Looking through this I notice something I have never seen before on line 83.end.map(&:chomp) so end is an object? (I realize that might be 100% wrong.) Can someone explain what and how that works there? What exactly is advantage?
No, end is not an object, but object.some_method do ... end is an object (or rather it's evaluated to an object) - namely the object returned by the some_method method.
So if you do object.some_method do ... end.some_other_method, you're calling some_other_method on the object returned by some_method.
The full code snippet you're referring to is below:
def initialize(dict_file)
#dict_arr = File.readlines(dict_file).select do |word|
!word.include?("-") && !word.include?("'")
end.map(&:chomp)
end
notice that the end you're talking about is the end of the block that starts on the 2nd line (it matches the do on line 2).
Perhaps if you see it parenthesized, and rewritten with curly braces, it will make more sense:
def initialize(dict_file)
#dict_arr = (File.readlines(dict_file).select { |word|
!word.include?("-") && !word.include?("'")
}).map(&:chomp)
end
It's often helpful to examine what Ruby is doing, step-by-step. Let's see what's going with the method ComputerPlayer#initialize:
def initialize(dict_file)
#dict_arr = File.readlines(dict_file).select do |word|
!word.include?("-") && !word.include?("'")
end.map(&:chomp)
end
First, create a file:
File.write("my_file", "cat\ndog's\n")
When we execute:
ComputerPlayer.new("my_file")
the class method IO#readlines is sent to File, which returns an array a:
a = File.readlines("my_file")
#=> ["cat\n", "dog's\n"]
Enumerable#select is sent to the array a to create an enumerator:
b = a.select
#=> #<Enumerator: ["cat\n", "dog's\n"]:select>
We can convert this enumerator to an array to see what it will pass to it's block:
b.to_a
=> ["cat\n", "dog's\n"]
The enumerator is invoked by sending it the method each with a block, and it returns an array c:
c = b.each { |word| !word.include?("-") && !word.include?("'") }
#=> ["cat\n"]
Lastly, we send Enumerable#map with argument &:chomp (the method String#chomp converted to a proc) to the array c:
c.map(&:chomp)
#=> ["cat"]
A final point: you can improve clarity by minimizing the use of !. For example, instead of
...select do |word|
!word.include?("-") && !word.include?("'")
consider
...reject do |word|
word.include?("-") || word.include?("'")
You might also use a regex.
Related
When using an accumulator, does the accumulator exist only within the reduce block or does it exist within the function?
I have a method that looks like:
def my_useless_function(str)
crazy_letters = ['a','s','d','f','g','h']
str.split.reduce([]) do |new_array, letter|
for a in 0..crazy_letters.length-1
if letter == crazy_letters[a]
new_array << letter
end
end
end
return true if (new_array == new_array.sort)
end
When I execute this code I get the error
"undefined variable new_array in line 11 (the return statement)"
I also tried assigning the new_array value to another variable as an else statement inside my reduce block but that gave me the same results.
Can someone explain to me why this is happening?
The problem is that new_array is created during the call to reduce, and then the reference is lost afterwards. Local variables in Ruby are scoped to the block they are in. The array can be returned from reduce in your case, so you could use it there. However, you need to fix a couple things:
str.split does not break a string into characters in Ruby 2+. You should use str.chars, or str.split('').
The object retained for each new iteration of reduce must be retained by returning it from the block each time. The simplest way to do this is to put new_array as the last expression in your block.
Thus:
def my_useless_function(str)
crazy_letters = ['a','s','d','f','g','h']
crazy_only = str.split('').reduce([]) do |new_array, letter|
for a in 0..crazy_letters.length-1
if letter == crazy_letters[a]
new_array << letter
end
end
new_array
end
return true if (crazy_only == crazy_only.sort)
end
Note that your function is not very efficient, and not very idiomatic. Here's a shorter version of the function that is more idiomatic, but not much more efficient:
def my_useless_function(str)
crazy_letters = %w[a s d f g h]
crazy_only = str.chars.select{ |c| crazy_letters.include?(c) }
crazy_only == crazy_only.sort # evaluates to true or false
end
And here's a version that's more efficient:
def efficient_useless(str)
crazy_only = str.scan(/[asdfgh]/) # use regex to search for the letters you want
crazy_only == crazy_only.sort
end
Block local variables
new_array doesn't exist outside the block of your reduce call. It's a "block local variable".
reduce does return an object, though, and you should use it inside your method.
sum = [1, 2, 3].reduce(0){ |acc, elem| acc + elem }
puts sum
# 6
puts acc
# undefined local variable or method `acc' for main:Object (NameError)
Your code
Here's the least amount of change for your method :
def my_useless_function(str)
crazy_letters = ['a','s','d','f','g','h']
new_array = str.split(//).reduce([]) do |new_array, letter|
for a in 0..crazy_letters.length-1
if letter == crazy_letters[a]
new_array << letter
end
end
new_array
end
return true if (new_array == new_array.sort)
end
Notes:
return isn't needed at the end.
true if ... isn't needed either
for loop should never be used in Ruby
reduce returns the result of the last expression inside the block. It was for in your code.
If you always need to return the same object in reduce, it might be a sign you could use each_with_object.
"test".split is just ["test"]
String and Enumerable have methods that could help you. Using them, you could write a much cleaner and more efficient method, as in #Phrogz answer.
I've got some code:
def my_each_with_index
return enum_for(:my_each_with_index) unless block_given?
i = 0
self.my_each do |x|
yield x, i
i += 1
end
self
end
It is my own code, but the line:
return enum_for(:my_each_with_index) unless block_given?
is found in solutions of other's. I can't get why they passed the function to enum_for as a parameter. When I invoke my function without a block, it won't return anything with or without enum_for. I could left sth like:
return unless block_given?
and it has the same result. Or am I wrong?
Being called without a block, it will return an enumerator:
▶ def my_each_with_index
▷ return enum_for(:my_each_with_index) unless block_given?
▷ end
#⇒ :my_each_with_index
▶ e = my_each_with_index
#⇒ #<Enumerator: main:my_each_with_index>
later on you might iterate on this enumerator:
▶ e.each { |elem| ... }
This behavior is specifically useful in some cases, like lazy iteration, passing block to this enumerator later etc.
Just returning nil cuts this ability off.
Think you for very precise answer. I recived also very good example to understand this issue for other new developers:
def iterator
yield 1
yield 2
yield 3
puts "koniec"
end
iterator { |v| puts v }
it = enum_for(:iterator)
puts it.next
puts it.next
puts it.next
puts it.next
Just run and analyze this code.
For any method that accepts a block, a good method implementation should have a well-defined behavior when the block is not given.
In the example shared by you, each_for_index is being re-implemented by author, may be to provide additional semantics or may be just for academic purpose given that its behavior is same as Ruby's Enumerable#each_with_index.
The documentation has following for Enumerable#each_with_index.
Calls block with two arguments, the item and its index, for each item
in enum. Given arguments are passed through to each().
If no block is given, an enumerator is returned instead.
In order to stay consistent with highlighted line indicating what should be the behavior if block is not given, one has to use something like
return enum_for(:my_each_with_index) unless block_given?
enum_for is interesting method
enum_for creates a new Enumerator which will enumerate by calling method on obj.
Below is an example reproduced from documentation:
str = "xyz"
enum = str.enum_for(:each_byte)
enum.each { |b| puts b }
# => 120
# => 121
# => 122
So, if one does not pass block to my_each_with_index, they have a chance to pass it later - just like one would have done with each_with_index.
e = obj.my_each_with_index
...
e.each { |x, i| # do something } # `my_each_with_index` executed later
In summary, my_each_with_index tries to be consistent with each_with_index and tries to be a well-behaved API.
i am working though LearnTocodethehardway.com http://ruby.learncodethehardway.org/book/ex25.html
On ex25. In the example there is a module that has a bunch of methods that return or print values. The Print_last_word method when supplied with an array of strings just puts nil. it does this even in his example output. My question would then be why?
To be precise, it doesn't puts nil - it puts the last word and returns nil. Here's the example output:
>> Ex25.print_last_word(words)
wait. # <- this is the output
=> nil # <- this is the return value
puts always returns nil.
UPDATE
There seems to be a bug in print_first_word:
module Ex25
def Ex25.print_first_word(words)
word = words.pop(0)
puts word
end
end
Ex25.print_first_word(["foo", "bar", "baz"])
#=> nil
This is because ["foo", "bar", "baz"].pop(0) returns an empty array and puts [] just returns nil without printing anything.
A working implementation (in the exercise's style) could look like this:
module Ex25
def Ex25.print_first_word(words)
word = words.shift
puts word
end
end
To make it more easy to understand:
"puts" never is used for its return value. It is used for its side effect if you wanted a method that would return a word, you would then do something with that word.
words = ["apple", "cucumber", "apple"]
def give_me_first_word(array_of_words)
array_of_words.first
end
variable_to_be_used_later = give_me_first_word(words)
This function would return "apple"(and in this case the variable would be assigned "apple"), but it would puts nothing to the screen. This value would then be used in another program or function.
If you puts a value, you would then not need to return it to any other program as it's already served its purpose. It's actually intuitive because if puts also returned the value, it would be doing two things. The counterpart to "puts" would be "return" that simply returns the value and does not display it to the screen.
I was trying to build a method that you take the first letter of every word and would capitalize it. I wrote it as
def titleize(name)
name.scan(/\w+/) { |x| x.capitalize! }
end
and it just wouldn't work properly. It wouldn't capitalize and letters. I did some searching and found the answer here Capitalizing titles eventually. It was written as
def titleize(name)
name.split(" ").each { |x| x.capitalize! }.join(" ")
end
How come my code didn't capitalize at all though? If I added a put statement and wrote it as
def titleize(name)
name.scan(/\w+/) { |x| puts x.capitalize! }
end
It would output "hi there" with capitals but the => would still be just "hi there" What did I miss?
Corrected code:
def titleize(name)
name.scan(/\w+/).each { |x| x.capitalize! }.join(' ')
end
p titleize("ayan roy") #=>"Ayan Roy"
Let's see why your one not worked:
def titleize(name)
name.scan(/\w+/)
end
p titleize("ayan roy") #=>["ayan", "roy"]
Now your line name.scan(/\w+/) { |x| x.capitalize! } , x is passed as "ayan", "roy". Now look at the below:
def titleize(name)
name.scan(/\w+/) { |x| p x.capitalize! }
end
p titleize("ayan roy")
Output:
"Ayan"
"Roy"
"ayan roy"
As String#scan says:
scan(pattern) {|match, ...| block } → str - if block is given,scan will return the receiver on which it is called. Both forms iterate through str, matching the pattern (which may be a Regexp or a String). For each match, a result is generated and either added to the result array or passed to the block.
scan returns/yields new strings and will never modify the source string. Perhaps you want gsub.
def titleize(name)
name.gsub(/\w+/) {|x| x.capitalize }
end
Or perhaps better to use a likely more correct implementation from the titleize gem.
Your code doesn't work because #scan returns new String objects which are the results of the Regexp and passes them to the block. So in your method you essentially took these new objects, mutated them by calling #capitalize! but never used them anywhere afterwards.
You should do instead:
def titleize(name)
name.scan(/\w+/).each { |x| x.capitalize! }.join(' ')
end
But this seems more readable to me:
def titleize2(name)
name.split(' ').each { |w| w.capitalize! }.join(' ')
end
Note however these methods do not mutate the original argument passed.
The block form of scan returns the original string, regardless of what you do in the block. (I think you may be able to alter the original string in the block by referring directly to it, but it's not recommended to alter the thing you're iterating over.) Instead, do your split variation, but instead of each, do collect followed by join:
name.split(" ").collect { |x| x.capitalize }.join(" ")
This works for titles containing numerals and punctuation, as well.
Can anyone help me to figure out the the use of yield and return in Ruby. I'm a Ruby beginner, so simple examples are highly appreciated.
Thank you in advance!
The return statement works the same way that it works on other similar programming languages, it just returns from the method it is used on.
You can skip the call to return, since all methods in ruby always return the last statement. So you might find method like this:
def method
"hey there"
end
That's actually the same as doing something like:
def method
return "hey there"
end
The yield on the other hand, excecutes the block given as a parameter to the method. So you can have a method like this:
def method
puts "do somthing..."
yield
end
And then use it like this:
method do
puts "doing something"
end
The result of that, would be printing on screen the following 2 lines:
"do somthing..."
"doing something"
Hope that clears it up a bit. For more info on blocks, you can check out this link.
yield is used to call the block associated with the method. You do this by placing the block (basically just code in curly braces) after the method and its parameters, like so:
[1, 2, 3].each {|elem| puts elem}
return exits from the current method, and uses its "argument" as the return value, like so:
def hello
return :hello if some_test
puts "If it some_test returns false, then this message will be printed."
end
But note that you don't have to use the return keyword in any methods; Ruby will return the last statement evaluated if it encounters no returns. Thus these two are equivelent:
def explicit_return
# ...
return true
end
def implicit_return
# ...
true
end
Here's an example for yield:
# A simple iterator that operates on an array
def each_in(ary)
i = 0
until i >= ary.size
# Calls the block associated with this method and sends the arguments as block parameters.
# Automatically raises LocalJumpError if there is no block, so to make it safe, you can use block_given?
yield(ary[i])
i += 1
end
end
# Reverses an array
result = [] # This block is "tied" to the method
# | | |
# v v v
each_in([:duck, :duck, :duck, :GOOSE]) {|elem| result.insert(0, elem)}
result # => [:GOOSE, :duck, :duck, :duck]
And an example for return, which I will use to implement a method to see if a number is happy:
class Numeric
# Not the real meat of the program
def sum_of_squares
(to_s.split("").collect {|s| s.to_i ** 2}).inject(0) {|sum, i| sum + i}
end
def happy?(cache=[])
# If the number reaches 1, then it is happy.
return true if self == 1
# Can't be happy because we're starting to loop
return false if cache.include?(self)
# Ask the next number if it's happy, with self added to the list of seen numbers
# You don't actually need the return (it works without it); I just add it for symmetry
return sum_of_squares.happy?(cache << self)
end
end
24.happy? # => false
19.happy? # => true
2.happy? # => false
1.happy? # => true
# ... and so on ...
Hope this helps! :)
def cool
return yield
end
p cool {"yes!"}
The yield keyword instructs Ruby to execute the code in the block. In this example, the block returns the string "yes!". An explicit return statement was used in the cool() method, but this could have been implicit as well.