Shareware Overload/ShartewareOverload.cdr/progm/ctutor1.zip/CHAP03.TXT

Text File | 1989-11-10 | 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 entry point main(),     =============
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 repeatedly 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 delimiting 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.  Note that the result of execution is given for
this program, (and will be given for all of the remaining

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example programs in this tutorial) so you do not need to
compile and execute every program to see the results.  Be sure
to compile and execute some of the programs however, to gain
experience with your compiler.


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 the do-while
loop. 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 the variable 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 loop.      FORLOOP.C
Load and display the file named FORLOOP.C on    =============
your monitor 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.

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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.)

The expression contained in the third field is executed each
time the loop is exercised 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

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suffice for now that if "data = 2" were used in this context,
it would mean a completely different thing.)


NOW FOR THE IF-ELSE
____________________________________________________________

The second if is similar to the first with the addition of a
new keyword, the else in line 15.  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 loop, 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 starting in line 12, 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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In this example, if the variable named truck contains the
value 3 during this pass of the switch statement, the printf()
in line 9 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 at line 13 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 line 17 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.
Note that the switch is not used as frequently as the loop and
the if statements, in fact the switch is used infrequently.
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.

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.

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                                  Chapter 3 - Program Control

It should not be abused however, as it is in the rest of the
program on your monitor.

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 files 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 fahrenheit
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.

Be sure to compile and execute this program.





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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 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.


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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