Decode the Coding Interview in Ruby: Real-World Examples/

...

/

Feature #11: Directory Iterator

Feature #11: Directory Iterator

Implementing the "Directory Iterator" feature for our "Operating System" project.

We'll cover the following...

Description

In this feature, we will create a directory tree iterator. A directory can contain files or other directories. Similarly, subdirectories can contain both files and other directories. We will be given a directory structure for a specific directory in the file system. This directory will be available as a list. Each element of this list is either a file represented as a scalar element, or a directory represented as a nested list. We will have to iterate over all of the files one by one, using an iterator.

The task is to implement the DirectoryIterator class:

  • initialize(nested_list) initializes the iterator with the nested list nested_list.
  • next() returns the next file in the nested directories.
  • has_next() returns true if there are still some files in the nested list. If there are no files left in the nested list, it returns false.

Solution

We will use a stack to solve this feature. The stack will be used to store the directories and files on the iterator object. In the constructor, we will push all the files and the directories in the stack in reverse order.

The has_next function checks if the top element of the stack is a file or directory. If so, then it returns true. Otherwise, if the top element is a list of files or directories, then it pops from the stack and pushes each element of the list in the stack in reverse order. This way, the lists at the top of the stack are converted into individual files or a directory whenever the has_next function is called.

The next function first calls the has_next function to check if there is any file or directory in the stack. If the has_next function returns true, then it pops from the stack and returns the topmost value.

Here is an example of how this function works:

Let’s look at the code for this solution:

require 'set'
class NestedDirectories 
  # Constructor initializes a single file if a value has been passed
  # else initializes an empty list
  attr_accessor :n_list, :file
  def initialize(opt_parm = nil) 
    if opt_parm != nil 
      @file = opt_parm #cce
    else
      @n_list = []# Can cause error later on
      @file =  nil # Added later on
    end
  end
  
  # return true if this NestedDirectories holds a single file, rather than a nested list.
  def is_file() 
    if @file != nil 
      return true
    end
    return false
  end
  
  # return the single file that this NestedDirectories holds, if it holds a single file
  # Return null if this NestedDirectories holds a nested list
  def get_file() 
    return @file
  end
  
  #  Set this NestedDirectories to hold a single file.
  def set_file(value) 
    @n_list = nil
    @file =  value
  end
  
  # Set this NestedDirectories to hold a nested list and adds a nested file to it.
  def add(ni) 
    if @file != nil 
      @n_list = [] # Can cause error
      @n_list.push(NestedDirectories.new(@file)) # Can cause error # self added later on
      @file = nil
    end
    @n_list.push(ni) # self added later on
    
    # return the nested list that this NestedDirectories holds, if it holds a nested list
    # Return null if this NestedDirectories holds a single file
  end
  def get_list() 
    return @n_list # self added later on
  end
end
class DirectoryIterator 
  attr_accessor :stack
  def initialize(nested_list) 
    @stack = (nested_list.reverse())
  end
  
  def has_next() 
    while @stack.length() > 0   
      top = @stack[-1]
      if top.is_file() 
        return true
      end
      top_list = @stack.pop().get_list()
      i = top_list.length()-1
      while i >= 0 
        @stack.push(top_list[i])
        i-=1
      end
    end
    return false
  end
  
  def next()  # can cause problems later on
    if has_next() == true
      return @stack.pop().get_file()
    end
    return nil
  end
end
# Driver Code
nested_list = []
l1 = NestedDirectories.new
nested_list.push(NestedDirectories.new("F1"))
l1.add(NestedDirectories.new("F2"));
l1.add(NestedDirectories.new("D1"));
nested_list.push(l1)
nested_list.push(NestedDirectories.new("D2"))
itr = DirectoryIterator.new(nested_list)
puts("Original structure: [F1, [F2, D1], D2]")
puts("")
puts("Output:")
while itr.has_next() 
  puts("itr.next(): " + itr.next().to_s);
end
Directory Iterator

Complexity measures

Time complexity Space complexity
O(n+l)O(n + l)
...