The Structure of a C++ Program

• The following contains lies of commission
  • At various places I will say "you must include the following element"
  • And that is true 90% of the time.
  • But sometimes it is not true.
  • Do it, until you know better.
• It contains things you must do without explanation
  • I will just say "Include this element"
  • But I will not tell you why.
  • Or I will give you a mostly incoherent discussion
  • You need to learn just a bit more before we can properly explain things.
  • Our goal is eventually you understand EVERYTHING involved in programming.
• Comments
  • Comments in a program are notes to people who are reading the program.
  • They are ignored by the compiler, and have no effect on the executable.
  • They are used to describe what is happening in the code
    • Or why the program was written
    • And who the wrote the program.
      • Comments are also used to remove code from the program without removing it from the file.
        • This is called "commenting out code"
      • There are two types of comments in C++
        • C style comments
          • Begin with two slashes //
          • End at the end of a line (newline character, enter key)
          • Only one line long
          • And can contain anything.
        • C++ style comments
          • Begin with /*
          • end with */
          • Can cover many lines
          • May not contain */
        • I tend to use C++ style comments for documentation at the beginning of a program and to comment out blocks of code
        • I tend to use C style comments to document lines of code
        • You should place a comment at the beginning of each program
          • Give your name
          • Describe the assignment, what the code does
          • Perhaps an algorithm
          • List any interesting features of the code
          • List any collaborators who should be referenced
        • You should place a comment describing major sections of the code
          • This is tricky and will require some practice to get it right.
          • Too many comments are distracting
          • Too few are a problem.
          • I will help you with this as the semester goes along.

      • /* 
          Program 1, How to write a C++ Program
          Dan Bennett
          September, 2015
        
          This program will print a joke on the screen.
        
          Algorithm: 
            Print the question
            Print a space
            Print the answer
        
          Joke Source: http://homepage.eircom.net/~cronews/elep/elep.html
        
          Collaborators:  The CSCI 130 class, MWF and TR
        */         
        	 

    • Header Files
      • There are many elements which get added to the C++ language
      • They are extensions if you will.
      • There are two components of adding something to the language
        • A header file
        • A library
      • Header files define the features contained in the library
      • Header files contain C++ code
      • Header files are located in /usr/include on unix like machines.
      • I will tell you when you need to include header files for various components we discuss.
      • And the compiler will tell you as well, but not as plainly...
      • You almost always need to include iostream
      • you usually include header files immediately following the opening comment.

    •   Joke Source: http://homepage.eircom.net/~cronews/elep/elep.html
      
        Collaborators:  The CSCI 130 class, MWF and TR
      */
      
      #include <iostream>
      

    • Namespaces
      • A namespace gives us the ability to have multiple things with the same name.
      • Sort of like how we know the difference between
        • Daniel Bennett
        • Daniel Boone
        • Daniel Webster
        • Daniel Day-Lewis
      • Some people will tell you never to use this
      • but for now just put it in.
      • More on this later,
      • It immediately follows the header files.

    •   Joke Source: http://homepage.eircom.net/~cronews/elep/elep.html
      
        Collaborators:  The CSCI 130 class, MWF and TR
      */
      
      #include <iostream>
      
      using namespace std;
      

    • Seicolons (;)
      • Mark the end of a c++ statement
      • A statement is the smallest standalone element of a programming language that expresses some action to be carried out. (Wikipedia)
      • The comment is not a statement, it causes no action to be carried out, so it doesn't need a ; at the end.
      • #include is not a statement
        • This is a bit more difficult.
        • It is a preprocessor directive
        • This means it gets replaced before the compiler sees it.
      • All statements end is a ;
      • But there are some other things that aren't statements that end in a ; as well.
      • And there are some that don't.
      • But it almost never hurts to put a ; at the end of a complete thought
        • #include <iostream>; is strange, but ok
        • //this is a comment; is strange but ok.
        • /* this is a comment too */;
        • ;; is ok
    • Reserved words
      • A reserved word is a word that has special meaning in c++ (page 49)
      • The list of reserved words are in Appendix A.
      • We will spend quite a bit of time learning the reserved words for C++
      • Both using and namespace are reserved words.
    • An identifier is a name associated with an object in a p rogram (somewhat from page 49)
      • In some sense, programmer make objects (things) in their program.
      • They need a way to name these objects
      • The names are identifiers
      • Some items are named by the people who created c++
        • This includes elements of libraries
        • Such identifiers are called standard identifiers
        • std is a standard identifier
      • Another class of identifiers are user declared identifiers
      • We will discuss those in a moment.
    • The main routine
      • Each program declares a number of functions
      • A function is a subprogram in c++ (page 44)
      • Each program must declare a main function.
      • The main function tells the compiler where to start executing code.
      • This is where we will put all of our code for a while.
      • More about this as you go along in the program.

    • #include <iostream>
      
      using namespace std;
      
      int main() {
      }
      

    • About this new statement
      • main is a standard identifier....
      • () are associated with the function
        • It is a parameter list.
        • Ignore it for now.
        • But do put it in.
      • int is a data type
        • A data type is a set of data values along with operations on those values (page 50)
        • int is a reserved word that indicates the integer data type.
          • The integer data type contains a subset of the integers.
          • C (and thus C++) refuses to define that range as it is hardware dependent.
          • But in our case it is probably -2³¹ to 2 ³¹-1
          • (More on this later)
          • The operations include +, -, *, /, % , ...
          • A full list of operators is in appendix B.
        • Other basic data types (char, float, boolean) and complex data types (string, file)
        • We will discuss data types for a long time. (Chapters 2-4, appendix G)
        • Then we will discuss control structures (5-9)
        • Then we will return to data types in CSCI 230 (10-17)
        • Then finally one more control structure (18)
      • { ... } marks a code block
        • At times we wish to group multiple statements into a single statement.
        • This is done with a code block
        • A code block begins with {
        • And ends with }
        • A code block is a statement which does not need a ; at the end
        • But you can put one if you wish.
        • More on this later.
      • Style
        • The way express your code is a matter of style.
        • I will give some guidance here, but style tends to differ by organization
        • Appendix F discusses style.
        • C/C++ is a free-form language
        • For the most part, whitespace (space, tab, newline) doesn't matter to the compiler
        • But it matters quite a bit to the programmers.
        • Matters of style are important to me and I will take points off for
          • Poor style
          • Inconsistent style
        • Style point 1: Placement of code blocks.
          • There are two main acceptable styles for this
            • The book uses a style called Allman style
            • See page 56 for example.

            • prefix to code block 
              {
                   code in code block
              }
              		     

          • I use a style called K&R style

            • prefix to code block {
                   code in code block
              }
              		  

          • You may use either, but be consistent.
        • Indentation.
          • Notice that in both cases above, the body of a code block was indented.
          • This is a REQUIRED style component.
          • A single space indentation is not sufficient
          • Two is probably not either.
          • A tab is probably too much.
          • I use three or four spaces, but be consistent.
        • Blank lines
          • I like to use plenty of blank lines
          • They separate ideas, much like paragraphs in a document
        • One statement per line
          • More of this later.
        • These two matters of style seem silly, but they will eventually make sense
        • Pay attention to them.
        • But in general, copy me or the book
    • Statements
      • In every C++ program, the line return 0 is required.

        • using namespace std;
          
          int main() {
          
              return 0;
          }
          

        • Return is a reserved word
        • It indicates that the routine should exit and return a value.
        • 0 is the value to be returned in this case.
        • Since the number 0 is hard coded in the program it is a
          • constant or a value that will not change. (p55)
          • literal value (page 55) or literal constant
          • magic number since it is a number in our program with no apparent meaning.
      • We want our code to do something so we will add a few other statements
        • cout << endl;
          • cout is the standard output stream
          • It is usually associated with the terminal
          • It is a standard identifier
          • It is part of iostream
          • It is part of the standard namespace
        • << is the stream insertion operator
          • It takes the expression on the right hand side of and places it on the stream on the left hand side
          • More on expressions and this operator later.
        • endl
          • Inserts a newline character and flushes the corresponding buffer
          • More on this later.
    • The full program is at ~dbennett/130/ch2/demo.C
      • You can copy this to your directory
      • cp ~dbennett/130/ch2/demo.C .