In Ruby, Why Does Nil[1]=1 Evaluate to Nil

nil result despite both #hash and #eql? evaluating to true, when looking up a key in a hash?

In this case, your hash becomes outdated. Use rehash to solve your problem.

my_hash.rehash

Checking if a variable is not nil and not zero in ruby

unless discount.nil? || discount == 0
  # ...
end

Why does Ruby include? evaluate to nil?

Minor nitpick: Array#include? always returns true or false, never nil.

To answer your question: x.include?(y) tests whether y is an element of x, not whether it's a subarray.

[1,1,1,5,1].include?([1,1,1]) returns false because [1,1,1] is not an element of the array [1,1,1,5,1]. [[1,1,1],[5,1]].include?([1,1,1])) would return true.

There is no method in ruby that checks whether an array is a subarray of a another array, but you can easily write it as arr1.each_cons(arr2.size).include?(arr2) (needs require 'enumerator' in 1.8.6). This is O(arr1.size*arr2.size) though.

If you want it in O(arr1.size + arr2.size), you can implement the Knuth-Morris-Pratt algorithm (which is meant to find substrings, but works equally well for finding subarrays as they are essentially the same thing).

Why is `a = a` `nil` in Ruby?

Ruby interpreter initializes a local variable with nil when it sees an assignment to it. It initializes the local variable before it executes the assignment expression or even when the assignment is not reachable (as in the example below). This means your code initializes a with nil and then the expression a = nil will evaluate to the right hand value.

a = 1 if false
a.nil? # => true

The first assignment expression is not executed, but a is initialized with nil.

You can find this behaviour documented in the Ruby assignment documentation.

Undeclared instance variables default to nil?

Per the Ruby doc:

Instance variables of ruby do not need declaration. This implies a flexible structure of objects. In fact, each instance variable is dynamically appended to an object when it is first referenced.

https://ruby-doc.org/docs/ruby-doc-bundle/UsersGuide/rg/instancevars.html

You can also get a list of all previously defined instance variables:

ClassName.instance_variables

or

a = ClassName.new 
a.instance_variables #specific to instance

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