Archive-name: c-notes/Lesson08.txt

                                 Lesson 8.

  This lesson will examine how to use the program structure - as opposed
to data structure - reserved words.

  Lets start with the looping structures:

  do repeated_statement while ( logical_expression );

  repeated_statement, which may be a block of code, will be executed
repetitively until the logical_expression, becomes false. If you have been
exposed to ( corrupted by? ) another language remember that there is no
`until' test at the end of a loop. Note that the repeated_statement is always
executed once irrespective of the state of the logical_expression.

  while ( logical_expression ) repeated_statement;

  repeated_statement is executed repetitively while the logical_expression
is true. Once again statement may be a block of code. Note that if the 
logical_expression evaluates to FALSE then the repeated_statement is NEVER
executed.

  Associated with the looping structures are the control words:

  break;
  continue;

  break; allows you to leave a loop in the middle of a block, and
  continue; allows you to re-start it from the top.

  Finally we must not forget the most common and useful looping construct:

  for ( initialising statement; logical_expression; incremental_statement )
  repeated_statement;

  Some further explanation is needed. The initialising statement is
executed once, but to allow for the need to initialise several separate
variables the assignment statements may be separated by commas. The
logical_expression must be true for the loop to run, and the
incremental_statement is executed once each time the loop is run.
The for statement is completely general and may, for example, be used to
manipulate a set of pointers to operate on a linked list.

Some examples.

  A do loop program.

#ident "@(#) do_demo.c - An example of the do loop"

#include <stdio.h>

main()
{
  char character;

  character = 'a';

  do printf ( "%c", character ); while ( character++ < 'z' );
  printf ( "\n" );
  }

  Fairly obviously it prints:

abcdefghijklmnopqrstuvwxyz

  A while loop example.

#ident "@(#) while_demo.c - An example of the while loop"

#include <stdio.h>

main()
{
  char character;

  character = 'a';

  while ( character <= 'z' ) printf ( "%c", character++ );
  printf ( "\n" );
  }

  Its output is exactly the same as the previous example:

abcdefghijklmnopqrstuvwxyz

  In this totally trivial case it is irrelevant which program structure
  you use, however you should note that in the `do' program structure the
  repeated statement is always executed at least once.
  A for loop example.

  The `for' looping structure.

#ident "@(#) for_demo.c - An example of the for loop"

#include <stdio.h>

main()
{
  char character;

  for ( character = 'a'; character <= 'z' ; character++ )
  {
    printf ( "%c", character );
    }
  printf ( "\n" );
  }

  Surprise, Surprise!

abcdefghijklmnopqrstuvwxyz

  You should be aware that in all the looping program structures, the
repeated statement can be a null statement ( either just a `;' or the
reserved word `continue;' ). This means that it is possible to - for
example - position a pointer, or count up some items of something or other.
It isn't particularly easy to think up a trivial little program which
demonstrates this concept, however the two `for' loops give some indication
of the idea.

#ident "@(#) pointer_demo.c - Pointer operations with the for loop"

#include <stdio.h>

main()
{
  char character, *character_pointer, alphabets [ 53 ];

  for ( character = 'a', character_pointer = alphabets;  /* Start conditions */
        character <= 'z';                                /* Run while true   */
        *character_pointer++ = character++               /* All the work     */
        ) continue;

  for ( character = 'A';  /* character_pointer is at the right place already */
        character <= 'Z';
        *character_pointer++ = character++
        ) continue;

  *character_pointer = (char) '\000'; /* NULL character to terminate string. */

  printf ( "%s\n\n", alphabets );
  }

  Another Surprise!

abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ

  So much for the looping structures provided by the `C' language. The
other main structures required to program a computer are the ones which
alter the program flow. These are the switch, and the if and its extension
the if ... else combination. More demo programs are much the best way of
getting the message across to you, so here they are, first the if construct.

#ident "if_demo.c"

#include <stdio.h>

main(argc, argv)
int argc;
char **argv;
{
  if ( argc > 1 ) printf ( "You have initiated execution with arguments."};
  }

  And the if ... else demo.

#ident "if_else_demo.c"
/*
** The Language #define could go in the compiler invocation line if desired.
*/

#define ENGLISH 

#include <stdio.h>

/*
** The message and text fragments output by the program.
*/

char *messages[] =
{
#if defined( ENGLISH )
#ident "@(#)ENGLISH Version"
  "\nUsage: if_else_demo <numeric argument 1> <numeric argument 2>\n\n",
  "The first argument is ",
  "the second",
  "equal to ",
  "bigger than ",
  "smaller than "
#endif

#if defined( FRANCAIS )
#ident "@(#)FRENCH Version"

  put the French translation in here so that we are ready to export to
  French speaking Countries. I'd be grateful if a French speaker could
  make the translation for me.

#endif
  };

/*
** Meaningful words defined to constants
*/

#define USAGE 0
#define FIRST 1
#define SECOND 2
#define EQUAL 3
#define BIGGER 4
#define SMALLER 5

#define SUCCESS 0
#define FAILURE 1

/*
** We need this more than once so it can be put in a function.
*/

void usage()
{
  printf ( messages[USAGE]);
  exit ( FAILURE );
  }
     
/*
** Main program function starts here. ( At the top of a page no less! )
*/

int main ( argc, argv )
int argc;
char **argv;
{
  int message_index;
  double i, j, strtod();
  char *ptr;

  if ( argc != 3 ) usage();                  /* have we been given the right */
                                             /* number of arguments. */
  i = strtod ( argv[1], &ptr);               /* Convert to a double float. */
  if ( ptr == argv[1] ) usage();             /* Successful conversion? */
  j = strtod ( argv[2], &ptr);               /* Convert to a double float. */
  if ( ptr == argv[2] ) usage();             /* Successful conversion? */

/*
** This statement uses the "ternary conditional assignment" language 
** construction to assign the value required to the message indexing variable.
** Note that this concept is efficient in both the generation of machine code
** output ( compile the program with a -S switch and have a look ) and in the
** ease with which it can be understood. The assignment is obvious instead of
** being buried under a litter of `if' and `else' keywords.
*/

  message_index = ( i == j ) ? EQUAL : ( i > j ) ? BIGGER : SMALLER;

/*
** Now print the message.
*/

  (void) printf ( "\n%s%s%s\n\n",     /* Format string specifying 3 strings. */
                  messages[ FIRST ],                   /* Address of string. */
                  messages[ message_index ],           /*        ditto.      */
                  messages[ SECOND ]                   /*        ditto.      */
                  );
  return ( SUCCESS );
  }

  Well as you can no doubt gather it simply compares two numbers on the 
command line and ejects a little message depending on the relative magnitude
of the numbers. In the UNIX tradition the help message is perhaps somewhat
terse, but it serves the purpose of getting you - the student - to think
about the importance of creating programs which always cope with nonsensical
input in a civilised way. Here are the lines of output.

Usage: if_else_demo <numeric argument 1> <numeric argument 2>

The first argument is equal to the second

The first argument is smaller than the second

The first argument is bigger than the second

  Now that the international community is shrinking with vastly improved
telecommunications, it is perhaps a good idea to think carefully about 
creating programs which can talk in many languages to the users. The method
of choice is - I believe - that presented above. The #if defined( LANGUAGE )
gives us an easy method of changing the source code to suit the new sales
area. Another possibility is to put all the text output needed from a program
into a file. The file would have to have a defined layout and some consistent
way of `getting at' the message strings.

  From a commercial point of view this may or may not be a good business plan.
Quite definitely it is an absolute no no to scatter a mass of string literals
containing the messages and message fragments all over your program script.

  There are two more methods of altering the program flow.

  1 ) The goto a label.
  2 ) The setjump / longjmp library routines.

  The concept of the go to a label construction has had reams of literary
verbiage written about it and this author does not intend to add to the pile.
Suffice it to say that a goto is a necessary language construct. There are a
few situations which require the language to have ( in practice ) some form of
unconditional jump. Treat this statement with great caution if you wish your
code to be readable by others. An example of legitimate use.

  for ( a = 0; a < MATRIX_SIZE; a++ )
  {
    for ( b = 0; b < MATRIX_SIZE; b++ )
    {
      if ( process ( matrix, a, b )) goto bad_matrix;
      }
    }
  return ( OK );

bad_matrix:

   perror ( progname, "The data in the matrix seems to have been corrupted" );
   return ( BAD );

  This is one of the very few "legitimate" uses of goto, as there is no
"break_to_outer_loop" in `C'. Note that some compilers complain if the label
is not immediately followed by a statement. If your compiler is one of these
naughty ones, you can put either a `;' or a pair of braces `{}' after the
`:' as a null statement.

  An example of a program package which makes extensive use of the goto is the
rz and sz modem communications protocol implementation by Chuck Forsberg of
Omen Technology. You should download it and study the code, but do remember
that the proof of the pudding argument must apply as the rz & sz system has
become extremely popular in its application because it works so well.

  The other method of changing program flow is the setjump and longjmp pair of
library functions. The idea is to provide a method of recovery from errors
which might be detected anywhere within a large program - perhaps a compiler,
interpreter or large data acquisition system. Here is the trivial example:

#ident "set_jmp_demo.c"

#include <stdio.h>
#include <setjmp.h>

jmp_buf save;

main()
{
  char c;

  for ( ;; )                     /* This is how you set up a continuous loop. */
  {
    switch ( setjmp( save ))
    {
case 0:
      printf ( "We get a zero returned from setjmp on setup.\n\n");
      break;                   /* This is the result from setting up. */

case 1:
      printf ( "NORMAL PROGRAM OPERATION\n\n" );
      break;

case 2:
      printf ( "WARNING\n\n" );
      break;

case 3:
      printf ( "FATAL ERROR PROGRAM TERMINATED\n\nReally Terminate? y/n: " );
      fflush ( stdout );
      scanf ( "%1s", &c );
      c = tolower ( c );
      if ( c == 'y' ) return ( 1 );
      printf ( "\n" );
      break;

default:
      printf ( "Should never return here.\n" );
      break;
      }
    process ();
    }
  }

process ()
{
  int i;

  printf ( "Input a number to simulate an error condition: " );
  fflush ( stdout );
  scanf ( "%d", &i );
  i %= 3;
  i++;                /* So that we call longjmp with  0 < i < 4 */
  longjmp ( save, i);
  }

  Although in this silly little demo the call to longjmp is in the same file
as the call to setjmp, this does not have to be the case, and in the practical
situation the call to longjmp will be a long way from the call to setjmp. The
mechanism is that setjmp saves the entire state of the computer's CPU in a
buffer declared in the jmp_buf save; statement and longjmp restores it exactly
with the exception of the register which carries the return value from longjmp.
This value is the same as the second argument in the longjmp call - i in our
little demo. This means, of course, that the stack and frame pointer registers
are reset to the old values and all the local variables being used at the time
of the longjmp call are going to be lost forever. One consequence of this is
that any pointer to memory allocated from the heap will also be lost, and
you will be unable to access the data stored in the buffer. This is what the
jargonauts call "memory leakage", and is really very difficult bug to find.
Your program runs out of dynamic memory long before it should. Take care.
So you have to keep a record of the buffers' addresses and free them
before the call to longjmp.

More details later on when we learn about the heap memory allocation routines.



Copyright notice:-

(c) 1993 Christopher Sawtell.

I assert the right to be known as the author, and owner of the
intellectual property rights of all the files in this material,
except for the quoted examples which have their individual
copyright notices. Permission is granted for onward copying,
storage, but not modification, of this course in electronic data
retrieval systems, and its use for personal study only, provided
all the copyright notices are left in the text and are printed
in full on any subsequent paper reproduction.

In other words you may pass it around to your friends and print it
out in full on paper, but you may not steal my text and pretend
you wrote it, change the text in any way, or print it as a bound book.

--
 +----------------------------------------------------------------------+
 | NAME   Christopher Sawtell                                           |
 | SMAIL  215 Ollivier's Road, Linwood, Christchurch, 8001. New Zealand.|
 | EMAIL  chris@gerty.equinox.gen.nz                                    |
 | PHONE  +64-3-389-3200   ( gmt +13 - your discretion is requested )   |
 +----------------------------------------------------------------------+
