Ruby: <<- operator [duplicate] - ruby-on-rails

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What are <-- Ruby Strings called? And how do I insert variables in them?
(3 answers)
Closed 6 years ago.
I'm working on Rails. In my code base, I see a line that using Arel::SqlLiteral like this:
result = Arel::Nodes::SqlLiteral.new(<<-SQL
CASE WHEN condition1 THEN calculation1
WHEN condition2 THEN calculation2
WHEN condition3 THEN calculation3
ELSE default_calculation END
SQL)
I understand what this code piece do. The thing I don't understand is its grammar, at this point:
Arel::Nodes::SqlLiteral.new(<<-SQL
...
SQL
)
So in ruby, what is the grammar of <<- follow by name, and then at last block we call that name.
thanks

The keyword you're looking for is "Heredoc".
https://ruby-doc.org/core-2.2.0/doc/syntax/literals_rdoc.html#label-Here+Documents
It's mainly used to prettify large texts and common practice for shells/shellscripts. The marker on top indicates the beginning of a heredoc and the marker on bottom (which must not be indented unless you place a “-” before the opening marker) specifies the end.

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Started learning Ruby from scratch, why is user_input equivalent to gets.chomp? [closed]

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I started learning Ruby from scratch, from the preliminary preparation there is a certain knowledge of HTML and CSS. For training I use Code Academy. I have questions and can't always find an answer I can understand I need help understanding the following:
user_input = gets.chomp
user_input.downcase!
Explain why user_input is equivalent to gets.chomp and what that means, thanks in advance!
In Ruby = is used to assign values to variables, as in:
x = 1
y = x
Where y assumes the value of x at the moment that line is executed. This is not to be confused with "equivalence" as in x=y in a mathematical sense where you're establishing some kind of permanent relationship.
In Ruby methods return a value, even if that value is "nothing", or nil. In the case of gets, it returns a String. You can call chomp on that, or any other thing you need to achieve your objective, like chaining on downcase.
On its own gets.chomp will read a line of input, strip off the trailing linefeed character, and then throw the result in the trash. Assigning this to a variable preserves that output.
To understand it, break it down first
Accept user input
Clean the user input (using chomp https://apidock.com/ruby/String/chomp)
Downcase it
user_input = gets # will return the value entered by the user
user_input = user_input.chomp # will remove the trailing \n
# A more idiomatic way to achieve the above steps in a single line
user_input = gets.chomp
# Finally downcase
user_input.downcase!
# By that same principle the entire code can be written in a single line
user_input = gets.chomp.downcase
user_input is equivalent to gets.chomp
Remember, everything in Ruby is an object. So gets returns a String object, so does chomp and so does downcase. Hence with this logic you are essentially calling instance methods on the String class
String.new("hello") == "hello" # true
# "hello".chomp is same as String.new("hello").chomp

What does "&:name" mean in Ruby [duplicate]

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What does map(&:name) mean in Ruby?
(17 answers)
Closed 8 years ago.
I saw it in some sample Ruby code someone had posted. It was something like:
a.sort_by(&:name)
where a is an array or ActiveRecord objects and :name is one of the attributes.
I have never seen &:name and Ruby's Symbol class documentation says nothing about it. Probably something really simple. :)
Unary Ampersand is address of a function/block/lambda
In this case, it means that the .sort_by function will use each a's element's function named name for comparison
Mostly it used for something else, like this:
[1,2,3].map{ |x| x.to_s } # ['1','2','3']
That could be shortened as:
[1,2,3].map(&:to_s)
So, in your case, a.sort_by(&:name) is a shorthand to:
a.sort_by{ |x| x.name }

Swift error: prefix/postfix '=' is reserved [duplicate]

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Is this response from the compiler valid?
(3 answers)
Closed 8 years ago.
I am getting error message:
prefix/postfix '=' is reserved
for below simple in swift.
var c=0,a=2,b=4
c= a+b
any idea why I am getting this error?
Check this:
Is this response from the compiler valid?
Swift isn't entirely whitespace-agnostic like C... in particular, it uses whitespace to distinguish prefix from postfix operators (because ++i++ in C is a grammar oddity). But it's not ridiculously strict about whitespace like Python either.
P.S. So you have to add whitespace before =.
If I use a single space after variable "c" name, this error is get removed.
c = a+b

How does this regex work? [closed]

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"Rubymonk Is Pretty Brilliant".match(/ ./, 9)
How is the answer "P" calculated from this regex?
use the match method on the string
passes two arguments, a regular expression and the position in the string to begin the search.
returns the character 'P'
The criteria you posted from the Rubymonk grader answer this succinctly:
passes two arguments, a regular expression and the position in the
string to begin the search
But let's examine that in more detail. match is being passed two arguments:
/ ./, a regular expression
9, the starting position in the string
The regular expression tells us that we're looking for a space () followed by any character (.).
The starting position tells us to start at position 9 (I). So instead of applying that regex against "Rubymonk Is Pretty Brilliant", we're applying it against "Is Pretty Brilliant".
In the string "Is Pretty Brilliant", where is the first place we encounter a space followed by another character? "Is[ P]retty Brilliant", right? Thus match finds a result of P (that's space-P, matching the regex, not just P.)
To see this more clearly and to experiment further with regexes, you can try it in an irb session or in your browser using Rubular.
(Just google for RegEx + ruby, You will find explanation of regex syntax)
/ANYTHING-HERE/
Will look for ANYTHING-HERE in the text.
In Your example its (/ ./,9):
/SPACE DOT/
So it will look for space followed by single character (Dot -> single character).
9 will be "I" from the string. And that is not space, so it will go on 2 characters right. Will find space, and then will find single character "P".
That is the result.

Print out an user input correctly [duplicate]

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How to modify user input?
(4 answers)
Closed 9 years ago.
I would like to replace text in an user input with an variable.
I wrote an little demo code, to show you my problem:
puts "Enter your feeling"
a = gets.chomp
#feel = "good"
puts a
SO when it comes to the input, i type in:
Actually i fell very #{#feel}
Then i hope to get this output:
Actually i fell very good
But instead i get this output:
Actually i fell very #{#feel}
What did i make wrong?
You can make use of Kernal#eval
eval ("a")
#{variable} works only double quotes (" "). So you try inside of "". if you use this ' ', it will consider as string

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