Programming Languages - Review for Test

1. Explain the difference between machine language and a high level language.
   -- A CPU (microprocessor) can only understand instructions written in machine language.  These are numbers that people cannot understand.  A High Level Language uses commands written in plain text (similar to English) that people can understand.  In machine language, each single instruction is very simple and does very little, so programs consist of lots and lots of simple instructions.  In HLLs, commands are more powerful (and understandable for people), so programs can be shorter.
    
2. Explain the difference between assembly language and machine language.
   -- These are very similar.  For each machine language instruction code (numbers) there is a corresponding mnemonic (symbol), so the translation is quite simple.  But people find it much easier to remember the mnemonics than the machine code numbers.
    
3. If a CPU (microprocessor) can only execute machine code, outline the process
   that enables a Java program to be executed by the CPU.
   -- A compiler translates the Java source code into "p-code".  Then the Java Virtual Machine can run and execute the p-code.  The JVM must send appropriate commands to the CPU so it can do what the p-code program says.
    
4. Explain the brilliant idea that John von Neumann suggested that is still
   used in virtually every modern computer.
   -- von Neumann is given credit for the "stored program" concept.  This means that a program is stored in the memory as numbers (machine code).  This is the same memory that also holds data.  The CPU fetches instructions from the memory and executes them.  
    
5. Approximately when were each of these invented:
   (a) the first electronic computer  - - 1940
   (b) the fist compiler -- around 1960
   (c) the first microprocessor -- 1970
   (d) the first personal computer -- 1980
    
6. Structured High Level Languages belong to the 3rd generation of
   programming languages.  Outline what we should expect from the 4th generation.
   -- We expect a natural language interface, and probably also a natural language syntax for programming languages.  This includes a more fault tolerant attitude from the computer - e.g. it can still run a program if I forget to type a semi-colon.
    
7. List the following in order from oldest to newest:
         Java         Basic      C++      Fortran
   --   Fortran (1950's)  -->   Basic (1960's)  -->  C++ (1980's/90's) -->  Java (1995 to present)
    
8. Outline two advantages of Object Oriented Program over structured programming.
   --  By combining data and methods into a single object, OOP improves re-usability.  My program can simply create an object and use it, without worrying about side-effects.  So programs are more robust (reliable).  This is not only true for single programmers - it allows groups of programmers (teams) to easily share classes (modules), and thus speeds development. All the currently popular software applications (Windows, MS-Office, etc) could only be created and refined over 20 years by using OOP techniques. 
9. Explain 3 differences between a compiler and interpreter.
   --  Compilers examine the whole program, whereas interpreters look at one command at a time.
   Compilers find all the syntax errors before the program runs - interpreters start running the program and then crash when they find a syntax error.
   Compilers produce a machine-code file which can be run over and over again, whereas interpreters must re-interpret the program every time it runs.  That means a compiled program can run stand-alone, whereas interpreted programs can only run together with the interpreter.
    
10. Outline the difference between syntax and semantics, including specific examples.
    -- Syntax is about the way commands must be written - especially punctuation.  For example, leaving out a semi-colon is a syntax error.  Semantics is about the meaning of commands - for example, print means to print something into the text-console, whereas output causes a GUI dialog to pop up in the middle of the screen.  Print and output sound similar, but have different meanings.
     
11. Explain why the Java Virtual Machine is essential for ensuring portability
    of Java programs.
    -- Since different platforms (machines) contain different CPUs (microprocessors) and different operating systems, they require different machine code to function correctly.  Each machine has its own version of the JVM, which translates a compiled Java class (p-code) into exactly the correct machine code for the particular machien.  The Java class (p-code) can stay the same, but run correctly on lots of different machines.
     
12. Explain the difference between a keyword and a variable identifier.
    --  A keyword is reserved - this means in can only be used for the purpose defined in Java.  
    A variable name can be used for any purpose the programmer decides.  But it is not possible to redefine the meaning or function of a keyword.  For example,  for can only be used to make loops - not for any other purpose.

 13.  State the output of the following code fragment:

String  command = "Add 2 plus 4";
String[]  words = commands.split("\\s");
output(words.length);                       --> 4
output(words[2]);                           --> "plus"                                       

 14.  Rewrite this method so that it would respond correctly
      to the command:  "add 12 and 20"

public void tryAdd(String command)
{
   String[] info = command.split();
   if (info[0].equals("add") && info[2].equals("and") )
   {
      int a = Integer.parseInt(info[1]);
      int b = Integer.parseInt(info[3]);
      int c = a + b;
      output(c);
   }
}

15.  Explain why it makes sense to use an interpreter for JavaScript rather than a compiler. 
-- In the WWW, we want web-pages to work immediately.  We don't want the browser to say, "wait a minute while I compile this code."  It also makes it quite simple to change programs and pages, which happens often in the WWW.  And since web-pages are not "mission critical", it doesn't matter too much if a syntax error crashes the page, so compiling is not essential.