                      Chapter 2 - Getting started in C


                            YOUR FIRST C PROGRAM

             The  best way to get started with C is to actually look 

        at  a program,  so load the file named TRIVIAL.C  into  your 

        editor  and display it on the monitor.   You are looking  at 

        the  simplest  possible  C  program.   There is  no  way  to 

        simplify   this   program   or  to   leave   anything   out.  

        Unfortunately, the program doesn't do anything.

             The  word  "main" is very important,  and  must  appear 

        once,  and only once in every C program.   This is the point 

        where execution is begun when the program is run.   We  will 

        see  later that this does not have to be the first statement 

        in  the  program  but  it must exist  as  the  entry  point.  

        Following  the "main" program name is a pair of  parentheses 

        which  are  an  indication to the compiler that  this  is  a 

        function.   We will cover exactly what a function is in  due 

        time.   For now,  I suggest that you simply include the pair 

        of parentheses. 

             The  two curly brackets,  properly called  braces,  are 

        used to define the limits of the program itself.  The actual 

        program  statements  go between the two braces and  in  this 

        case,  there  are  no  statements because the  program  does 

        absolutely nothing.   You can compile and run this  program, 

        but since it has no executable statements,  it does nothing.  

        Keep in mind however, that it is a valid C program.

                       A PROGRAM THAT DOES SOMETHING

             For  a much more interesting program,  load the program 

        named WRTSOME.C and display it on your monitor.   It is  the 

        same  as  the  previous  program  except  that  it  has  one 

        executable statement between the braces.

             The  executable  statement is another  function.   Once 

        again,  we will not worry about what a function is, but only 

        how  to  use  this one.   In order to  output  text  to  the 

        monitor,  it  is  put  within the function  parentheses  and 

        bounded by quotation marks.  The end result is that whatever 

        is included between the quotation marks will be displayed on 

        the monitor when the program is run. 

             Notice the semi-colon at the end of the line.  C uses a 

        semi-colon as a statement terminator,  so the semi-colon  is 

        required  as  a  signal to the compiler that  this  line  is 

        complete.   This  program  is also executable,  so  you  can 

        compile  and  run  it to see if it does what  you  think  it 

        should.





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                      Chapter 2 - Getting started in C


                      ANOTHER PROGRAM WITH MORE OUTPUT

             Load  the  program  WRTMORE.C and display  it  on  your 

        monitor  for an example of more output and another small but 

        important concept.  You will see that there are four program 

        statements  in  this  program,  each one  being  a  "printf" 

        function  statement.   The top line will be executed  first, 

        then the next, and so on, until the fourth line is complete.  

        The statements are executed in order from top to bottom.

             Notice  the funny character near the end of  the  first 

        line,  namely  the backslash.   The backslash is used in the 

        printf statement to indicate a special control character  is 

        following.  In this case, the "n" indicates that a "newline" 

        is requested.  This is an indication to return the cursor to 

        the left side of the monitor and move down one line.   It is 

        commonly  referred to as a carriage return/line  feed.   Any 

        place  within  text that you desire,  you can put a  newline 

        character  and start a new line.   You could even put it  in 

        the  middle of a word and split the word between two  lines.  

        The  C compiler considers the combination of  the  backslash 

        and letter n as one character.

             A complete description of this program is now possible.  

        The  first  printf  outputs a line of text and  returns  the 

        carriage.   The  second printf outputs a line but  does  not 

        return the carriage so the third line is appended to that of 

        the second, then followed by two carriage returns, resulting 

        in  a blank line.   Finally the fourth printf outputs a line 

        followed by a carriage return and the program is complete.

             Compile and run this program to see if it does what you 

        expect  it to do.   It would be a good idea at this time for 

        you to experiment by adding additional lines of printout  to 

        see if you understand how the statements really work.

                          LETS PRINT SOME NUMBERS

             Load  the  file  named ONEINT.C and display it  on  the 

        monitor for our first example of how to work with data in  a 

        C program.  The entry point "main" should be clear to you by 

        now as well as the beginning brace.   The first new thing we 

        encounter is the line containing "int index;", which is used 

        to define an integer variable named "index".  The "int" is a 

        reserved  word  in  C,  and can therefore not  be  used  for 

        anything else.   It defines a variable that can have a value 

        from  -32768 to 32767 on most microcomputer  implementations 

        of  C.   Consult your users manual for the exact  definition 

        for your compiler.   The variable name,  "index", can be any 

        name that follows the rules for an identifier and is not one 

        of  the reserved words for C.   Consult your manual  for  an 



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                      Chapter 2 - Getting started in C


        exact  definition of an identifier for your  compiler.   The 

        final  character  on  the  line,   the  semi-colon,  is  the 

        statement terminator used in C.

             We will see in a later chapter that additional integers 

        could  also  be defined on the same line,  but we  will  not 

        complicate the present situation. 

             Observing the main body of the program, you will notice 

        that  there are three statements that assign a value to  the 

        variable  "index",  but only one at a time.   The first  one 

        assigns the value of 13 to "index", and its value is printed 

        out.   (We will see how shortly.)  Later, the value of 27 is 

        assigned to "index",  and finally 10 is assigned to it, each 

        value  being  printed out.   It should be intuitively  clear 

        that  "index"  is  indeed  a variable  and  can  store  many 

        different  values.   Please note that many times  the  words 

        "printed  out" are used to mean "displayed on the  monitor".  

        You  will  find that in many cases  experienced  programmers 

        take  this  liberty,  probably due to the "printf"  function 

        being used for monitor display.

                          HOW DO WE PRINT NUMBERS

             To  keep  our promise,  let's return  to  the  "printf" 

        statements  for a definition of how they work.   Notice that 

        they are all identical and that they all begin just like the 

        "printf"  statements  we  have  seen  before.    The   first 

        difference occurs when we come to the % character.   This is 

        a  special character that signals the output routine to stop 

        copying characters to the output and do something different, 

        namely output a variable.   The % sign is used to signal the 

        start  of  many different types of variables,  but  we  will 

        restrict  ourselves  to  only one  for  this  example.   The 

        character following the % sign is a "d",  which signals  the 

        output routine to get a decimal value and output it.   Where 

        the decimal value comes from will be covered shortly.  After 

        the  "d",  we  find the familiar \n,  which is a  signal  to 

        return the video "carriage", and the closing quotation mark.

             All  of  the  characters between  the  quotation  marks 

        define  the pattern of data to be output by this  statement, 

        and  after  the pattern,  there is a comma followed  by  the 

        variable name "index".  This is where the "printf" statement 

        gets  the decimal value which it will output because of  the 

        "%d"  we saw earlier.   We could add more "%d" output  field 

        descriptors within the brackets and more variables following 

        the  description  to cause more data to be printed with  one 

        statement.   Keep in mind however, that it is important that 

        the  number of field descriptors and the number of  variable 





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                      Chapter 2 - Getting started in C


        definitions must be the same or the runtime system will  get 

        confused and probably quit with a runtime error.

             Much  more  will  be  covered at a later  time  on  all 

        aspects of input and output formatting.   A reasonably  good 

        grasp  of this topic is necessary in order to understand the 

        following  lessons.   It  is  not  necessary  to  understand 

        everything about output formatting at this time, only a fair 

        understanding of the basics.

             Compile and run ONEINT.C and observe the output.

                        HOW DO WE ADD COMMENTS IN C

             Load the file COMMENTS.C and observe it on your monitor 

        for an example of how comments can be added to a C  program.  

        Comments  are  added to make a program more readable to  you 

        but the compiler must ignore the comments.   The slash  star 

        combination  is used in C for comment delimiters.   They are 

        illustrated  in the program at hand.   Please note that  the 

        program does not illustrate good commenting practice, but is 

        intended  to illustrate where comments can go in a  program.  

        It is a very sloppy looking program.

             The  first slash star combination introduces the  first 

        comment  and  the star slash at the end of  the  first  line 

        terminates this comment.  Note that this comment is prior to 

        the beginning of the program illustrating that a comment can 

        precede the program itself.  Good programming practice would 

        include  a  comment  prior  to  the  program  with  a  short 

        introductory  description of the program.   The next comment 

        is  after the "main()" program entry point and prior to  the 

        opening brace for the program code itself.

             The  third  comment starts after the  first  executable 

        statement and continues for four lines.   This is  perfectly 

        legal  because  a comment can continue for as many lines  as 

        desired  until  it is terminated.   Note carefully  that  if 

        anything  were included in the blank spaces to the  left  of 

        the  three  continuation lines of the comment,  it would  be 

        part  of the comment and would not be  compiled.   The  last 

        comment  is located following the completion of the program, 

        illustrating  that  comments can go nearly anywhere in  a  C 

        program. 

             Experiment  with  this program by  adding  comments  in 

        other places to see what will happen. Comment out one of the 

        printf  statements by putting comment delimiters both before 

        and after it and see that it does not get printed out.





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                      Chapter 2 - Getting started in C


             Comments are very important in any programming language 

        because  you will soon forget what you did and why  you  did 

        it.   It  will  be  much  easier to modify  or  fix  a  well 

        commented  program  a year from now than one with few or  no 

        comments.   You will very quickly develop your own  personal 

        style of commenting.

             Some  compilers allow you to "nest" comments which  can 

        be very handy if you need to "comment out" a section of code 

        during debugging.  Check your compiler documentation for the 

        availability  of this feature with you particular  compiler.  

        Compile and run COMMENTS.C at this time.

                           GOOD FORMATTING STYLE

             Load  the  file  GOODFORM.C  and  observe  it  on  your 

        monitor.   It  is  an example of a well  formatted  program.  

        Even though it is very short and therefore does very little, 

        it  is very easy to see at a glance what it does.   With the 

        experience  you have already gained in  this  tutorial,  you 

        should  be  able  to very quickly grasp the meaning  of  the 

        program in it's entirety.  Your C compiler ignores all extra 

        spaces  and  all carriage returns  giving  you  considerable 

        freedom  concerning how you format your program.   Indenting 

        and  adding spaces is entirely up to you and is a matter  of 

        personal  taste.   Compile and run the program to see if  it 

        does what you expect it to do.

             Now load and display the program UGLYFORM.C and observe 

        it.   How  long  will it take you to figure  out  what  this 

        program  will do?   It doesn't matter to the compiler  which 

        format style you use, but it will matter to you when you try 

        to  debug  your program.   Compile this program and run  it.  

        You may be surprised to find that it is the same program  as 

        the  last  one,  except for the formatting.   Don't get  too 

        worried about formatting style yet.  You will have plenty of 

        time  to  develop  a  style of your own  as  you  learn  the 

        language.   Be observant of styles as you see C programs  in 

        magazines, books, and other publications. 

             This  should  pretty well cover the basic  concepts  of 

        programming  in  C,  but as there are many other  things  to 

        learn, we will forge ahead to additional program structure.

        PROGRAMMING EXERCISES

        1. Write a program to display your name on the monitor.

        2. Modify  the  program to display your address  and  phone 

           number  on  separate  lines  by  adding  two  additional 

           "printf" statements.



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