© 2019 Martin J. Dürst 青山学院大学

ミニテスト

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Today's Schedule

• Minitest
• Last week's exercises and homework
• Iterators and blocks for loops, with demo
• Dynamic Web sites with CGI, with demo
• Today's exercises

How to Be Successful in this Course

• Contents is progressive ⇒ Don't forget previous lecture's content
• Contents is not necessarily complete, on purpose
  (no simple typing or filling in the blanks)
• Learn to ask questions (better use Moodle)
• Learn to search the Web when needed
• Learn to distinguish what you know and what you don't know
• Work to make sure to plug gaps in your knowledge

Last Lecture's Exercises

Exercise 3a: Calendar Conversion

• Write a function to_japanese_calendar in a file japanese_calendar.rb.
• The function takes a year (Integer) in the Western calendar as an argument.
• The function returns a String indicating the year in the Japanese calendar.
• Solution: def with if statement with a branch for each era and for the exceptions.

Exercise 3b: Temperature Conversion

• Write a class Temperature in a file temperature.rb.
• An object of a class Temperature represents a temperature.
• Set the temperature to 20℃ on initialization (this is a nice temperature).
• Create methods for access (celsius, fahrenheit, kelvin) and setting (celsius=, fahrenheit=, kelvin=).
• Solution: Very similar to example class Length

Homework

• Review the most important Ruby classes: Integer, Float, String, Array (incl. Enumerable), Hash
• Study a few methods for each class

Course Overview

• 1st week: HTML markup ⇒ structure
• 2nd week: CSS stylesheet ⇒ styling
• Starting in 3rd week: Programming on the server side with Ruby
• Today: First time we use a Web server and create a Web page with a program

Loop Example

(1..5).each do |n|
    puts n
end

Range

• a..b is a Range from a to b (inclusive).
• Because of precedence, Ranges are usually written in parentheses.
• Ranges can be used for simple loops.

Loops in Ruby

• For loops in Ruby, use iterators
• There are many convenient iterators, but the most important one is each
• Many iterators are defined in Enumerable, so they work for Arrays, Ranges, Hashes,…
• Iterators take blocks that do the actual work in each iteration

Ruby Blocks

• Part of arguments to a function/method call
• Appear after all other arguments
• Start/end with do end or { }
• General style:
  • do end used for multiline blocks
  • { } used for inline blocks
• Block parameters (loop variables,…) appear between | |, after do or {
• Blocks make Ruby very powerful, you can use this power, too

More Loop Examples

(1..5).each  { |n| puts n }

(1..5).each  { |i| puts i }

(1..5).each  { |n| puts n+3, n*3 }

(1..10).each { |n| puts n }

(1...5).each { |n| puts n }

(1..5).each do |n| puts n end

[2,3,5,7,11].each { |n| puts n }

[1,2,4,8].each { |n| puts n**n }

Many of these examples use Ranges, but use on Arrays is even more common.

Iterator Demo

Dynamic Web Sites

• With HTML, we can create static documents
• But on a Web page, we also want to show dynamic results
• One way to show dynamic results is to create a Web page on the Web server with a program
• This can be done with CGI

CGI Basics

• CGI: Common Gateway Interface (see RFC 3875 for details)
• Program to generate Web page is called externally from Web server
• Old and well established, but slow
• Easy to understand
• Traditionally used with Perl (and C), we will use Ruby
• Traditional extension is .cgi, we will use .rb
• Not very flexible, and slow

How CGI Works

---------------               --------------------------------------
| client      |               |             server machine         |      
| machine     |               |                                    |
|             | (1)           |              (2) set ENV           |
| ----------- | HTTP request  | ------------ call script --------- |
| |         |------------------>|          |------------>|       | |
| | client  | |               | |Web server|             | CGI   | |
| |(browser)| |               | |(webrick) |             | script| |
| |         |<------------------|          |<------------|       | |
| ----------- | HTTP response | ------------ (3) stdout  --------- |
--------------- (4)           --------------------------------------

CGI Input and Output

• Input: Environment variables (環境変数) carefully set by the Web server
  (accessible in Ruby via ENV)
• Output: To standard output
  • Some HTTP headers, including a Content-Type header:
    Content-Type: text/html;charset=UTF-8
  • Empty line
    (HTTP headers and empty lines have to end with \r\n, not just \n)
  • Web page itself (from <!DOCTYPE to </html>)

Web Servers

• A Web server
  • Waits for a TCP connection on a specific port
  • Analyses the input
  • If necessary, calls other programs
  • Returns a Web page or other resource (e.g. stylesheet, image,…)
• There are many Web servers:
  • General Web servers: Apache httpd, ngnix, IIS
  • Web servers often used with Ruby/Rails: mongrell, unicorn, puma
  • Web server that comes with Ruby: webrick

CGI Demo

• Open a new "Start Command Prompt with Ruby"
  (you can open as many of these as you want;
  I recommend one for irb, one for webrick, and one for general commands)
• cd to the directory you use for this practice
• Create a directory for starting the server, and cd to it:
  mkdir webrick; cd webrick
• Download the startup script webrick_start.rb from Moodle
• Start webrick with: ruby webrick_start.rb
• Create a Ruby CGI script in the same directory (e.g. my_first_cgi.rb)
• Open a browser and go to http://localhost:12000/my_first_cgi.rb
• Watch the output in the command prompt window, and check headers with browser development tools

CGI Security

• Any Web server that executes code is a potential security problem
• Carefully check Web server settings
• Limit CGI scripts to specific directories and extensions
• Carefully check CGI scripts before activation
• Carefully escape and check all input

More on Ruby Output

• A CGI script outputs a lot of text
• Using puts for each line of a Web page is quite tedious
• String literals can extend to multiple lines:
  "This is a long,
long, long string."
• Here documents are better:
  

  html_end = <<HTMLEND
    </body>
  </html>
  HTMLEND
  

Exercise 4a: A Program to Convert Marks

• Write a program in a file convert_marks.rb to output a conversion table for all points from 0 to 100, in the following form:
  

    0 points ->   0 points
    1 points ->  10 points
    ...
    ...
  100 points -> 100 points

• The following function can be used to change students' marks:

  def convert_mark(mark, exponent)
    (mark**exponent * (100.0/100**exponent)).round
  end

  The argument mark is a mark between 0 and 100. The argument exponent changes how the conversion is done. If the exponentis smaller than 1, then the marks get better. If the exponent is greater than 1, then the marks get worse. But they always stay between 0 and 100.

• Hints:
  • For formatting, use the % operator on the String class: "%3d" % integer.
  • Check out and use ARGV[0] to change the exponent. As an example, calling the program with
    ruby convert_marks.rb 0.7
    should set the exponent to 0.7.
  • Use an exponent of 0.5 if there is no exponent given.
• Submit the final program convert_marks.rb to Moodle (deadline: 17:30).

Exercise 4b: A Web Page to Convert Marks

• Change the program from exercise 4a to a program in a file convert_marks_page.rb. This program creates (prints out) a Web page with a table showing the marks.
• Hints:
  • Use your knowledge from lecture 1 to create the Web page.
  • Use your knowledge from lecture 2 to add some table borders using styling. Put your style rules into a style element inside the head element.
  • Use 'Before' and 'After' as table headings (th).
  • Make sure you have a good title and h1.
  • Use redirect to create a Web page: ruby convert_marks_page.rb 0.8 > convert_marks_page.html
  • Open your page in a Web browser to check it. But do not rely on the Web browser too much.
  • Validate your Web page the way you have learned it in lecture 1.
• Submit the final program convert_marks_page.rb to Moodle (deadline: 18:00).

Exercise 4c: A CGI Script to Convert Marks

• Change the program from exercise 4b to a CGI program in a file convert_marks_cgi.rb.
• Hints:
  • Add output of CGI headers at the start of the output.
  • Use ENV[QUERY_STRING] to get the exponent.
  • Make sure webrick is started in the same directory as convert_marks_cgi.rb.
  • You can change the program and reload the page without shutting down webrick.
    You can also shut down and restart webrick any time you want.
  • Use a browser to access http://localhost:12000/convert_marks_cgi.rb
  • Add the exponent at the end of the URI after a question mark, e.g.:
    http://localhost:12000/convert_marks_cgi.rb?0.6
• Submit the final program convert_marks_cgi.rb to Moodle (deadline: 18:30).

Exercise 4d: Advanced Exercise (発展問題):
An Advanced CGI Script to Convert Marks

• Create an improved CGI script in a file convert_marks_advanced.rb, with someone of the following or your own ideas:
  • Provide links to pages with different exponents.
  • Add a form element to make it easier to set the exponent.
  • Use require 'cgi', cgi = CGI.new, and cgi[field_name] to access form fields.
  • Show conversions for more than one exponent in a single table.
  • Shorten the table by showing conversions only for even marks, or for marks divisible by 5.
  • Show the results for more than one exponent in additional rows in the table.
• Add text explaining the output to your Web page.
• Submit the final program convert_marks_advanced.rb to Moodle (deadline: Wednesday 19:00).

Homework

• Install rails with the following command in "Start Command Prompt with Ruby"
  gem install rails -v 5.2.3
• Check the installed version with
  rails --version
• Use irb to play with arrays and iterators (each, map, select, reject, inject,…).