HamCall (October 1991)/HamCall (Whitehall Publishing)(1991).bin/prgming/ctutor/chap03.txt

Text File | 1990-10-14 | 17KB | 410 lines

                                                     Chapter 3

                                               PROGRAM CONTROL




THE WHILE LOOP
______________________________________________________________

The C programming language has several structures for looping
and conditional branching.  We will cover them all in this
chapter and we will begin with the while loop.

The while loop continues to loop while some condition is true.
When the condition becomes false, the looping is discontinued.
It therefore does just what it says it does, the name of the
loop being very descriptive.

Load the program WHILE.C and display it for        ===========
an example of a while loop.  We begin with a         WHILE.C
comment and the program name, then go on to        ===========
define an integer variable "count" within the
body of the program.  The variable is set to zero and we come
to the while loop itself.  The syntax of a while loop is just
as shown here.  The keyword "while" is followed by an
expression of something in parentheses, followed by a compound
statement bracketed by braces.  As long as the expression in
parenthesis is true, all statements within the braces will be
executed.  In this case, since the variable count is
incremented by one every time the statements are executed, it
will eventually reach 6.  At that time the statement will not
be executed, and the loop will be terminated.  The program
control will resume at the statement following the statements
in braces.

We will cover the compare expression, the one in parentheses,
in the next chapter.  Until then, simply accept the
expressions for what you think they should do and you will
probably be correct.

Several things must be pointed out regarding the while loop.
First, if the variable count were initially set to any number
greater than 5, the statements within the loop would not be
executed at all, so it is possible to have a while loop that
never is executed.  Secondly, if the variable were not
incremented in the loop, then in this case, the loop would
never terminate, and the program would never complete.
Finally, if there is only one statement to be executed within
the loop, it does not need braces but can stand alone.
Compile and run this program after you have studied it enough
to assure yourself that you understand its operation
completely.



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THE DO-WHILE LOOP
______________________________________________________________

A variation of the while loop is illustrated     =============
in the program DOWHILE.C, which you should         DOWHILE.C
load and display.  This program is nearly        =============
identical to the last one except that the
loop begins with the keyword "do", followed by a compound
statement in braces, then the keyword "while", and finally an
expression in parentheses.  The statements in the braces are
executed repeatedly as long as the expression in parentheses
is true.  When the expression in parentheses becomes false,
execution is terminated, and control passes to the statements
following this statement.

Several things must be pointed out regarding this statement.
Since the test is done at the end of the loop, the statements
in the braces will always be executed at least once.
Secondly, if "i" were not changed within the loop, the loop
would never terminate, and hence the program would never
terminate.  Finally, just like the while loop, if only one
statement will be executed within the loop, no braces are
required.  Compile and run this program to see if it does what
you think it should do.

It should come as no surprise to you that these loops can be
nested.  That is, one loop can be included within the compound
statement of another loop, and the nesting level has no limit.


THE FOR LOOP
______________________________________________________________

The "for" loop is really nothing new, it is      =============
simply a new way to describe the "while"           FORLOOP.C
loop.  Load and edit the file named FORLOOP.C    =============
for an example of a program with a "for"
loop. The "for" loop consists of the keyword "for" followed
by a rather large expression in parentheses.  This expression
is really composed of three fields separated by semi-colons.

The first field contains the expression "index = 0" and is
an initializing field.  Any expressions in this field are
executed prior to the first pass through the loop.  There is
essentially no limit as to what can go here, but good
programming practice would require it to be kept simple.
Several initializing statements can be placed in this field,
separated by commas.

The second field, in this case containing "index < 6", is the
test which is done at the beginning of each loop through the
program.  It can be any expression which will evaluate to a
true or false.  (More will be said about the actual value of
true and false in the next chapter.)

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The expression contained in the third field is executed each
time the loop is executed but it is not executed until after
those statements in the main body of the loop are executed.
This field, like the first, can also be composed of several
operations separated by commas.

Following the for() expression is any single or compound
statement which will be executed as the body of the loop.  A
compound statement is any group of valid C statements enclosed
in braces.  In nearly any context in C, a simple statement can
be replaced by a compound statement that will be treated as
if it were a single statement as far as program control goes.
Compile and run this program.

You may be wondering why there are two statements available
that do exactly the same thing because the "while" and the
"for" loop do exactly the same thing.  The "while" is
convenient to use for a loop that you don't have any idea how
many times the loop will be executed, and the "for" loop is
usually used in those cases when you are doing a fixed number
of iterations.  The "for" loop is also convenient because it
moves all of the control information for a loop into one
place, between the parentheses, rather than at both ends of
the code.  It is your choice as to which you would rather use.



THE IF STATEMENT
______________________________________________________________

Load and display the file IFELSE.C for an         ============
example of our first conditional branching          IFELSE.C
statement, the "if".  Notice first, that          ============
there is a "for" loop with a compound
statement as its executable part containing two "if"
statements.  This is an example of how statements can be
nested.  It should be clear to you that each of the "if"
statements will be executed 10 times.

Consider the first "if" statement.  It starts with the keyword
"if" followed by an expression in parentheses.  If the
expression is evaluated and found to be true, the single
statement following the "if" is executed, and if false, the
following statement is skipped.  Here too, the single
statement can be replaced by a compound statement composed of
several statements bounded by braces.   The expression "data
== 2" is simply asking if the value of data is equal to 2,
this will be explained in detail in the next chapter.  (Simply
suffice for now that if "data = 2" were used in this context,
it would mean a completely different thing.)





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                                   Chapter 3 - Program Control

NOW FOR THE IF-ELSE
______________________________________________________________

The second "if" is similar to the first with the addition of
a new keyword, the "else" following the first printf
statement.  This simply says that if the expression in the
parentheses evaluates as true, the first expression is
executed, otherwise the expression following the "else" is
executed.  Thus, one of the two expressions will always be
executed, whereas in the first example the single expression
was either executed or skipped.  Both will find many uses in
your C programming efforts.  Compile and run this program to
see if it does what you expect.


THE BREAK AND CONTINUE
______________________________________________________________

Load the file named BREAKCON.C for an           ==============
example of two new statements.  Notice that       BREAKCON.C
in the first "for", there is an if              ==============
statement that calls a break if xx equals
8.  The break will jump out of the loop you are in and begin
executing statements following the loop, effectively
terminating the loop.  This is a valuable statement when you
need to jump out of a loop depending on the value of some
results calculated in the loop.  In this case, when xx reaches
8, the loop is terminated and the last value printed will be
the previous value, namely 7.

The next "for" loop, contains a continue statement which does
not cause termination of the loop but jumps out of the present
iteration.  When the value of xx reaches 8 in this case, the
program will jump to the end of the loop and continue
executing the loop, effectively eliminating the printf
statement during the pass through the loop when xx is eight.
Compile and run this program.


THE SWITCH STATEMENT
______________________________________________________________

Load and display the file SWITCH.C for an         ============
example of the biggest construct yet in the         SWITCH.C
C language, the switch.  The switch is not        ============
difficult, so don't let it intimidate you.
It begins with the keyword "switch" followed by a variable in
parentheses which is the switching variable, in this case
"truck".  As many cases as desired are then enclosed within
a pair of braces.  The reserved word "case" is used to begin
each case, followed by the value of the variable, then a
colon, and the statements to be executed.


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                                   Chapter 3 - Program Control

In this example, if the variable "truck" contains the value
3 during this pass of the switch statement, the printf will
cause "The value is three" to be displayed, and the "break"
statement will cause us to jump out of the switch.

Once an entry point is found, statements will be executed
until a "break" is found or until the program drops through
the bottom of the switch braces.  If the variable has the
value 5, the statements will begin executing where "case 5 :"
is found, but the first statements found are where the case
8 statements are.  These are executed and the break statement
in the "case 8" portion will direct the execution out the
bottom of the switch.  The various case values can be in any
order and if a value is not found, the default portion of the
switch will be executed.

It should be clear that any of the above constructs can be
nested within each other or placed in succession, depending
on the needs of the particular programming project at hand.
Be sure to compile and run SWITCH.C and examine the results.


THE EVIL GOTO STATEMENT
______________________________________________________________

Load and display the file GOTOEX.C for an         ============
example of a file with some "goto" statements       GOTOEX.C
in it.  To use a "goto" statement, you simply     ============
use the reserved word "goto" followed by the
symbolic name to which you wish to jump.  The name is then
placed anywhere in the program followed by a colon.  You can
jump nearly anywhere within a function, but you are not
permitted to jump into a loop, although you are allowed to
jump out of a loop.  Also, you are not allowed to jump out of
any function into another.  These attempts will be flagged by
your C compiler as an error if you attempt any of them.

This particular program is really a mess but it is a good
example of why software writers are trying to eliminate the
use of the "goto" statement as much as possible.  The only
place in this program where it is reasonable to use the "goto"
is the one in line 18 where the program jumps out of the three
nested loops in one jump.  In this case it would be rather
messy to set up a variable and jump successively out of all
three loops but one "goto" statement gets you out of all three
in a very concise manner.

Some persons say the "goto" statement should never be used
under any circumstances, but this is rather narrow minded
thinking.  If there is a place where a "goto" will clearly do
a neater control flow than some other construct, feel free to
use it.  It should not be abused however, as it is in the rest
of the program on your monitor.

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                                   Chapter 3 - Program Control

Entire books are written on "gotoless" programming, better
known as Structured Programming.  These will be left to your
study.  One point of reference is the Visual Calculator
described in Chapter 14 of this tutorial.  This program is
contained in four separately compiled programs and is a rather
large complex program.  If you spend some time studying the
source code, you will find that there is not a single "goto"
statement anywhere in it.  Compile and run GOTOEX.C and study
its output.  It would be a good exercise to rewrite it and see
how much more readable it is when the statements are listed
in order.


FINALLY, A MEANINGFUL PROGRAM
______________________________________________________________

Load the file named TEMPCONV.C for an example   ==============
of a useful, even though somewhat limited         TEMPCONV.C
program.  This is a program that generates a    ==============
list of centigrade and farenheit temperatures
and prints a message out at the freezing point of water and
another at the boiling point of water.

Of particular importance is the formatting.  The header is
simply several lines of comments describing what the program
does in a manner that catches the readers attention and is
still pleasing to the eye. You will eventually develop your
own formatting style, but this is a good way to start.  Also
if you observe the for loop, you will notice that all of the
contents of the compound statement are indented 3 spaces to
the right of the "for" keyword, and the closing brace is lined
up under the "f" in "for".  This makes debugging a bit easier
because the construction becomes very obvious.  You will also
notice that the "printf" statements that are in the "if"
statements within the big "for" loop are indented three
additional spaces because they are part of another construct.

This is the first program in which we used more than one
variable.  The three variables are simply defined on three
different lines and are used in the same manner as a single
variable was used in previous programs.  By defining them on
different lines, we have an opportunity to define each with
a comment.


ANOTHER POOR PROGRAMMING EXAMPLE
______________________________________________________________

Recalling UGLYFORM.C from the last chapter,     ==============
you saw a very poorly formatted program.  If      DUMBCONV.C
you load and display DUMBCONV.C you will have   ==============
an example of poor formatting which is much
closer to what you will find in practice.  This is the same
program as TEMPCONV.C with the comments removed and the

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                                   Chapter 3 - Program Control

variable names changed to remove the descriptive aspect of the
names.  Although this program does exactly the same as the
last one, it is much more difficult to read and understand.
You should begin to develop good programming practices now.
Compile and run this program to see that it does exactly what
the last one did.


PROGRAMMING EXERCISES
______________________________________________________________

1.   Write a program that writes your name on the monitor ten
     times.  Write this program three times, once with each
     looping method.

2.   Write a program that counts from one to ten, prints the
     values on a separate line for each, and includes a
     message of your choice when the count is 3 and a
     different message when the count is 7.


































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