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Ruby Code Snippets

• Watch a File for Changes

  This script will watch a file for changes and execute a callback whenever the file is changed. It works by caching an md5 hash of the file and comparing it with the file periodically.

        require 'digest/md5'
  
        $md5 = ''
  
        def watch(file, timeout, &cb)
          $md5 = Digest::MD5.hexdigest(File.read(file))
          loop do
            sleep timeout
            if ( temp = Digest::MD5.hexdigest(File.read(file)) ) != $md5
              $md5 = temp
              cb.call(file)
            end
          end
        end
      

  Usage Example:

        $num = 0
        watch '../num_diff.rb', 1 do |file|
          puts "\e[31m MODIFIED: \e[0m \e[34m #{file} \e[0m (#{$num}, #{Time.now})"
          puts "\e[32m     -- EXECUTING -- \e[0m"
          puts
          puts `ruby #{file}`
          puts
          puts "\e[32m -- FINISHED EXECUTION -- \e[0m"
          puts
          $num += 1
        end
      

  

• Octave's diag

  GNU Octave has a diag function which behaves like this:

      octave> diag([1,2,3])
      ans =
  
      Diagonal Matrix
  
         1   0   0
         0   2   0
         0   0   3
      

  To be more clear, diag \left( \begin{array}{ccc} 1 & 2 & 3 \end{array} \right) = \left( \begin{array}{ccc} 1 & 0 & 0 \\ 0 & 2 & 0 \\ 0 & 0 & 3 \end{array} \right)

  This is an implementation of the diag function in Ruby.

        # Create a diagonal matrix given a vector (or an array), 
        # placing the entries of the vector on the diagonal of 
        # the matrix.
        # For example, diag([1,2,3]) would give:
        # [ 1 0 0 ]
        # [ 0 2 0 ]
        # [ 0 0 3 ]
        # That would actually be a returned as a 2D array like this: 
        #  [[1, 0, 0], [0, 2, 0], [0, 0, 3]]
        # The array returned has to be converted to a matrix by doing 
        #  Matrix[ *diag(vector) ]
        def diag(vector)
          n = vector.size
          matrix = []
          n.times do |i|
            matrix << Array.new(n) { |x| (x == i) ? vector[x] : 0 }
          end
          matrix
        end
      

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