Copyright © 2008-2018
Robert W. Hasker

Note 2: Basic Ruby

How to learn a language?

• Syntax: rules to write things - key is to notice what's different from other languages
• Types: rules for expressions, function calls

Ruby Basics

• (run irb)
• Arithmetic: 3 + 4 * 5, 4 ** 3, 1.5 / 23e18, Math.sin(0.5)
• Parentheses are optional for functions: Math.sin 0.25
• Comparisons: 3 < 5, 3 >= 5, 3 != 4
• Assignments: x = 4 - note no need to declare x
  • x = x * 2
  • x *= 2
• Conditions:

  • if x > 4
    5
    else
    8
    end
    

  • if x < 4
      5
    elsif x < 50
      9
    else
      100
    end
    

  • note: if returns a value (more like ?: in Java)
  • note: end marks the end of the if statement
  • note: no semicolons, though they are valid in sequences
    • end of line serves as a terminator
    • can use then with if to put it on single line - avoid
  • note: no braces - they're used very differently
• Printing: puts
  • puts "hello"
  • puts 5.0 / 3
  • (print is also available; read up online)
  • can use single quotes on strings: puts 'hello'
  • difference: " allows embedding expressions:

        x = 42
        puts "#{x} is the meaning of life, the universe, and everything"
        puts "but not #{x / 5.0}"
        puts 'but not #{x / 5.0}'
    

• What is this?
  • Known as the "read-eval-print" loop
  • Exit: two alternatives: exit, quit
  • simple experimentation, testing
• Can also type expressions into ruby itself (from the DOS command prompt):

          C:\web\cs2040\samples>ruby
          puts 3 * 4
          Control-D
  

  • A program: a file with commands in it
  • Running sum.rb at command prompt:

            ruby sum.rb
            5
            4
            3
            Control-Z
    

    Control-Z marks the end of input in Dos; use Control-D in Unix.
  • Better way to run a program at the command prompt for testing:

            d:\> ruby sum.rb < fib100.txt
    

    • Ensures consistency.
    • The file extension is arbitrary: ruby sum.rb < sum.rb works just fine!
    • Note the direction of the "arrow": data is read from the file fib100.txt and given to the program.
    • This is similar to reading from the keyboard in Java: Display.java - code to read from input and write to output with numbered lines
      • Eg: java Display.java < Display.java
• How to edit Ruby files?
  • Any IDE will do - you can even use IntelliJ, though there are bits that will not work as expected
  • Preferable: install Notepad++ or Visual Studio Code (VSCode)
    • Both are lightweight
    • For VSCode, install the Ruby language package
  • Demonstrate creating a hello world Ruby program in Notepad++ and VSCode
    • Show running the program after

Functions and Loops

• Defining functions (back in irb):

          def max(a, b)
            if a > b
              a
            else
              b
            end
          end
  

  • usage: max(3, 8)
  • also: max 43, 12
    • but do be cautious about dropping parens - can get you into trouble!
  • Note: if a file contains a mix of functions and statements that are not part of a function, then the functions have to appear before those statements.
    • For example, see gpa.rb
• Loops:

          count = 10
          while count >= 1
            puts count
            count -= 1
          end
  

  • There are much better ways to do a task like this! More later.
• Reading data
  • gets: returns next string
    • x = gets
    • Roughly equivalent to System.in.nextLine() in Java
    • This implicitly uses the global variable $stdin - gets is equivalent to writing $stdin.gets
  • How to read an integer?
    • Strings have an operation to convert to an integer:

              "-183".to_i
      

      and a float:

              "15.18".to_f
      

    • If there's additional text after the number, it's ignored
      • "-1.5junk".to_f
      • "-1.5junk".to_i
      • a general Ruby principle: do what you can with what you have
      • Reading an integer, if guaranteed you have an integer in the input:

                num = gets.to_i
        

  • nil works like false in conditions:

            if false then 'a' else 'b' end
            if nil then 'a' else 'b' end
    

    • Warning: this is the sort of thing that's easy to ask questions about!
    • 0 is not false!

              if 0 then 'a' else 'z' end
      
              # or...
      
              !false
              !nil
              !0
      

  • Largest item in a non-empty list of integers - note this is a complete program...
    • Uses the fact that gets returns nil when no more input

            largest = gets.to_i
            while (str = gets)
              n = str.to_i
              if n >= largest
                largest = n
              end
            end
    

Review

• basic Ruby operations (arithmetic, comparisons): what you'd expect with Java experience
  • And includes several features from C++!
• no type declarations for variables
• if statement: always ends
• same for while
• defining functions:

      def name(...parameters...) body end