Let's say I want to access an element of an array at a random index this way:
[1, 2, 3, 4].at(rand(4))
Is there a way to pass the size of the array like the following?
[1, 2, 3, 4].at(rand(le_object.self.size))
Why would I do that?--A great man once said:
Science isn't about why, it is about why not.
Not recommended, but instance_eval would somehow work:
[1, 2, 3, 4].instance_eval { at(rand(size)) }
And you can also break out of tap:
[1, 2, 3, 4].tap { |a| break a.at(rand(a.size)) }
There's an open feature request to add a method that yields self and returns the block's result. If that makes it into Ruby, you could write:
[1, 2, 3, 4].insert_method_name_here { |a| a.at(rand(a.size)) }
No, you can't do that. Receiver of a method (that array) is not accessible by some special name at the call site. Your best bet is assigning a name to that object.
ary = [1, 2, 3, 4]
ary.at(rand(ary.size))
Of course, if all you need is a random element, then .sample should be used. Which does not require evaluation of any arguments at the call site and its self is the array.
You can use instance_eval to execute ruby code with the binding of the array variable
[1, 2, 3, 4].instance_eval { at(rand(size)) }
Assuming you are interested in a random element as Array#at returns an element at given index, you can use Array#sample to pick a random element from an array.
[1,2,3,4].sample
#=> 3
If you do not want to use instance_eval (or any form of eval), then, you can add a method to Array class by monkey patching - generally speaking, I am not sure whether it's a wise idea to monkey patch though
class Array
def random_index
rand(size)
end
end
["a","b","c","d"].random_index
#=> 2
You could do something similar with lambda:
getrand = ->(x) { x[rand(x.count)] }
getrand.call [1,2,3]
# => 2
Related
I've searched extensively but sadly couldn't find a solution to this surely often-asked question.
In Perl I can reassign an entire array within a function and have my changes reflected outside the function:
#!/usr/bin/perl -w
use v5.20;
use Data::Dumper;
sub foo {
my ($ref) = #_;
#$ref = (3, 4, 5);
}
my $ref = [1, 2];
foo($ref);
say Dumper $ref; # prints [3, 4, 5]
Now I'm trying to learn Ruby and have written a function where I'd like to change an array items in-place by filtering out elements matching a condition and returning the removed items:
def filterItems(items)
removed, items = items.partition { ... }
After running the function, items returns to its state before calling the function. How should I approach this please?
I'd like to change an array items in-place by filtering out elements matching a condition and returning the removed items [...] How should I approach this please?
You could replace the array content within your method:
def filter_items(items)
removed, kept = items.partition { |i| i.odd? }
items.replace(kept)
removed
end
ary = [1, 2, 3, 4, 5]
filter_items(ary)
#=> [1, 3, 5]
ary
#=> [2, 4]
I would search for pass by value/reference in ruby. Here is one I found first https://mixandgo.com/learn/is-ruby-pass-by-reference-or-pass-by-value.
You pass reference value of items to the function, not the reference to items. Variable items is defined out of method scope and always refers to same value, unless you reassign it in the variable scope.
Also filterItems is not ruby style, see https://rubystyle.guide/
TL;DR
To access or modify an outer variable within a block, declare the variable outside the block. To access a variable outside of a method, store it in an instance or class variable. There's a lot more to it than that, but this covers the use case in your original post.
Explanation and Examples
In Ruby, you have scope gates and closures. In particular, methods and blocks represent scope gates, but there are certainly ways (both routine and meta) for accessing variables outside of your local scope.
In a class, this is usually handled by instance variables. So, as a simple example of String#parition (because it's easier to explain than Enumerable#partition on an Array):
def filter items, separator
head, sep, tail = items.partition separator
#items = tail
end
filter "foobarbaz", "bar"
#=> "baz"
#items
#=> "baz"
Inside a class or within irb, this will modify whatever's passed and then assign it to the instance variable outside the method.
Partitioning Arrays Instead of Strings
If you really don't want to pass things as arguments, or if #items should be an Array, then you can certainly do that too. However, Arrays behave differently, so I'm not sure what you really expect Array#partition (which is inherited from Enumerable) to yield. This works, using Enumerable#slice_after:
class Filter
def initialize
#items = []
end
def filter_array items, separator
#items = [3,4,5].slice_after { |i| i == separator }.to_a.pop
end
end
f = Filter.new
f.filter_array [3, 4, 5], 4
#=> [5]
Look into the Array class for any method which mutates the object, for example all the method with a bang or methods that insert elements.
Here is an Array#push:
ary = [1,2,3,4,5]
def foo(ary)
ary.push *[6, 7]
end
foo(ary)
ary
#=> [1, 2, 3, 4, 5, 6, 7]
Here is an Array#insert:
ary = [1,2,3,4,5]
def baz(ary)
ary.insert(2, 10, 20)
end
baz(ary)
ary
#=> [1, 2, 10, 20, 3, 4, 5]
Here is an example with a bang Array#reject!:
ary = [1,2,3,4,5]
def zoo(ary)
ary.reject!(&:even?)
end
zoo(ary)
ary
#=> [1, 3, 5]
Another with a bang Array#map!:
ary = [1,2,3,4,5]
def bar(ary)
ary.map! { |e| e**2 }
end
bar(ary)
ary
#=> [1, 4, 9, 16, 25]
I want to write an Array method in ruby that takes the successive values in the array and returns their differences as a new array (unshifting a '0' in at the beginning).
So feeding the array [4,7,11,16] into the method returns a new array [4,3,4,5].
1) does such a method already exist?
If not, then I think I know how to write it. However,
2) does a method already exist which allows me to test the input array and make sure it only consists of integers and/or floats?
Again, if not, I think I know how to write one.
p [4,7,11,16].unshift(0).each_cons(2).map{|a,b| b-a} # => [4, 3, 4, 5]
Keep it simple:
arr = [4,7,11,16]
last = 0
arr.map { |e| new=e-last; last=e; new }
#=> [4, 3, 4, 5]
Another way:
a = [arr.first]
enum = arr.each
loop do
a << -enum.next + enum.peek
end
a
#=> [4, 3, 4, 5]
Enumerator#peek raises a StopIteration exception when enum is at its last element. Kernel#loop handles the exception by breaking from the loop.
Regarding the first method, I am not aware of any such method in the Ruby Array class.
Regarding the second one, you can do it as explained in this answer:
your_array.all? {|i| i.is_a? Numeric }
In Ruby, is there a functional difference between these two Enumerators?
irb> enum_map = [1,2,3].map
=> #<Enumerator: [1, 2, 3]:map> # ends with "map>"
irb> enum_group_by = [1,2,3].group_by
=> #<Enumerator: [1, 2, 3]:group_by> # ends with "group_by>"
irb> enum_map.methods == enum_group_by.methods
=> true # they have the same methods
What can #<Enumerator: [1, 2, 3]:map> do that <Enumerator: [1, 2, 3]:group_by> can't do, and vice versa?
Thanks!
From the documentation of group_by:
Groups the collection by result of the block. Returns a hash where the
keys are the evaluated result from the block and the values are arrays
of elements in the collection that correspond to the key.
If no block is given an enumerator is returned.
(1..6).group_by { |i| i%3 } #=> {0=>[3, 6], 1=>[1, 4], 2=>[2, 5]}
From the documentation of map:
Returns a new array with the results of running block once for every
element in enum.
If no block is given, an enumerator is returned instead.
(1..4).map { |i| i*i } #=> [1, 4, 9, 16]
(1..4).collect { "cat" } #=> ["cat", "cat", "cat", "cat"]
As you can see, each does something different, which serves a different purpose. Concluding that two APIs are the same because they expose the same interface seems to miss the entire purpose of Object Oriented Programming - different services are supposed to expose the same interface to enable polymorphism.
There's a difference in what they do, but fundamentally they are both of the same class: Enumerator.
When they're used the values emitted by the enumerator will be different, yet the interface to them is identical.
Two objects of the same class generally have the same methods. It is possible to augment an instance with additional methods, but this is not normally done.
I would like to do something like this:
#residenciais, #comerciais = TipoImovel.all.split { |t| t.residencial? }
The problem is that #comerciais is always empty because it never returns the object, since the condition is false.
Is there a better way of doing this?
You're looking for the standard method Enumerable#partition, rather than the Rails split add-on.
#residenciais, #comerciais = TipoImovel.all.partition { |t| t.residencial? }
Which can also be written like this, since the condition is a single method call:
#residenciais, #comerciais = TipoImovel.all.partition(&:residencial?)
Some more explanation:
The Rails Array#split method is used to separate an array into ordered groups delimited by elements which return true for a given block. It's a generalization of the standard String method. For example:
[1,2,3,4,5,6].split(&:odd?) #=> [[], [2], [4], [6]]
Any odd number is a delimiter, so it returns the portions of the array between the odd numbers, in order.
Whereas this is closer to what you're doing:
odds, evens = [1,2,3,4,5,6].partition(&:odd?) #=> [[1, 3, 5], [2, 4, 6]]
If the partition condition is not simply Boolean, or if you want to key off the values regardless, then you can use Enumerable#group_by, which returns a Hash of Arrays instead of a pair:
[1,2,3,4,5,6].group_by(&:odd?) #=> {true=>[1, 3, 5], false=>[2, 4, 6]}
You can use group_by:
#residenciais, #comerciais = TipoImovel.all.group_by { |t| t.residencial }.values
Given this irb session:
[2.0.0p195]> arr = [{count: 5}, {count: 6}, {count: 7}]
=> [{:count=>5}, {:count=>6}, {:count=>7}]
[2.0.0p195]> arr.collect(&:count)
=> [1, 1, 1]
wat
[2.0.0p195]> arr.collect(&:count).reduce(:+)
=> 3
[2.0.0p195]> arr.collect {|e| e[:count]}.reduce(:+)
=> 18
Can I exclude methods on Hash when collecting or is using a block the only way around this problem?
& means call #to_proc on its argument, and the Symbol class implements this by creating a Proc that calls the method name based on the symbol - so &:symbol means "Call the #symbol method on the passed in object". Essentially, what you've got is the equivalent of this:
arr.collect{|obj| obj.send(:count)}
Since Hash won't respond to the "count" method at all to get the value of the :count key - that is, Hash#count is not the same as Hash#[](:count), (though OpenStruct does do this for you), you're stuck with the block method.
Another alternative is to create a lambda, useful if you are writing the same block many times:
fetch_count = -> x{x[:count]}
arr.collect(&fetch_count) #=> [5, 6, 7]
# If hash only has one value as in example:
arr.collect(&values).flatten #=> [5, 6, 7]
The implementation of calling & on a symbol is as follows (more or less):
class Symbol
def to_proc
Proc.new { |obj| obj.send self }
end
end
You can see that all it is doing (when combined with a #map) is calling the method corresponding to the provided symbol on each member of the enumerable.
You could fix this if you really wanted by using OpenStructs instead of hashes, they have method-style access of elements:
[{test: 1}].map { |h| OpenStruct.new(h) }.map &:test
#=> [1]
Or invent an operator that does what you want for hash access in addition to &, I may revisit this challenge if I have a spare moment later!
EDIT: I have returned
This is hacky but you could monkey-patch symbol to provide the functionality that you wish for by augmenting with unary ~:
# Patch
class Symbol
def ~#
->(obj){ obj[self] }
end
end
# Example usage:
[{count: 5}, {count: 6}, {count: 7}].map &~:count
#=> [5, 6, 7]
If a free-for-all language such as Ruby doesn't have a feature that you wish for, you can always build it in :-)
Disclaimer: This is probably a terrible idea.