PC World Interactive 7/PC World Interactive 7.iso/program/ctutor.exe/TEXT/CHAP03.TXT

Text File | 1994-05-15 | 19KB | 406 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 named
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 the
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 because count is not less than 6, 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 be correct for these
simple cases.

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 incre-
mented 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 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.  Once


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again, you may get compiler warnings in this chapter which can be
ignored temporarily.

You should also modify any programs that are not completely clear
to you until you understand them completely.  The best way to
learn is to try various modifications yourself.


THE DO-WHILE LOOP
-----------------------------------------------------------------
A variation of the while loop is illustrated in   ===============
the program DOWHILE.C, which you should load         DOWHILE.C
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 paren-
theses 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.

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.
This will be illustrated later.

Compile and run this program to see if it does what you think it
should do.


THE FOR LOOP
-----------------------------------------------------------------
The for loop is really nothing new, it is simply  ===============
a new way to describe the while loop.  Load and      FORLOOP.C
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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                                      Chapter 3 - Program Control

The second field, in this case containing "index < 6", is the
test which is done at the beginning of each pass through the
loop.  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 when 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. You must use the double equal sign for comparing values.)

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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 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 in the          BREAKCON.C
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 break
always jumps out of the loop just past the terminating brace.

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.  The
continue statement always jumps to the end of the loop just prior
to the terminating brace.  At that time the loop is terminated or
continues based on the result of the loop test.

Be sure to compile and execute 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 needed 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 for that case, 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.  The break
statement here works in much the same manner as the loop, it
jumps out just past the closing brace.

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 of the bottom of the
switch just past the closing brace.  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 but should
be completely understood by the serious C programmer.  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 in       GOTOEX.C
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 each of the three nested 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


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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.  It would be possible to define them on one line, but
to do so would remove the ability to include a comment on each
line.  This is illustrated in the next program.  Be sure to
compile and execute this program.







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                                      Chapter 3 - Program Control

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 by studying this
program to learn what not to do.


OUR FIRST STYLE PROGRAM
-----------------------------------------------------------------
This program does nothing practical except to    ================
illustrate various styles of programming and         STYLE1.C
how to combine some of the constructs            ================
introduced in this chapter.  There is nothing
in this program that we have not studied so far in this tutorial.
The program is heavily commented and should be studied in detail
by the diligent C student to begin learning proper C programming
style.  Like all other example programs, this one can be compiled
and executed, and should be.


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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