an empty hash when the previous key search fails.
Solution2
This solution is similar in spirit to mu is too short's answer in that it uses a subclass, but still somewhat different. In case there is no value for a certain key, it does not use a default value, but rather creates a value of empty hash, so that it does not have the problem of confusion in assigment that DigitalRoss's answer has, as was pointed out by mu is too short.
class NilFreeHash < Hash
def [] key; key?(key) ? super(key) : self[key] = NilFreeHash.new end
end
structure = NilFreeHash.new
structure[:a][:b] = 3
p strucrture[:a][:b] # => 3
It departs from the specification given in the question, though. When an undefined key is given, it will return an empty hash instread of nil.
p structure[:c] # => {}
If you build an instance of this NilFreeHash from the beginning and assign the key-values, it will work, but if you want to convert a hash into an instance of this class, that may be a problem.
You could just build a Hash subclass with an extra variadic method for digging all the way down with appropriate checks along the way. Something like this (with a better name of course):
class Thing < Hash
def find(*path)
path.inject(self) { |h, x| return nil if(!h.is_a?(Thing) || h[x].nil?); h[x] }
end
end
Then just use Things instead of hashes:
>> x = Thing.new
=> {}
>> x[:a] = Thing.new
=> {}
>> x[:a][:b] = 'k'
=> "k"
>> x.find(:a)
=> {:b=>"k"}
>> x.find(:a, :b)
=> "k"
>> x.find(:a, :b, :c)
=> nil
>> x.find(:a, :c, :d)
=> nil
This monkey patch function for Hash should be easiest (at least for me). It also doesn't alter structure i.e. changing nil's to {}. It would still also apply even if you're reading a tree from a raw source e.g. JSON. It also doesn't need to produce empty hash objects as it goes or parse a string. rescue nil was actually a good easy solution for me as I'm brave enough for such a low risk but I find it to essentially have a drawback with performance.
class ::Hash
def recurse(*keys)
v = self[keys.shift]
while keys.length > 0
return nil if not v.is_a? Hash
v = v[keys.shift]
end
v
end
end
Example:
> structure = { :a => { :b => 'foo' }}
=> {:a=>{:b=>"foo"}}
> structure.recurse(:a, :b)
=> "foo"
> structure.recurse(:a, :x)
=> nil
What's also good is that you can play around saved arrays with it:
> keys = [:a, :b]
=> [:a, :b]
> structure.recurse(*keys)
=> "foo"
> structure.recurse(*keys, :x1, :x2)
=> nil
The XKeys gem will read and auto-vivify-on-write nested hashes (::Hash) or hashes and arrays (::Auto, based on the key/index type) with a simple, clear, readable, and compact syntax by enhancing #[] and #[]=. The sentinel symbol :[] will push onto the end of an array.
require 'xkeys'
structure = {}.extend XKeys::Hash
structure[:a, :b] # nil
structure[:a, :b, :else => 0] # 0 (contextual default)
structure[:a] # nil, even after above
structure[:a, :b] = 'foo'
structure[:a, :b] # foo
You can use the andand gem, but I'm becoming more and more wary of it:
>> structure = { :a => { :b => 'foo' }} #=> {:a=>{:b=>"foo"}}
>> require 'andand' #=> true
>> structure[:a].andand[:b] #=> "foo"
>> structure[:c].andand[:b] #=> nil
There is the cute but wrong way to do this. Which is to monkey-patch NilClass to add a [] method that returns nil. I say it is the wrong approach because you have no idea what other software may have made a different version, or what behavior change in a future version of Ruby can be broken by this.
A better approach is to create a new object that works a lot like nil but supports this behavior. Make this new object the default return of your hashes. And then it will just work.
Alternately you can create a simple "nested lookup" function that you pass the hash and the keys to, which traverses the hashes in order, breaking out when it can.
I would personally prefer one of the latter two approaches. Though I think it would be cute if the first was integrated into the Ruby language. (But monkey-patching is a bad idea. Don't do that. Particularly not to demonstrate what a cool hacker you are.)
Not that I would do it, but you can Monkeypatch in NilClass#[]:
> structure = { :a => { :b => 'foo' }}
#=> {:a=>{:b=>"foo"}}
> structure[:x][:y]
NoMethodError: undefined method `[]' for nil:NilClass
from (irb):2
from C:/Ruby/bin/irb:12:in `<main>'
> class NilClass; def [](*a); end; end
#=> nil
> structure[:x][:y]
#=> nil
> structure[:a][:y]
#=> nil
> structure[:a][:b]
#=> "foo"
Go with #DigitalRoss's answer. Yes, it's more typing, but that's because it's safer.
In my case, I needed a two-dimensional matrix where each cell is a list of items.
I found this technique which seems to work. It might work for the OP:
$all = Hash.new()
def $all.[](k)
v = fetch(k, nil)
return v if v
h = Hash.new()
def h.[](k2)
v = fetch(k2, nil)
return v if v
list = Array.new()
store(k2, list)
return list
end
store(k, h)
return h
end
$all['g1-a']['g2-a'] << '1'
$all['g1-a']['g2-a'] << '2'
$all['g1-a']['g2-a'] << '3'
$all['g1-a']['g2-b'] << '4'
$all['g1-b']['g2-a'] << '5'
$all['g1-b']['g2-c'] << '6'
$all.keys.each do |group1|
$all[group1].keys.each do |group2|
$all[group1][group2].each do |item|
puts "#{group1} #{group2} #{item}"
end
end
end
The output is:
$ ruby -v && ruby t.rb
ruby 1.9.2p0 (2010-08-18 revision 29036) [x86_64-linux]
g1-a g2-a 1
g1-a g2-a 2
g1-a g2-a 3
g1-a g2-b 4
g1-b g2-a 5
g1-b g2-c 6
I am currently trying out this:
# --------------------------------------------------------------------
# System so that we chain methods together without worrying about nil
# values (a la Objective-c).
# Example:
# params[:foo].try?[:bar]
#
class Object
# Returns self, unless NilClass (see below)
def try?
self
end
end
class NilClass
class MethodMissingSink
include Singleton
def method_missing(meth, *args, &block)
end
end
def try?
MethodMissingSink.instance
end
end
I know the arguments against try, but it is useful when looking into things, like say, params.

What is the difference between print and puts?

For example in this line of code I wrote, print and puts produce different results.
1.upto(1000).each { |i| print i if i % 2 == 0 }
puts adds a new line to the end of each argument if there is not one already.
print does not add a new line.
For example:
puts [[1,2,3], [4,5,nil]] Would return:
1
2
3
4
5
Whereas print [[1,2,3], [4,5,nil]]
would return:
[[1,2,3], [4,5,nil]]
Notice how puts does not output the nil value whereas print does.
A big difference is if you are displaying arrays.
Especially ones with NIL.
For example:
print [nil, 1, 2]
gives
[nil, 1, 2]
but
puts [nil, 1, 2]
gives
1
2
Note, no appearing nil item (just a blank line) and each item on a different line.
print outputs each argument, followed by $,, to $stdout, followed by $\. It is equivalent to args.join($,) + $\
puts sets both $, and $\ to "\n" and then does the same thing as print. The key difference being that each argument is a new line with puts.
You can require 'english' to access those global variables with user-friendly names.
The API docs give some good hints:
print() → nil
print(obj, ...) → nil
Writes the given object(s) to ios. Returns nil.
The stream must be opened for writing. Each given object that isn't a
string will be converted by calling its to_s method. When
called without arguments, prints the contents of $_.
If the output field separator ($,) is not nil, it
is inserted between objects. If the output record separator
($\) is not nil, it is appended to the output.
...
puts(obj, ...) → nil
Writes the given object(s) to ios. Writes a newline after any that
do not already end with a newline sequence. Returns nil.
The stream must be opened for writing. If called with an array argument,
writes each element on a new line. Each given object that isn't a
string or array will be converted by calling its to_s method.
If called without arguments, outputs a single newline.
Experimenting a little with the points given above, the differences seem to be:
Called with multiple arguments, print separates them by the 'output field separator' $, (which defaults to nothing) while puts separates them by newlines. puts also puts a newline after the final argument, while print does not.
2.1.3 :001 > print 'hello', 'world'
helloworld => nil
2.1.3 :002 > puts 'hello', 'world'
hello
world
=> nil
2.1.3 :003 > $, = 'fanodd'
=> "fanodd"
2.1.3 :004 > print 'hello', 'world'
hellofanoddworld => nil
2.1.3 :005 > puts 'hello', 'world'
hello
world
=> nil
puts automatically unpacks arrays, while print does not:
2.1.3 :001 > print [1, [2, 3]], [4]
[1, [2, 3]][4] => nil
2.1.3 :002 > puts [1, [2, 3]], [4]
1
2
3
4
=> nil
print with no arguments prints $_ (the last thing read by gets), while puts prints a newline:
2.1.3 :001 > gets
hello world
=> "hello world\n"
2.1.3 :002 > puts
=> nil
2.1.3 :003 > print
hello world
=> nil
print writes the output record separator $\ after whatever it prints, while puts ignores this variable:
mark#lunchbox:~$ irb
2.1.3 :001 > $\ = 'MOOOOOOO!'
=> "MOOOOOOO!"
2.1.3 :002 > puts "Oink! Baa! Cluck! "
Oink! Baa! Cluck!
=> nil
2.1.3 :003 > print "Oink! Baa! Cluck! "
Oink! Baa! Cluck! MOOOOOOO! => nil
puts call the to_s of each argument and adds a new line to each string, if it does not end with new line.
print just output each argument by calling their to_s.
for example:
puts "one two":
one two
{new line}
puts "one two\n":
one two
{new line} #puts will not add a new line to the result, since the string ends with a new line
print "one two":
one two
print "one two\n":
one two
{new line}
And there is another way to output: p
For each object, directly writes obj.inspect followed by a newline to the program’s standard output.
It is helpful to output debugging message.
p "aa\n\t": aa\n\t
If you would like to output array within string using puts, you will get the same result as if you were using print:
puts "#{[0, 1, nil]}":
[0, 1, nil]
But if not withing a quoted string then yes. The only difference is between new line when we use puts.

Binary or "|" in ruby

Why isnt that working:
>> s = "hi"
=> "hi"
>> s == ("hi"|"ho")
NoMethodError: undefined method `|' for "hi":String
from (irb):2
>>
I don't get it.. Is there a solution for this kind of syntax? Because
s == ("hi"|"ho")
#is shorther than
s == "hi" || s == "ho"
Yes, the bitwise operator | is not defined in the String class: http://ruby-doc.org/core/classes/String.html
Consider this for expressiveness:
["hi", "ho"].include? myStr
irb(main):001:0> s = "hi"
=> "hi"
irb(main):002:0> ["hi", "ho"]
=> ["hi", "ho"]
irb(main):003:0> ["hi", "ho"].include? s
=> true
irb(main):004:0> s = "foo"
=> "foo"
irb(main):005:0> ["hi", "ho"].include? s
=> false
In most high level languages that syntax will not work, you have to stick to the longer syntax of:
s == "hi" || s == "ho"
Note that | is a bitwise or, whereas || is a regular or
You could use the include? method on array if you've got several == tests to do:
["hi", "ho"].include?(s)
Not shorter for two checks admittedly but it will be shorter for three or more.
This syntax doesn't exist in any language as far as I know.
What you are saying
s == ("hi"|"ho")
Literally translates to 'bitwise OR the strings "hi" and "ho" together and then compare them with s'. If you can't see why this is not what you are looking for, try writing down the ASCII codes for "hi" and "ho" and then bitwise ORing them together. You are going to get complete gibberish.
You could make it work that way:
irb> class Pair
def initialize(strA,strB)
#strA,#strB = strA,strB
end
def ==(string)
string == #strA || string == #strB
end
def |(other)
Pair.new(self,other)
end
end
#=> nil
irb> class String
def |(other)
Pair.new(self,other)
end
alias old_equals :==
def ==(other)
if other.kind_of? Pair
other == self
else
old_equals other
end
end
end
#=> nil
irb> ("one"|"two") == "one"
#=> true
irb> ("one"|"two") == "two"
#=> true
irb> ("one"|"two") == "three"
#=> false
irb> "one" == ("one"|"two")
#=> true
irb> "three" == ("one"|"two"|"three")
#=> true
But since this involves some monkey-patching of a fairly lowlevel class, I wouldn't advise relying on it. Other people will hate reading your code.
Ruby supports binary 'or' and other binary operations on values of type Fixnum and Bignum, meaning any integer. Bitwise operations aren't supported on strings or any other type, as far as I know.
As other people have mentioned, you probably want something other than binary operations altogether. However, you can easily get integer representations of characters, so you can compare characters like so:
a = "Cake"
b = "Pie"
puts a[0] | b[0] # Prints "83" - C is 67 and P is 80.
You can get an array of the comparisons easily with some conversions.
a = "Cake"
b = "Pie " # Strings of uneven length is trivial but more cluttered.
a_arr = a.split(//)
b_arr = b.split(//)
c_arr = []
a.each_with_index { |char, i| c.push(a[i].to_i | b[i].to_i) }
# If you *really* want an ASCII string back...
c = c_arr.collect(&:chr).join
You could use a regex:
Like so:
regex = /hi|ho/
s = "hi"
t = "foo"
s =~ regex
#=> 0
t =~ regex
#=> nil

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