I'm trying to figure out how I can create a method in Ruby where I can retrieve values from the method's parameters such as strings/integers.
For example, if this were a function coded in C, it might be done similar to this:
main()
{
int value;
GetAnIntegerValue(value);
printf("The value is %d", value);
}
// The "value" integer variable is passed to it, and updated accordingly because of the use of the ampersand prior to the parameter
GetAnIntegerValue(&int value)
{
value = 5;
}
// The output would be "The value is 5"
I think the term for this is pass by value but I'm not sure. My mind is a little vague on this area and I couldn't find many decent results.
Here's my example Ruby function, the array that the parameters are being assigned to is only local to the class which is the reason for this usage:
def getRandomWordAndHint(&RandomWord, &RandomHint)
randIndex = rand(7)
RandomWord = EnglishLevel1Word[randIndex]
RandomHint = EnglishLevel1Hint[randIndex]
end
Cheers!i
Ruby is pass-by-value. Always. No exceptions. You cannot do pass-by-reference in Ruby.
What you can do, is put the object you want to change into some sort of mutable container:
class MutableCell
attr_accessor :val
def initialize(val)
self.val = val
end
end
def change_the_value(cell)
cell.val = 5
end
value = MutableCell.new(42)
change_the_value(value)
value.val
# => 5
Of course, you can just use an Array instead of writing your own MutableCell class, this is just for demonstration.
However, mutable state is a bad idea in general, and mutating arguments passed to methods is a really bad idea especially. Methods know about their own object (i.e. self) and thus can safely modify it, but for other objects, that's generally a no-go.
i have following code:
def mymethod(a)
a.replace("a")
end
mystring = "b"
mymethod(mystring)
p mystring # => "a"
but i want to perform same with Integer
is that possible?
Short answer: no.
Long answer: no, it's not possible. Integer is a type primitive enough to not have state (and state modifying operations). Every operation on integer generates a new integer.
Probably, if you drop down to C level, you could be able to modify underlying value in-place. But I'm not sure. Anyway, this seems like an overkill and a wrong thing to do.
-1 for "wrong thing to do". This can be a perfectly reasonable issue -- I need to do this right now: multiple objects need a shared counter.
It seems to me that the best way is to create a wrapper class and have the integer as an instance variable:
class Counter
def initialize(i = 0)
#i = i
end
def get
#i
end
def set(i)
#i = i
end
def inc(delta = 1)
#i += delta
end
end
My background is in PHP and C#, but I'd really like to learn RoR. To that end, I've started reading the official documentation. I have some questions about some code examples.
The first is with iterators:
class Array
def inject(n)
each { |value| n = yield(n, value) }
n
end
def sum
inject(0) { |n, value| n + value }
end
def product
inject(1) { |n, value| n * value }
end
end
I understand that yield means "execute the associated block here." What's throwing me is the |value| n = part of the each. The other blocks make more sense to me as they seem to mimic C# style lambdas:
public int sum(int n, int value)
{
return Inject((n, value) => n + value);
}
But the first example is confusing to me.
The other is with symbols. When would I want to use them? And why can't I do something like:
class Example
attr_reader #member
# more code
end
In the inject or reduce method, n represents an accumulated value; this means the result of every iteration is accumulated in the n variable. This could be, as is in your example, the sum or product of the elements in the array.
yield returns the result of the block, which is stored in n and used in the next iterations. This is what makes the result "cumulative."
a = [ 1, 2, 3 ]
a.sum # inject(0) { |n, v| n + v }
# n == 0; n = 0 + 1
# n == 1; n = 1 + 2
# n == 3; n = 3 + 3
=> 6
Also, to compute the sum you could also have written a.reduce :+. This works for any binary operation. If your method is named symbol, writing a.reduce :symbol is the same as writing a.reduce { |n, v| n.symbol v }.
attr and company are actually methods. Under the hood, they dynamically define the methods for you. It uses the symbol you passed to work out the names of the instance variable and the methods. :member results in the #member instance variable and the member and member = methods.
The reason you can't write attr_reader #member is because #member isn't an object in itself, nor can it be converted to a symbol; it actually tells ruby to fetch the value of the instance variable #member of the self object, which, at class scope, is the class itself.
To illustrate:
class Example
#member = :member
attr_accessor #member
end
e = Example.new
e.member = :value
e.member
=> :value
Remember that accessing unset instance variables yields nil, and since the attr method family accepts only symbols, you get: TypeError: nil is not a symbol.
Regarding Symbol usage, you can sort of use them like strings. They make excellent hash keys because equal symbols always refer to the same object, unlike strings.
:a.object_id == :a.object_id
=> true
'a'.object_id == 'a'.object_id
=> false
They're also commonly used to refer to method names, and can actually be converted to Procs, which can be passed to methods. This is what allows us to write things like array.map &:to_s.
Check out this article for more interpretations of the symbol.
For the definition of inject, you're basically setting up chained blocks. Specifically, the variable n in {|value| n = yield(n, value)} is essentially an accumulator for the block passed to inject. So, for example, for the definition of product, inject(1) {|value| n * value}, let's assume you have an array my_array = [1, 2, 3, 4]. When you call my_array.product, you start by calling inject with n = 1. each yields to the block defined in inject, which in turns yields to the block passed to inject itself with n (1) and the first value in the array (1 as well, in this case). This block, {|n, value| n * value} returns 1 == 1 * 1, which is set it inject's n variable. Next, 2 is yielded from each, and the block defined in inject block yields as yield(1, 2), which returns 2 and assigns it to n. Next 3 is yielded from each, the block yields the values (2, 3) and returns 6, which is stored in n for the next value, and so forth. Essentially, tracking the overall value agnostic of the calculation being performed in the specialised routines (sum and product) allows for generalization. Without that, you'd have to declare e.g.
def sum
n = 0
each {|val| n += val}
end
def product
n = 1
each {|val| n *= val}
end
which is annoyingly repetitive.
For your second question, attr_reader and its family are themselves methods that are defining the appropriate accessor routines using define_method internally, in a process called metaprogramming; they are not language statements, but just plain old methods. These functions expect to passed a symbol (or, perhaps, a string) that gives the name of the accessors you're creating. You could, in theory, use instance variables such as #member here, though it would be the value to which #member points that would be passed in and used in define_method. For an example of how these are implemented, this page shows some examples of attr_* methods.
def inject(accumulator)
each { |value| accumulator = yield(accumulator, value) }
accumulator
end
This is just yielding the current value of accumulator and the array item to inject's block and then storing the result back into accumulator again.
class Example
attr_reader #member
end
attr_reader is just a method whose argument is the name of the accessor you want to setup. So, in a contrived way you could do
class Example
#ivar_name = 'foo'
attr_reader #ivar_name
end
to create an getter method called foo
Your confusion with the first example may be due to your reading |value| n as a single expression, but it isn't.
This reformatted version might be clearer to you:
def inject(n)
each do |value|
n = yield(n, value)
end
return n
end
value is an element in the array, and it is yielded with n to whatever block is passed to inject, the result of which is set to n. If that's not clear, read up on the each method, which takes a block and yields each item in the array to it. Then it should be clearer how the accumulation works.
attr_reader is less weird when you consider that it is a method for generating accessor methods. It's not an accessor in itself. It doesn't need to deal with the #member variable's value, just its name. :member is just the interned version of the string 'member', which is the name of the variable.
You can think of symbols as lighter weight strings, with the additional bonus that every equal label is the same object - :foo.object_id == :foo.object_id, whereas 'foo'.object_id != 'foo'.object_id, because each 'foo' is a new object. You can try that for yourself in irb. Think of them as labels, or primitive strings. They're surprisingly useful and come up a lot, e.g. for metaprogramming or as keys in hashes. As pointed out elsewhere, calling object.send :foo is the same as calling object.foo
It's probably worth reading some early chapters from the 'pickaxe' book to learn some more ruby, it will help you understand and appreciate the extra stuff rails adds.
First you need to understand where to use symbols and where its not..
Symbol is especially used to represent something. Ex: :name, :age like that. Here we are not going to perform any operations using this.
String are used only for data processing. Ex: 'a = name'. Here I gonna use this variable 'a' further for other string operations in ruby.
Moreover, symbol is more memory efficient than strings and it is immutable. That's why ruby developer's prefers symbols than string.
You can even use inject method to calculate sum as (1..5).to_a.inject(:+)
I was trying to see if I could reconstruct the Array class' delete_if iterator as my own method in order to see if I understood methods and blocks correctly. Here is what I coded:
def delete_if(arr)
for x in 0...arr.length
if (yield arr[x])
arr[x]=arr[x+1,arr.length]
redo
end
end
end
arr = [0,1,2,3,4,5]
delete_if(arr) {|value| value % 2 == 0}
This resulted in an error saying that the % method could not be identified in the last line. I know that value is going to be an integer so I am not sure why it would say this error. Can someone please explain? Also, in Ruby in general, how can you be sure that someone passes the correct type into a method? What if the method is supposed to take a string but they pass in an integer -- how do you prevent that??
Thanks!
def delete_if arr
for x in 0...arr.length
return if x >= arr.length
if yield arr[x]
arr[x..-1] = arr[(x + 1)..-1]
redo
end
end
end
Things I fixed:
it's necessary to mutate the array, if all you do is assign to the parameter, your changes will be local to the method. And for that matter, you were assigning your calculated array object to an element of the original array, which was the immediate cause of the error message.
since the array may become shorter, we need to bail out at the (new) end
of course you could just use arr.delete_at x but I couldn't correct the slice assignment without keeping the code pattern
In Ruby, is it possible to pass by reference a parameter with value-type semantics (e.g. a Fixnum)?
I'm looking for something similar to C#'s 'ref' keyword.
Example:
def func(x)
x += 1
end
a = 5
func(a) #this should be something like func(ref a)
puts a #should read '6'
Btw. I know I could just use:
a = func(a)
You can accomplish this by explicitly passing in the current binding:
def func(x, bdg)
eval "#{x} += 1", bdg
end
a = 5
func(:a, binding)
puts a # => 6
Ruby doesn't support "pass by reference" at all. Everything is an object and the references to those objects are always passed by value. Actually, in your example you are passing a copy of the reference to the Fixnum Object by value.
The problem with the your code is, that x += 1 doesn't modify the passed Fixnum Object but instead creates a completely new and independent object.
I think, Java programmers would call Fixnum objects immutable.
In Ruby you can't pass parameters by reference. For your example, you would have to return the new value and assign it to the variable a or create a new class that contains the value and pass an instance of this class around. Example:
class Container
attr_accessor :value
def initialize value
#value = value
end
end
def func(x)
x.value += 1
end
a = Container.new(5)
func(a)
puts a.value
You can try following trick:
def func(x)
x[0] += 1
end
a = [5]
func(a) #this should be something like func(ref a)
puts a[0] #should read '6'
http://ruby-doc.org/core-2.1.5/Fixnum.html
Fixnum objects have immediate value. This means that when they are assigned or
passed as parameters, the actual object is passed, rather than a reference to
that object.
Also Ruby is pass by value.
However, it seems that composite objects, like hashes, are passed by reference:
fp = {}
def changeit(par)
par[:abc] = 'cde'
end
changeit(fp)
p fp
gives
{:abc=>"cde"}