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"}
Related
#user.update_languages(params[:language][:language1],
params[:language][:language2],
params[:language][:language3])
lang_errors = #user.errors
logger.debug "--------------------LANG_ERRORS----------101-------------"
+ lang_errors.full_messages.inspect
if params[:user]
#user.state = params[:user][:state]
success = success & #user.save
end
logger.debug "--------------------LANG_ERRORS-------------102----------"
+ lang_errors.full_messages.inspect
if lang_errors.full_messages.empty?
#user object adds errors to the lang_errors variable in the update_lanugages method.
when I perform a save on the #user object I lose the errors that were initially stored in the lang_errors variable.
Though what I am attempting to do would be more of a hack (which does not seem to be working). I would like to understand why the variable values are washed out. I understand pass by reference so I would like to know how the value can be held in that variable without being washed out.
The other answerers are all correct, but a friend asked me to explain this to him and what it really boils down to is how Ruby handles variables, so I thought I would share some simple pictures / explanations I wrote for him (apologies for the length and probably some oversimplification):
Q1: What happens when you assign a new variable str to a value of 'foo'?
str = 'foo'
str.object_id # => 2000
A: A label called str is created that points at the object 'foo', which for the state of this Ruby interpreter happens to be at memory location 2000.
Q2: What happens when you assign the existing variable str to a new object using =?
str = 'bar'.tap{|b| puts "bar: #{b.object_id}"} # bar: 2002
str.object_id # => 2002
A: The label str now points to a different object.
Q3: What happens when you assign a new variable = to str?
str2 = str
str2.object_id # => 2002
A: A new label called str2 is created that points at the same object as str.
Q4: What happens if the object referenced by str and str2 gets changed?
str2.replace 'baz'
str2 # => 'baz'
str # => 'baz'
str.object_id # => 2002
str2.object_id # => 2002
A: Both labels still point at the same object, but that object itself has mutated (its contents have changed to be something else).
How does this relate to the original question?
It's basically the same as what happens in Q3/Q4; the method gets its own private copy of the variable / label (str2) that gets passed in to it (str). It can't change which object the label str points to, but it can change the contents of the object that they both reference to contain else:
str = 'foo'
def mutate(str2)
puts "str2: #{str2.object_id}"
str2.replace 'bar'
str2 = 'baz'
puts "str2: #{str2.object_id}"
end
str.object_id # => 2004
mutate(str) # str2: 2004, str2: 2006
str # => "bar"
str.object_id # => 2004
In traditional terminology, Ruby is strictly pass-by-value. But that's not really what you're asking here.
Ruby doesn't have any concept of a pure, non-reference value, so you certainly can't pass one to a method. Variables are always references to objects. In order to get an object that won't change out from under you, you need to dup or clone the object you're passed, thus giving an object that nobody else has a reference to. (Even this isn't bulletproof, though — both of the standard cloning methods do a shallow copy, so the instance variables of the clone still point to the same objects that the originals did. If the objects referenced by the ivars mutate, that will still show up in the copy, since it's referencing the same objects.)
Ruby uses "pass by object reference"
(Using Python's terminology.)
To say Ruby uses "pass by value" or "pass by reference" isn't really descriptive enough to be helpful. I think as most people know it these days, that terminology ("value" vs "reference") comes from C++.
In C++, "pass by value" means the function gets a copy of the variable and any changes to the copy don't change the original. That's true for objects too. If you pass an object variable by value then the whole object (including all of its members) get copied and any changes to the members don't change those members on the original object. (It's different if you pass a pointer by value but Ruby doesn't have pointers anyway, AFAIK.)
class A {
public:
int x;
};
void inc(A arg) {
arg.x++;
printf("in inc: %d\n", arg.x); // => 6
}
void inc(A* arg) {
arg->x++;
printf("in inc: %d\n", arg->x); // => 1
}
int main() {
A a;
a.x = 5;
inc(a);
printf("in main: %d\n", a.x); // => 5
A* b = new A;
b->x = 0;
inc(b);
printf("in main: %d\n", b->x); // => 1
return 0;
}
Output:
in inc: 6
in main: 5
in inc: 1
in main: 1
In C++, "pass by reference" means the function gets access to the original variable. It can assign a whole new literal integer and the original variable will then have that value too.
void replace(A &arg) {
A newA;
newA.x = 10;
arg = newA;
printf("in replace: %d\n", arg.x);
}
int main() {
A a;
a.x = 5;
replace(a);
printf("in main: %d\n", a.x);
return 0;
}
Output:
in replace: 10
in main: 10
Ruby uses pass by value (in the C++ sense) if the argument is not an object. But in Ruby everything is an object, so there really is no pass by value in the C++ sense in Ruby.
In Ruby, "pass by object reference" (to use Python's terminology) is used:
Inside the function, any of the object's members can have new values assigned to them and these changes will persist after the function returns.*
Inside the function, assigning a whole new object to the variable causes the variable to stop referencing the old object. But after the function returns, the original variable will still reference the old object.
Therefore Ruby does not use "pass by reference" in the C++ sense. If it did, then assigning a new object to a variable inside a function would cause the old object to be forgotten after the function returned.
class A
attr_accessor :x
end
def inc(arg)
arg.x += 1
puts arg.x
end
def replace(arg)
arg = A.new
arg.x = 3
puts arg.x
end
a = A.new
a.x = 1
puts a.x # 1
inc a # 2
puts a.x # 2
replace a # 3
puts a.x # 2
puts ''
def inc_var(arg)
arg += 1
puts arg
end
b = 1 # Even integers are objects in Ruby
puts b # 1
inc_var b # 2
puts b # 1
Output:
1
2
2
3
2
1
2
1
* This is why, in Ruby, if you want to modify an object inside a function but forget those changes when the function returns, then you must explicitly make a copy of the object before making your temporary changes to the copy.
Is Ruby pass by reference or by value?
Ruby is pass-by-value. Always. No exceptions. No ifs. No buts.
Here is a simple program which demonstrates that fact:
def foo(bar)
bar = 'reference'
end
baz = 'value'
foo(baz)
puts "Ruby is pass-by-#{baz}"
# Ruby is pass-by-value
Ruby is pass-by-value in a strict sense, BUT the values are references.
This could be called "pass-reference-by-value". This article has the best explanation I have read: http://robertheaton.com/2014/07/22/is-ruby-pass-by-reference-or-pass-by-value/
Pass-reference-by-value could briefly be explained as follows:
A function receives a reference to (and will access) the same object in memory as used by the caller. However, it does not receive the box that the caller is storing this object in; as in pass-value-by-value, the function provides its own box and creates a new variable for itself.
The resulting behavior is actually a combination of the classical definitions of pass-by-reference and pass-by-value.
There are already some great answers, but I want to post the definition of a pair of authorities on the subject, but also hoping someone might explain what said authorities Matz (creator of Ruby) and David Flanagan meant in their excellent O'Reilly book, The Ruby Programming Language.
[from 3.8.1: Object References]
When you pass an object to a method in Ruby, it is an object reference that is passed to the method. It is not the object itself, and it is not a reference to the reference to the object. Another way to say this is that method arguments are passed by value rather than by reference, but that the values passed are object references.
Because object references are passed to methods, methods can use those references to modify the underlying object. These modifications are then visible when the method returns.
This all makes sense to me until that last paragraph, and especially that last sentence. This is at best misleading, and at worse confounding. How, in any way, could modifications to that passed-by-value reference change the underlying object?
Is Ruby pass by reference or by value?
Ruby is pass-by-reference. Always. No exceptions. No ifs. No buts.
Here is a simple program which demonstrates that fact:
def foo(bar)
bar.object_id
end
baz = 'value'
puts "#{baz.object_id} Ruby is pass-by-reference #{foo(baz)} because object_id's (memory addresses) are always the same ;)"
=> 2279146940 Ruby is pass-by-reference 2279146940 because object_id's (memory addresses) are always the same ;)
def bar(babar)
babar.replace("reference")
end
bar(baz)
puts "some people don't realize it's reference because local assignment can take precedence, but it's clearly pass-by-#{baz}"
=> some people don't realize it's reference because local assignment can take precedence, but it's clearly pass-by-reference
Parameters are a copy of the original reference. So, you can change values, but cannot change the original reference.
Try this:--
1.object_id
#=> 3
2.object_id
#=> 5
a = 1
#=> 1
a.object_id
#=> 3
b = 2
#=> 2
b.object_id
#=> 5
identifier a contains object_id 3 for value object 1 and identifier b contains object_id 5 for value object 2.
Now do this:--
a.object_id = 5
#=> error
a = b
#value(object_id) at b copies itself as value(object_id) at a. value object 2 has object_id 5
#=> 2
a.object_id
#=> 5
Now, a and b both contain same object_id 5 which refers to value object 2.
So, Ruby variable contains object_ids to refer to value objects.
Doing following also gives error:--
c
#=> error
but doing this won't give error:--
5.object_id
#=> 11
c = 5
#=> value object 5 provides return type for variable c and saves 5.object_id i.e. 11 at c
#=> 5
c.object_id
#=> 11
a = c.object_id
#=> object_id of c as a value object changes value at a
#=> 11
11.object_id
#=> 23
a.object_id == 11.object_id
#=> true
a
#=> Value at a
#=> 11
Here identifier a returns value object 11 whose object id is 23 i.e. object_id 23 is at identifier a, Now we see an example by using method.
def foo(arg)
p arg
p arg.object_id
end
#=> nil
11.object_id
#=> 23
x = 11
#=> 11
x.object_id
#=> 23
foo(x)
#=> 11
#=> 23
arg in foo is assigned with return value of x.
It clearly shows that argument is passed by value 11, and value 11 being itself an object has unique object id 23.
Now see this also:--
def foo(arg)
p arg
p arg.object_id
arg = 12
p arg
p arg.object_id
end
#=> nil
11.object_id
#=> 23
x = 11
#=> 11
x.object_id
#=> 23
foo(x)
#=> 11
#=> 23
#=> 12
#=> 25
x
#=> 11
x.object_id
#=> 23
Here, identifier arg first contains object_id 23 to refer 11 and after internal assignment with value object 12, it contains object_id 25. But it does not change value referenced by identifier x used in calling method.
Hence, Ruby is pass by value and Ruby variables do not contain values but do contain reference to value object.
It should be noted that you do not have to even use the "replace" method to change the value original value. If you assign one of the hash values for a hash, you are changing the original value.
def my_foo(a_hash)
a_hash["test"]="reference"
end;
hash = {"test"=>"value"}
my_foo(hash)
puts "Ruby is pass-by-#{hash["test"]}"
Two references refer to same object as long as there is no reassignment.
Any updates in the same object won't make the references to new memory since it still is in same memory.
Here are few examples :
a = "first string"
b = a
b.upcase!
=> FIRST STRING
a
=> FIRST STRING
b = "second string"
a
=> FIRST STRING
hash = {first_sub_hash: {first_key: "first_value"}}
first_sub_hash = hash[:first_sub_hash]
first_sub_hash[:second_key] = "second_value"
hash
=> {first_sub_hash: {first_key: "first_value", second_key: "second_value"}}
def change(first_sub_hash)
first_sub_hash[:third_key] = "third_value"
end
change(first_sub_hash)
hash
=> {first_sub_hash: {first_key: "first_value", second_key: "second_value", third_key: "third_value"}}
Ruby is interpreted. Variables are references to data, but not the data itself. This facilitates using the same variable for data of different types.
Assignment of lhs = rhs then copies the reference on the rhs, not the data. This differs in other languages, such as C, where assignment does a data copy to lhs from rhs.
So for the function call, the variable passed, say x, is indeed copied into a local variable in the function, but x is a reference. There will then be two copies of the reference, both referencing the same data. One will be in the caller, one in the function.
Assignment in the function would then copy a new reference to the function's version of x. After this the caller's version of x remains unchanged. It is still a reference to the original data.
In contrast, using the .replace method on x will cause ruby to do a data copy. If replace is used before any new assignments then indeed the caller will see the data change in its version also.
Similarly, as long as the original reference is in tact for the passed in variable, the instance variables will be the same that the caller sees. Within the framework of an object, the instance variables always have the most up to date reference values, whether those are provided by the caller or set in the function the class was passed in to.
The 'call by value' or 'call by reference' is muddled here because of confusion over '=' In compiled languages '=' is a data copy. Here in this interpreted language '=' is a reference copy. In the example you have the reference passed in followed by a reference copy though '=' that clobbers the original passed in reference, and then people talking about it as though '=' were a data copy.
To be consistent with definitions we must keep with '.replace' as it is a data copy. From the perspective of '.replace' we see that this is indeed pass by reference. Furthermore, if we walk through in the debugger, we see references being passed in, as variables are references.
However if we must keep '=' as a frame of reference, then indeed we do get to see the passed in data up until an assignment, and then we don't get to see it anymore after assignment while the caller's data remains unchanged. At a behavioral level this is pass by value as long as we don't consider the passed in value to be composite - as we won't be able to keep part of it while changing the other part in a single assignment (as that assignment changes the reference and the original goes out of scope). There will also be a wart, in that instance variables in objects will be references, as are all variables. Hence we will be forced to talk about passing 'references by value' and have to use related locutions.
Lots of great answers diving into the theory of how Ruby's "pass-reference-by-value" works. But I learn and understand everything much better by example. Hopefully, this will be helpful.
def foo(bar)
puts "bar (#{bar}) entering foo with object_id #{bar.object_id}"
bar = "reference"
puts "bar (#{bar}) leaving foo with object_id #{bar.object_id}"
end
bar = "value"
puts "bar (#{bar}) before foo with object_id #{bar.object_id}"
foo(bar)
puts "bar (#{bar}) after foo with object_id #{bar.object_id}"
# Output
bar (value) before foo with object_id 60
bar (value) entering foo with object_id 60
bar (reference) leaving foo with object_id 80 # <-----
bar (value) after foo with object_id 60 # <-----
As you can see when we entered the method, our bar was still pointing to the string "value". But then we assigned a string object "reference" to bar, which has a new object_id. In this case bar inside of foo, has a different scope, and whatever we passed inside the method, is no longer accessed by bar as we re-assigned it and point it to a new place in memory that holds String "reference".
Now consider this same method. The only difference is what with do inside the method
def foo(bar)
puts "bar (#{bar}) entering foo with object_id #{bar.object_id}"
bar.replace "reference"
puts "bar (#{bar}) leaving foo with object_id #{bar.object_id}"
end
bar = "value"
puts "bar (#{bar}) before foo with object_id #{bar.object_id}"
foo(bar)
puts "bar (#{bar}) after foo with object_id #{bar.object_id}"
# Output
bar (value) before foo with object_id 60
bar (value) entering foo with object_id 60
bar (reference) leaving foo with object_id 60 # <-----
bar (reference) after foo with object_id 60 # <-----
Notice the difference? What we did here was: we modified the contents of the String object, that variable was pointing to. The scope of bar is still different inside of the method.
So be careful how you treat the variable passed into methods. And if you modify passed-in variables-in-place (gsub!, replace, etc), then indicate so in the name of the method with a bang !, like so "def foo!"
P.S.:
It's important to keep in mind that the "bar"s inside and outside of foo, are "different" "bar". Their scope is different. Inside the method, you could rename "bar" to "club" and the result would be the same.
I often see variables re-used inside and outside of methods, and while it's fine, it takes away from the readability of the code and is a code smell IMHO. I highly recommend not to do what I did in my example above :) and rather do this
def foo(fiz)
puts "fiz (#{fiz}) entering foo with object_id #{fiz.object_id}"
fiz = "reference"
puts "fiz (#{fiz}) leaving foo with object_id #{fiz.object_id}"
end
bar = "value"
puts "bar (#{bar}) before foo with object_id #{bar.object_id}"
foo(bar)
puts "bar (#{bar}) after foo with object_id #{bar.object_id}"
# Output
bar (value) before foo with object_id 60
fiz (value) entering foo with object_id 60
fiz (reference) leaving foo with object_id 80
bar (value) after foo with object_id 60
Yes but ....
Ruby passes a reference to an object and since everything in ruby is an object, then you could say it's pass by reference.
I don't agree with the postings here claiming it's pass by value, that seems like pedantic, symantic games to me.
However, in effect it "hides" the behaviour because most of the operations ruby provides "out of the box" - for example string operations, produce a copy of the object:
> astringobject = "lowercase"
> bstringobject = astringobject.upcase
> # bstringobject is a new object created by String.upcase
> puts astringobject
lowercase
> puts bstringobject
LOWERCASE
This means that much of the time, the original object is left unchanged giving the appearance that ruby is "pass by value".
Of course when designing your own classes, an understanding of the details of this behaviour is important for both functional behaviour, memory efficiency and performance.
There are many examples how to pass Ruby block as an argument, but these solutions pass the block itself.
I need a solution that takes some variable, executes an inline code block passing this variable as a parameter for the block, and the return value as an argument to the calling method. Something like:
a = 555
b = a.some_method { |value|
#Do some stuff with value
return result
}
or
a = 555
b = some_method(a) { |value|
#Do some stuff with value
return result
}
I could imagine a custom function:
class Object
def some_method(&block)
block.call(self)
end
end
or
def some_method(arg, &block)
block.call(arg)
end
but are there standard means present?
I think, you are looking for instance_eval.
Evaluates a string containing Ruby source code, or the given block, within the context of the receiver (obj). In order to set the context, the variable self is set to obj while the code is executing, giving the code access to obj’s instance variables. In the version of instance_eval that takes a String, the optional second and third parameters supply a filename and starting line number that are used when reporting compilation errors.
a = 55
a.instance_eval do |obj|
# some operation on the object and stored it to the
# variable and then returned it back
result = obj / 5 # last stament, and value of this expression will be
# returned which is 11
end # => 11
This is exactly how #Arup Rakshit commented. Use tap
def compute(x)
x + 1
end
compute(3).tap do |val|
logger.info(val)
end # => 4
I'm getting familiar with ruby send method, but for some reason, I can't do something like this
a = 4
a.send(:+=, 1)
For some reason this doesn't work. Then I tried something like
a.send(:=, a.send(:+, 1))
But this doesn't work too. What is the proper way to fire plus equals through 'send'?
I think the basic option is only:
a = a.send(:+, 1)
That is because send is for messages to objects. Assignment modifies a variable, not an object.
It is possible to assign direct to variables with some meta-programming, but the code is convoluted, so far the best I can find is:
a = 1
var_name = :a
eval "#{var_name} = #{var_name}.send(:+, 1)"
puts a # 2
Or using instance variables:
#a = 2
var_name = :#a
instance_variable_set( var_name, instance_variable_get( var_name ).send(:+, 1) )
puts #a # 3
See the below :
p 4.respond_to?(:"+=") # false
p 4.respond_to?(:"=") # false
p 4.respond_to?(:"+") # true
a+=1 is syntactic sugar of a = a+1. But there is no direct method +=. = is an assignment operator,not the method as well. On the other hand Object#send takes method name as its argument. Thus your code will not work,the way you are looking for.
It is because Ruby doesn't have = method. In Ruby = don't work like in C/C++ but it rather assign new object reference to variable, not assign new value to variable.
You can't call a method on a, because a is not an object, it's a variable, and variables aren't objects in Ruby. You are calling a method on 4, but 4 is not the thing you want to modify, a is. It's just not possible.
Note: it is certainly possible to define a method named = or += and call it, but of course those methods will only exist on objects, not variables.
class Fixnum
define_method(:'+=') do |n| self + n end
end
a = 4
a.send(:'+=', 1)
# => 5
a
# => 4
This might miss the mark a bit, but I was trying to do this where a is actually a method dynamically called on an object. For example, with attributes like added_count and updated_count for Importer I wrote the following
class Importer
attr_accessor :added_count, :updated_count
def increment(method)
send("#{method}=", (send(method) + 1))
end
end
So I could use importer.increment(:added_count) or importer.increment(:updated_count)
Now this may seem silly if you only have these 2 different counters but in some cases we have a half dozen or more counters and different conditions on which attr to increment so it can be handy.
I know a bit about ruby way to handle objects and references. The replace stuff, ect ...
I know it d'ont work on fixnum, cause the var is the fixnum. But i wish to change the value of a fixnum inside a function, and that the value changed in the ouside var.
How can i do this ?
I guess i can use a string like this "1" but that's quite dirty.
Ruby will always pass-by-reference (because everything is an object) but Fixnum lacks any methods that allow you to mutate the value. See "void foo(int &x) -> Ruby? Passing integers by reference?" for more details.
You can either return a value that you then assign to your variable, like so:
a = 5
def do_something(value)
return 1 #this could be more complicated and depend on the value passed in
end
a = do_something(a)
or you could wrap your value in an object such as a Hash and have it updated that way.
a = {:value => 5}
def do_something(dict)
dict[:value] = 1
end
do_something(a) #now a[:value] is 1 outside the function
Hope this helps.
You could pass an array with a single number, like [1], or a hash like {value: 1}. Less ugly than a string, as your number itself remains a number, but less overhead than a new class...
When I was building a game I had the same problem you have. There was a numeric score that represented how many zombies you've killed and I needed to manually keep it in sync between Player (that incremented the score), ScoreBar and ScoreScreen (that displayed the score). The solution I've found was creating a separate class for the score that will wrap the value and mutate it:
class Score
def initialize(value = 0)
#value = value
end
def increment
#value += 1
end
def to_i
#value
end
def to_s
#value.to_s
end
end
I am trying to mess around a little bit with Ruby. Therefor I try to implement the algorithms (given in Python) from the book "Programming Collective Intelligence" Ruby.
In chapter 8 the author passes a method a as parameter. This seems to work in Python but not in Ruby.
I have here the method
def gaussian(dist, sigma=10.0)
foo
end
and want to call this with another method
def weightedknn(data, vec1, k = 5, weightf = gaussian)
foo
weight = weightf(dist)
foo
end
All I got is an error
ArgumentError: wrong number of arguments (0 for 1)
The comments referring to blocks and Procs are correct in that they are more usual in Ruby. But you can pass a method if you want. You call method to get the method and .call to call it:
def weightedknn( data, vec1, k = 5, weightf = method(:gaussian) )
...
weight = weightf.call( dist )
...
end
You want a proc object:
gaussian = Proc.new do |dist, *args|
sigma = args.first || 10.0
...
end
def weightedknn(data, vec1, k = 5, weightf = gaussian)
...
weight = weightf.call(dist)
...
end
Just note that you can't set a default argument in a block declaration like that. So you need to use a splat and setup the default in the proc code itself.
Or, depending on your scope of all this, it may be easier to pass in a method name instead.
def weightedknn(data, vec1, k = 5, weightf = :gaussian)
...
weight = self.send(weightf)
...
end
In this case you are just calling a method that is defined on an object rather than passing in a complete chunk of code. Depending on how you structure this you may need replace self.send with object_that_has_the_these_math_methods.send
Last but not least, you can hang a block off the method.
def weightedknn(data, vec1, k = 5)
...
weight =
if block_given?
yield(dist)
else
gaussian.call(dist)
end
end
...
end
weightedknn(foo, bar) do |dist|
# square the dist
dist * dist
end
But it sounds like you would like more reusable chunks of code here.
You can pass a method as parameter with method(:function) way. Below is a very simple example:
def double(a)
return a * 2
end
=> nil
def method_with_function_as_param( callback, number)
callback.call(number)
end
=> nil
method_with_function_as_param( method(:double) , 10 )
=> 20
The normal Ruby way to do this is to use a block.
So it would be something like:
def weightedknn(data, vec1, k = 5)
foo
weight = yield(dist)
foo
end
And used like:
weightedknn(data, vec1) { |dist| gaussian( dist ) }
This pattern is used extensively in Ruby.
You can use the & operator on the Method instance of your method to convert the method to a block.
Example:
def foo(arg)
p arg
end
def bar(&block)
p 'bar'
block.call('foo')
end
bar(&method(:foo))
More details at http://weblog.raganwald.com/2008/06/what-does-do-when-used-as-unary.html
You have to call the method "call" of the function object:
weight = weightf.call( dist )
EDIT: as explained in the comments, this approach is wrong. It would work if you're using Procs instead of normal functions.
I would recommend to use ampersand to have an access to named blocks within a function. Following the recommendations given in this article you can write something like this (this is a real scrap from my working program):
# Returns a valid hash for html form select element, combined of all entities
# for the given +model+, where only id and name attributes are taken as
# values and keys correspondingly. Provide block returning boolean if you
# need to select only specific entities.
#
# * *Args* :
# - +model+ -> ORM interface for specific entities'
# - +&cond+ -> block {|x| boolean}, filtering entities upon iterations
# * *Returns* :
# - hash of {entity.id => entity.name}
#
def make_select_list( model, &cond )
cond ||= proc { true } # cond defaults to proc { true }
# Entities filtered by cond, followed by filtration by (id, name)
model.all.map do |x|
cond.( x ) ? { x.id => x.name } : {}
end.reduce Hash.new do |memo, e| memo.merge( e ) end
end
Afterwerds, you can call this function like this:
#contests = make_select_list Contest do |contest|
logged_admin? or contest.organizer == #current_user
end
If you don't need to filter your selection, you simply omit the block:
#categories = make_select_list( Category ) # selects all categories
So much for the power of Ruby blocks.
Similarly to a Proc or a method call, you can also pass a lambda as weightf parameter :
def main
gaussian = -> (params) {
...
}
weightedknn(data, vec1, k = 5, gaussian, params)
# Use symbol :gaussian if method exists instead
end
def weightedknn(data, vec1, k = 5, weightf, params)
...
weight = weightf.call(params)
...
end
you also can use "eval", and pass the method as a string argument, and then simply eval it in the other method.