Beginner on Ruby here...below are two similar codes, I understand the procedures of this very basic code but I'd like to understand the theory behind it...
First
def stats(ppg)
if ppg > 20
puts "The PG is considered elite"
else
puts "The PG is not considered elite"
end
end
stats(28)
Second
def stats(ppg)
if ppg > 20
"The PG is considered elite"
else
"The PG is not considered elite"
end
end
puts stats(28)
So the first piece automatically writes out the strings whereas the second piece does not - is stats(ppg) from the first piece considered an object or a method with the variable as the argument?
def stats(ppg) in both cases is the same thing, i.e. the beginning of a method definition for a method named stats that takes one argument or parameter that will be named ppg inside the method body.
stats(28) in both cases is the same thing, i.e. a call to a method named stats with the literal Fixnum 28 passed as its argument, whereupon it will be assigned to the variable ppg inside the method body from the definition.
In Ruby, every expression involves two distinct phenomena: side effects, and return values. The expression a = 1 has a side effect of assigning the value of 1 to the variable a, and a return value of 1. Not every method in Ruby has a side effect, but every method does have a return value — either an explicit return value (by using return), or else the return value of the last expression evaluated in the method body.
puts is an unfortunate method in Ruby, because it is used in so many beginner examples, yet its behavior is confusing. It has a side effect of printing its argument to stdout, but its return value is nil (which often confuses beginners, who expect it to return the value of its argument).
The difference between your first method and second method is that the first method, because it uses puts internally, has the side effect of printing a string to stdout and a return value of nil, while the second method has no side effect, but a return value of the string itself.
Therefore, when you call your first method without puts in front, the side effect of printing the string occurs and you see the output. When you call your second method, there is no printing side effect, so in order to have the string printed to stdout, you have to call puts. The argument to puts is the return value of your stats method, i.e. the string you wanted to print.
Related
I'm unclear on why there is a need to pass block arguments when calling a function.
why not just pass in as function arguments and what happens to the block arguments, how are they passed and used?
m.call(somevalue) {|_k, v| v['abc'] = 'xyz'}
module m
def call ( arg1, *arg2, &arg3)
end
end
Ruby, like almost all mainstream programming languages, is a strict language, meaning that arguments are fully evaluated before being passed into the method.
Now, imagine you want to implement (a simplified version of) Integer#times. The implementation would look a little bit like this:
class Integer
def my_times(action_to_be_executed)
raise ArgumentError, "`self` must be non-negative but is `#{inspect}`" if negative?
return if zero?
action_to_be_executed
pred.my_times(action_to_be_executed)
end
end
3.my_times(puts "Hello")
# Hello
0.my_times(puts "Hello")
# Hello
-1.my_times(puts "Hello")
# Hello
# ArgumentError (`self` must be non-negative but is `-1`)
As you can see, 3.my_times(puts "Hello") printed Hello exactly once, instead of thrice, as it should do. Also, 0.my_times(puts "Hello") printed Hello exactly once, instead of not at all, as it should do, despite the fact that it returns in the second line of the method, and thus action_to_be_executed is never even evaluated. Even -1.my_times(puts "Hello") printed Hello exactly once, despite that fact that it raises an ArgumentError exception as the very first thing in the method and thus the entire rest of the method body is never evaluated.
Why is that? Because Ruby is strict! Again, strict means that arguments are fully evaluated before being passed. So, what this means is that before my_times even gets called, the puts "Hello" is evaluated (which prints Hello to the standard output stream), and the result of that evaluation (which is just nil because Kernel#puts always returns nil) is passed into the method.
So, what we need to do, is somehow delay the evaluation of the argument. One way we know how to delay evaluation, is by using a method: methods are only evaluated when they are called.
So, we take a page out of Java's playbook, and define a Single Abstract Method Protocol: the argument that is being passed to my_each must be an object which implements a method with a specific name. Let's call it call, because, well, we are going to call it.
This would look a little bit like this:
class Integer
def my_times(action_to_be_executed)
raise ArgumentError, "`self` must be non-negative but is `#{inspect}`" if negative?
return if zero?
action_to_be_executed.call
pred.my_times(action_to_be_executed)
end
end
def (hello = Object.new).call
puts "Hello"
end
3.my_times(hello)
# Hello
# Hello
# Hello
0.my_times(hello)
-1.my_times(hello)
# ArgumentError (`self` must be non-negative but is `-1`)
Nice! It works! The argument that is passed is of course still strictly evaluated before being passed (we can't change the fundamental nature of Ruby from within Ruby itself), but this evaluation only results in the object that is bound by the local variable hello. The code that we want to run is another layer of indirection away and will only be executed at the point where we actually call it.
It also has another advantage: Integer#times actually makes the index of the current iteration available to the action as an argument. This was impossible to implement with our first solution, but here we can do it, because we are using a method and methods can take arguments:
class Integer
def my_times(action_to_be_executed)
raise ArgumentError, "`self` must be non-negative but is `#{inspect}`" if negative?
__my_times_helper(action_to_be_executed)
end
protected
def __my_times_helper(action_to_be_executed, index = 0)
return if zero?
action_to_be_executed.call(index)
pred.__my_times_helper(action_to_be_executed, index + 1)
end
end
def (hello = Object.new).call(i)
puts "Hello from iteration #{i}"
end
3.my_times(hello)
# Hello from iteration 0
# Hello from iteration 1
# Hello from iteration 2
0.my_times(hello)
-1.my_times(hello)
# ArgumentError (`self` must be non-negative but is `-1`)
However, this is not actually very readable. If you didn't want to give a name to this action that we are trying to pass but instead simply literally write it down inside the argument list, it would look something like this:
3.my_times(Object.new.tap do |obj|
def obj.call(i)
puts "Hello from iteration #{i}"
end
end)
# Hello from iteration 0
# Hello from iteration 1
# Hello from iteration 2
or on one line:
3.my_times(Object.new.tap do |obj| def obj.call; puts "Hello from iteration #{i}" end end)
# Hello from iteration 0
# Hello from iteration 1
# Hello from iteration 2
# or:
3.my_times(Object.new.tap {|obj| def obj.call; puts "Hello from iteration #{i}" end })
# Hello from iteration 0
# Hello from iteration 1
# Hello from iteration 2
Now, I don't know about you, but I find that pretty ugly.
In Ruby 1.9, Ruby added Proc literals aka stabby lambda literals to the language. Lambda literals are a concise literal syntax for writing objects with a call method, specifically Proc objects with Proc#call.
Using lambda literals, and without any changes to our existing code, it looks something like this:
3.my_times(-> i { puts "Hello from iteration #{i}" })
# Hello from iteration 0
# Hello from iteration 1
# Hello from iteration 2
This does not look bad!
When Yukihiro "matz" Matsumoto designed Ruby almost thirty years ago in early 1993, he did a survey of the core libraries and standard libraries of languages like Smalltalk, Scheme, and Common Lisp to figure out how such methods that take a piece of code as an argument are actually used, and he found that the overwhelming majority of such methods take exactly one code argument and all they do with that argument is call it.
So, he decided to add special language support for a single argument that contains code and can only be called. This argument is both syntactically and semantically lightweight, in particular, it looks syntactically exactly like any other control structure, and it is semantically not an object.
This special language feature, you probably guessed it, are blocks.
Every method in Ruby has an optional block parameter. I can always pass a block to a method. It's up to the method to do anything with the block. Here, for example, the block is useless because Kernel#puts doesn't do anything with a block:
puts("Hello") { puts "from the block" }
# Hello
Because blocks are not objects, you cannot call methods on them. Also, because there can be only one block argument, there is no need to give it a name: if you refer to a block, it's always clear which block because there can be only one. But, if the block doesn't have methods and doesn't have a name, how can we call it?
That's what the yield keyword is for. It temporarily "yields" control flow to the block, or, in other words, it calls the block.
With blocks, our solution would look like this:
class Integer
def my_times(&action_to_be_executed)
raise ArgumentError, "`self` must be non-negative but is `#{inspect}`" if negative?
return enum_for(__callee__) unless block_given?
__my_times_helper(&action_to_be_executed)
end
protected
def __my_times_helper(&action_to_be_executed, index = 0)
return if zero?
yield index
pred.__my_times_helper(&action_to_be_executed, index + 1)
end
end
3.my_times do
puts "Hello from iteration #{i}"
end
# Hello from iteration 0
# Hello from iteration 1
# Hello from iteration 2
0.my_times do
puts "Hello from iteration #{i}"
end
-1.my_times do
puts "Hello from iteration #{i}"
end
# ArgumentError (`self` must be non-negative but is `-1`)
Okay, you might notice that I simplified a bit when I wrote above that the only thing you can do with a block is call it. There are two other things you can do with it:
You can check whether a block argument was passed using Kernel#block_given?. Since blocks are always optional, and blocks have no names, there must be a way to check whether a block was passed or not.
You can "roll up" a block (which is not an object and doesn't have a name) into a Proc object (which is an object) and bind it to a parameter (which gives it a name) using the & ampersand unary prefix sigil in the parameter list of the method. Now that we have an object, and a way to refer to it, we can store it in a variable, return it from a method, or (as we are doing here) pass it along as an argument to a different method, which otherwise wouldn't be possible.
There is also the opposite operation: with the & ampersand unary prefix operator, you can "unroll" a Proc object into a block in an argument list; this makes it so that the method behaves as if you had passed the code that is stored inside the Proc as a literal block argument to the method.
And there you have it! That's what blocks are for: a semantically and syntactically lightweight form of passing code to a method.
There are other possible approaches, of course. The approach that is closest to Ruby is probably Smalltalk. Smalltalk also has a concept called blocks (in fact, that is where Ruby got both the idea and the name from). Similarly to Ruby, Smalltalk blocks have a syntactically light-weight literal form, but they are objects, and you can pass more than one to a method. Thanks to Smalltalk's generally light-weight and simple syntax, especially the keyword method syntax which intersperses parts of the method name with the arguments, even passing multiple blocks to a method call is very concise and readable.
For example, Smalltalk actually does not have an if / then / else conditional expression, in fact, Smalltalk has no control structures at all. Everything is done with methods. So, the way that a conditional works, is that the two boolean classes TrueClass and FalseClass each have a method named ifTrue:ifFalse: which takes two block arguments, and the two implementations will simply either evaluate the first or the second block. For example, the implementation in TrueClass might look a little bit like this (note that Smalltalk has no syntax for classes or methods, instead classes and methods are created in the IDE by creating class objects and method objects via the GUI):
True>>ifTrue: trueBlock ifFalse: falseBlock
"Answer with the value of `trueBlock`."
↑trueBlock value
The corresponding implementation in FalseClass would then look like this:
FalseClass>>ifTrue: trueBlock ifFalse: falseBlock
"Answer with the value of `falseBlock`."
↑falseBlock value
And you would call it like this:
2 < 3 ifTrue: [ Transcript show: 'yes' ] ifFalse: [ Transcript show: 'no' ].
"yes"
4 < 3 ifTrue: [ Transcript show: 'yes' ] ifFalse: [ Transcript show: 'no' ].
"no"
In ECMAScript, you can simply use function definitions as expressions, and there is also lightweight syntax for functions.
In the various Lisps, code is just data, and data is code, so you can just pass the code as an argument as data, then inside the function, treat that data as code again.
Scala has call-by-name parameters which are only evaluated when you use their name, and they are evaluated every time you use their name. It would look something like this:
implicit class IntegerTimes(val i: Int) extends AnyVal {
#scala.annotation.tailrec
def times(actionToBeExecuted: => Unit): Unit = {
if (i < 0) throw new Error()
if (i == 0) () else { actionToBeExecuted; (i - 1).times(actionToBeExecuted) }
}
}
3.times { println("Hello") }
// Hello
// Hello
// Hello
I write a ruby program that prints 3 different values of variable a with different data types:
a = 5
puts a
a = true
puts a
a = 1.325
return a
puts a
In this case, last value didn't print. When i remove return a, program prints a last value, 1.325.
But how?
return controls program flow, calling it will exit the current method and pass the value of a into whatever expression called it.
Usually you would not write a method with an unconditional return and more code afterward, because that code is not reachable. The puts in your example will never be called. Just move it to before the return expression if you want it to run.
I was doing some reading on if/elsif/else in Ruby, and I ran into some differences in terminology when describing how control expressions work.
In the Ruby Programming Wikibooks (emphasis added):
A conditional Branch takes the result of a test expression and executes a block of code depending whether the test expression is true or false.
and
An if expression, for example, not only determines whether a subordinate block of code will execute, but also results in a value itself.
Ruby-doc.org, however, does not mention blocks at all in the definitions:
The simplest if expression has two parts, a “test” expression and a “then” expression. If the “test” expression evaluates to a true then the “then” expression is evaluated.
Typically, when I have read about 'blocks' in Ruby, it has almost always been within the context of procs and lambdas. For example, rubylearning.com defines a block:
A Ruby block is a way of grouping statements, and may appear only in the source adjacent to a method call; the block is written starting on the same line as the method call's last parameter (or the closing parenthesis of the parameter list).
The questions:
When talking about blocks of code in Ruby, are we talking about
the group of code that gets passed in to a method or are we simply
talking about a group of code in general?
Is there a way to easily differentiate between the two (and is there
a technical difference between the two)?
Context for these questions: I am wondering if referring to the code inside of conditionals as blocks will be confusing to to new Ruby programmers when they are later introduced to blocks, procs, and lambdas.
TL;DR if...end is an expression, not a block
The proper use of the term block in Ruby is the code passed to a method in between do...end or curly braces {...}. A block can be and often is implicitly converted into a Proc within a method by using the &block syntax in the method signature. This new Proc is an object with its own methods that can be passed to other methods, stored in variables and data structures, called repeatedly, etc...
def block_to_proc(&block)
prc = block
puts prc
prc.class
end
block_to_proc { 'inside the block' }
# "#<Proc:0x007fa626845a98#(irb):21>"
# => Proc
In the code above, a Proc is being implicitly created with the block as its body and assigned to the variable block. Likewise, a Proc (or a lambda, a type of Proc) can be "expanded" into blocks and passed to methods that are expecting them, by using the &block syntax at the end of an arguments list.
def proc_to_block
result = yield # only the return value of the block can be saved, not the block itself
puts result
result.class
end
block = Proc.new { 'inside the Proc' }
proc_to_block(&block)
# "inside the Proc"
# => String
Although there's somewhat of a two-way street between blocks and Procs, they're not the same. Notice that to define a Proc we had to pass a block to Proc.new. Strictly speaking a block is just a chunk of code passed to a method whose execution is deferred until explicitly called. A Proc is defined with a block, its execution is also deferred until called, but it is a bonafide object just like any other. A block cannot survive on its own, a Proc can.
On the other hand, block or block of code is sometimes casually used to refer to any discreet chunk of code enclosed by Ruby keywords terminating with end: if...else...end, begin...rescue...end, def...end, class...end, module...end, until...end. But these are not really blocks, per se, and only really resemble them on the surface. Often they also have deferred execution until some condition is met. But they can stand entirely on their own, and always have return values. Ruby-doc.org's use of "expression" is more accurate.
From wikipedia
An expression in a programming language is a combination of one or
more explicit values, constants, variables, operators, and functions
that the programming language interprets (according to its particular
rules of precedence and of association) and computes to produce ("to
return", in a stateful environment) another value.
This is why you can do things like this
return_value = if 'expression'
true
end
return_value # => true
Try doing that with a block
return_value = do
true
end
# SyntaxError: (irb):24: syntax error, unexpected keyword_do_block
# return_value = do
# ^
A block is not an expression on its own. It needs either yield or a conversion to a Proc to survive. What happens when we pass a block to a method that doesn't want one?
puts("indifferent") { "to blocks" }
# "indifferent"
# => nil
The block is totally lost, it disappears with no return value, no execution, as if it never existed. It needs yield to complete the expression and produce a return value.
class Object
def puts(*args)
super
yield if block_given?
end
end
puts("mindful") { "of blocks" }
# "mindful"
# => "of blocks"
I have the following code, which is supposed to provide a simple true-false wrapper over Array#detect, which is nil-element.
class Array
def any &expr
if (self.detect expr)
return true
else
return false
end
end
end
For some weird reason, no matter what is passed to &expr, it ALWAYS returns true! Why is this?
The documentation for Enumerable#detect says that it can optionally take one argument. If it doesn't find the element that matched your block, it returns this argument. In your case, you're passing a Proc object, expr to detect, and not passing a block. This causes detect to return an enumerator, which won't be interpreted as a "falsy" value.
I think instead you want self.detect &expr to pass an actual block instead of a Proc.
Can someone explain to me Ruby's use of pipe characters in a block? I understand that it contains a variable name that will be assigned the data as it iterates. But what is this called? Can there be more than one variable inside the pipes? Anything else I should know about it? Any good links to more information on it?
For example:
25.times { | i | puts i }
Braces define an anonymous function, called a block. Tokens between the pipe are the arguments of this block. The number of arguments required depends on how the block is used. Each time the block is evaluated, the method requiring the block will pass a value based on the object calling it.
It's the same as defining a method, only it's not stored beyond the method that accepts a block.
For example:
def my_print(i)
puts i
end
will do the same as this when executed:
{|i| puts i}
the only difference is the block is defined on the fly and not stored.
Example 2:
The following statements are equivalent
25.times &method(:my_print)
25.times {|i| puts i}
We use anonymous blocks because the majority of functions passed as a block are usually specific to your situation and not worth defining for reuse.
So what happens when a method accepts a block? That depends on the method. Methods that accept a block will call it by passing values from their calling object in a well defined manner. What's returned depends on the method requiring the block.
For example: In 25.times {|i| puts i} .times calls the block once for each value between 0 and the value of its caller, passing the value into the block as the temporary variable i. Times returns the value of the calling object. In this case 25.
Let's look at method that accepts a block with two arguments.
{:key1 => "value1", :key2 => "value2"}.each {|key,value|
puts "This key is: #{key}. Its value is #{value}"
}
In this case each calls the block ones for each key/value pair passing the key as the first argument and the value as the second argument.
The pipes specify arguments that are populated with values by the function that calls your block. There can be zero or more of them, and how many you should use depends on the method you call.
For example, each_with_index uses two variables and puts the element in one of them and the index in the other.
here is a good description of how blocks and iterators work
Block arguments follow all the same conventions as method parameters (at least as of 1.9): you can define optional arguments, variable length arg lists, defaults, etc. Here's a pretty decent summary.
Some things to be aware of: because blocks see variables in the scope they were defined it, if you pass in an argument with the same name as an existing variable, it will "shadow" it - your block will see the passed in value and the original variable will be unchanged.
i = 10
25.times { | i | puts i }
puts i #=> prints '10'
Will print '10' at the end. Because sometimes this is desirable behavior even if you are not passing in a value (ie you want to make sure you don't accidentally clobber a variable from surrounding scope) you can specify block-local variable names after a semicolon after the argument list:
x = 'foo'
25.times { | i ; x | puts i; x = 'bar' }
puts x #=> prints 'foo'
Here, 'x' is local to the block, even though no value is passed in.