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CHAP3.TXT
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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 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, 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.
THE DO-WHILE LOOP
A variation of the while loop is illustrated in the
program DOWHILE.C, which you should load and display. This
program is nearly identical to the last one except that the
loop begins with the reserved word "do", followed by a
compound statement in braces, then the reserved word
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Chapter 3 - Program Control
"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 for 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. Load and edit the
file named FORLOOP.C for an example of a program with a
"for" loop. The "for" loop consists of the reserved word
"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.)
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
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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.
THE IF STATEMENT
Load and display the file IFELSE.C for an example of
our first conditional branching 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.)
NOW FOR THE IF-ELSE
The second "if" is similar to the first with the
addition of a new reserved word, 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 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,
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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 the program to see if it does what
you expect.
THE SWITCH STATEMENT
Load and display the file SWITCH.C for an example of
the biggest construct yet in the 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 entered followed by the value of the variable,
then a colon, and the statements to be executed.
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.
Compile and run SWITCH.C to see if it does what you
expect it to after this discussion.
Load and display the file GOTOEX.C for an example of a
file with some "goto" statements in it. To use a "goto"
statement, you simply use the reserved word "goto" followed
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by the symbolic name to which you wish to jump. The name is
then placed anywhere in the program followed by a colon.
You are not allowed to jump into any loop, but 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 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 17 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.
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.
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
Calculater 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 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
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that all of the contents of the compound statement are
indented 3 spaces to the right of the "for" reserved word,
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 you load and display
DUMBCONV.C you will have an example of poor formatting which
is much closer to what you will actually 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.
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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