The Ruby Documentation says that "do/end is equivalent to curly braces", so why is it that when I attempt to do the following I do not receive any output:
a = [1, 2, 3]
for i in a {
puts i
}
When I perform the above I receive no output (but I don't receive an error message either). However, when I do the following everything is as it should be:
a = [1, 2, 3]
for i in a do
puts i
end
#=> 1
#=> 2
#=> 3
I know this can be done more idiomatically with the each statement, but that's not what I'm asking. What am I not understanding here?
The Ruby Documentation says that "do/end is equivalent to curly braces"
No, it doesn't. It says (bold emphasis mine):
In this context, do/end is equivalent to curly braces […]
What "in this context" means is defined directly before the half-sentence you quoted:
do
Paired with end, can delimit a code block
So, "in this context" here refers to the context of a block.
so why is it that when I attempt to do the following I do not receive any output
Because this is a completely different context, again quoting from the documentation you linked to:
do can also (optionally) appear at the end of a for/in statement. (See for for an example.)
The "also" in that sentence makes it very clear that this is a different usage of the keyword do that has nothing to do with the usage discussed in this section. And if you look at the documentation of for, you can see that there is no mention of curly braces being allowed.
When I perform the above I receive no output (but I don't receive an error message either).
That is not true. Your code is syntactically invalid because it is missing the end keyword to end the for/in expression, therefore you get a "syntax error, unexpected end-of-input" on line 4:
ruby -c -e 'a = [1, 2, 3]
for i in a {
puts i
}'
# -e:4: syntax error, unexpected end-of-input
And if you add the missing end, you get a in `<main>': undefined method `a' for main:Object (NoMethodError) on line 2:
ruby -e 'a = [1, 2, 3]
for i in a {
puts i
}
end'
# -e:2:in `<main>': undefined method `a' for main:Object (NoMethodError)
Again, this is expected because curly braces delimit a code block, so
a {
puts i
}
is interpreted as a code block being passed to a and since variables cannot receive arguments, only methods can, a must be a method. Therefore, Ruby rightfully complains about not finding a method named a.
There are three ways of delimiting the iterator expression from the loop body expression in a for/in loop (and the same applies to while and until loops, actually):
An expression separator. An expression separator can either be
a semicolon ;
a newline
The keyword do
So, the following would all be valid fixes for your code:
# non-idiomatic
for i in a; puts i end
# non-idiomatic
for i in a
puts i end
# the same but with idiomatic indentation and whitespace
for i in a
puts i
end
# idiomatic
for i in a do puts i end
# redundant, non-idiomatic
for i in a do
puts i
end
Note, that when I say "idiomatic" above, that is to be interpreted relative, since actually for/in loops as a whole are completely non-idiomatic, and you would rather do this:
a.each do |i|
puts i
end
or maybe
a.each(&method(:puts))
It is in general preferred to not mix I/O and data transformation, so another idiomatic solution would be to transform the data to the desired output first, then output it, like this:
puts a.join("\n")
Except that Kernel#puts will already treat Array arguments special and print each element on its own line (as documented at IO#puts), so the real correct idiomatic solution for your code would be just:
puts a
Take a look to the documentation here: For loop
It states:
Like while and until, the do is optional. The for loop is similar to
using each, but does not create a new variable scope.
And also
The for loop is rarely used in modern ruby programs.
So, be less Pythonic :) using Enumerator#each instead:
a.each { |a| puts a }
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'm very new to Ruby, so please bear with me.
Why is it a syntax error to have "test" {|s| print s}? How about "test" do |s| print s end?
Thanks
You can't say this:
"test" { |s| print s }
because "test" is a string literal, not a method. The same would apply to your do/end version. You could say:
["test"].each { |s| print s }
though because Arrays are Enumerable and Enumerable has an each method.
The {} are usually used for one liners.
do/end for multiple lines.
But there is no rule, do what you prefer.
Notice:
If ever you need to pass several instructions in a one liner, separate them with ;
A block is just a chunk of code enclosed in braces or keywords do/end. As mentioned already, you typically use braces for one liners, and do/end for multiple lines of code. Blocks can appear only immediately after the calling of some method. You can think of a block as an anonymous method (one that doesn't have a method name).
In your code, you were placing a block immediately after a string literal, not a method invocation. Blocks can be used for looping, as such:
2.times { puts "hello" } # => 2
# >> hello
# >> hello
In the above code, times is a method that belongs to all integers (that is to say, it is a instance method of the Integer class). The times method executes the code in the block twice, and returns the object (2 in this case) you called it on. You can pass a block to any method, although methods that are not expecting them will simply ignore the block.
Blocks can take parameters. The parameters are placed between pipes (the '|' character). It turns out, the first example could have accepted a parameter as seen here:
2.times { |i| puts i.to_s + " hello" } # => 2
# >> 0 hello
# >> 1 hello
I've only just scratched the surface of the power of blocks. You can read more about blocks for free in the online version of Programming Ruby: The Pragmatic Programmer's Guide (aka PickAx Book). It is a couple editions old now, but for an introduction to Ruby, you should find it sufficient. Once you understand blocks, you can start using power features of Enumerable which is included in Arrays and Hashes.
I recently started learning ruby, and I understood that you coud use code blocks with both of these syntaxes. But I just found a case which I dont understand:
#my_hash is a hash in which the keys are strings and the values arrays, but dont think about the specifics fo the code
#if I run my code like this, it works perfectly
my_hash.each do |art|
puts mystring.gsub(art[0]).each {
art[1][rand(art[1].length) -1]
}
end
#but if I use this, it prints "Enumerator"
my_hash.each do |art|
puts mystring.gsub(art[0]).each do
art[1][rand(art[1].length) -1]
end
end
Is it because you cant nest do-end pairs?
I am using 1.9
puts mystring.gsub(art[0]).each do
art[1][rand(art[1].length) -1]
end
Here you called puts without parens, the do ... end refers to the puts method, that does nothing with a block and prints mystring.gsub(art[0]).each (with is a Enumerator).
The { ... } is called with the nearest method. Becomes ugly, but you can do it with do ... end:
puts(mystring.gsub(art[0]).each do
art[1][rand(art[1].length) -1]
end)
Or, better, put the result in a variable and print the variable:
var = mystring.gsub(art[0]).each do
art[1][rand(art[1].length) -1]
end
puts var
Anyway, the each don't changes the object, it just iterate and returns the object itself. You may be wanting the map method, test it.
Expanding on Scott's reply, and quoting Jim Weirich:
The difference is where in the operator precedence table they fall. { } binds tighter than do/end. For example:
f g { }
is parsed as f(g { }), where the braces belong to the method g. On the other hand,
f g do end
is parsed as f(g) do end, where the braces belong to the method f. It only matters when you omit parenthesis and create ambiguities.
Nesting do/end pairs is perfectly legal in Ruby, but you are coming up against a subtle precedence issue between { } and do/end.
You can read a little more about that here.
In the book Rails Test Prescriptions (b10.0, page 176), there are examples of one-liner assertions like the following:
should "be successful" { assert_response :success }
This doesn’t appear to be valid ruby syntax to me, and ruby reports that the left curly brace is unexpected. In order for it to be parsed, I have to change it to
should "be successful"; do assert_response :success end
What's wrong with the syntax of the first example?
This is valid Ruby syntax. Well, sort of. It just doesn't make sense!
Since the precedence of a literal block using curly braces is higher than passing an argument without parentheses, the block gets bound to the argument instead of the method call. If the argument is itself a method call, then you won't even get a syntax error. You'll just scratch your head wondering why your block doesn't get called.
To fix this, you either put parentheses around the argument, since parentheses have higher precedence than curly braces, or use the do / end form, which is lower precedence than an argument list without parentheses.
def foo; yield if block_given?; 'foo' end
puts foo { puts 'block' }
# block
# foo
puts(foo) { puts 'block' }
# foo
puts foo do puts 'block' end
# foo
puts foo { puts 'block' }, foo { puts 'block' }
# block
# block
# foo
# foo
puts 'foo' { puts 'block' }
# SyntaxError: (irb):19: syntax error, unexpected '{', expecting $end
This may be a mistake on my part in trying to get the example to take up fewer lines. The block is probably binding to the argument and not the method call, as Jorg said.
I think that the proper way to rewrite this as a one-liner is:
should("be successful") { assert_response :success }
But really, the way to go is to use the shoulda macro:
should respond_with(:success)
Thanks,
Noel
I'm new to Ruby, and I'm trying the following:
mySet = numOfCuts.times.map{ rand(seqLength) }
but I get the 'yield called out of block' error. I'm not sure what his means. BTW, this question is part of a more general question I asked here.
The problem is that the times method expects to get a block that it will yield control to. However you haven't passed a block to it. There are two ways to solve this. The first is to not use times:
mySet = (1..numOfCuts).map{ rand(seqLength) }
or else pass a block to it:
mySet = []
numOfCuts.times {mySet.push( rand(seqLength) )}
if "numOfCuts" is an integer,
5.times.foo
is invalid
"times" expects a block.
5.times{ code here }
You're combining functions that don't seem to make sense -- if numOfCuts is an integer, then just using times and a block will run the block that many times (though it only returns the original integer:
irb(main):089:0> 2.times {|x| puts x}
0
1
2
map is a function that works on ranges and arrays and returns an array:
irb(main):092:0> (1..3).map { |x| puts x; x+1 }
1
2
3
[2, 3, 4]
I'm not sure what you're trying to achieve with the code - what are you trying to do? (as opposed to asking specifically about what appears to be invalid syntax)
Bingo, I just found out what this is. Its a JRuby bug.
Under MRI
>> 3.times.map
=> [0, 1, 2]
>>
Under JRuby
irb(main):001:0> 3.times.map
LocalJumpError: yield called out of block
from (irb):2:in `times'
from (irb):2:in `signal_status'
irb(main):002:0>
Now, I don't know if MRI (the standard Ruby implementation) is doing the right thing here. It probably should complain that this does not make sense, but when n.times is called in MRI it returns an Enumerator, whereas Jruby complains that it needs a block.
Integer.times expects a block. The error message means the yield statement inside the times method can not be called because you did not give it a block.
As for your code, I think what you are looking for is a range:
(1..5).map{ do something }
Here is thy rubydoc for the Integer.times and Range.