The if-else statement:
The
if-else statement is used to carry out a logical test and then take one of two
possible actions depending on the outcome of the test (i.e. whether the outcome
is true or false).The else position of the if-else statement is optional. Thus,
in its simplest general form, the statement can be written.
If (expression)
statement;
The
expression must be placed in parenthesis, as shown. In this form, the statement
will be executed only if the expression has non-zero value (i.e. true). If the
expression has a value of zero (i.e. expression is false), then the statement
will be ignored. The statement can be either simple or compound. In practice,
it is often compound statement, which may include other control statements.
The
general form of an if statement which include the else clause is
 |
If the
expression has a non-zero value (i.e. if expression is true), then statement 1
will be executed. Otherwise, (i.e.
expression is false), statement 2 will be executed. It is possible to next
if…else statement within one-another, just as we did with loops, there are several
different forms that nested if…else two-layer nesting is : In this situation,
one can complete if…else statement will be executed if expression is
nonzero(true) and another complete if else statement will be executed if
expression is false (zero). It is of course, possible that statement 1,
statement2, statement3 and statement4 will contain another if…else statements.
We would then have multi layer nesting. Some other forms of two layer are:
|
|
If<exp1>
{
statement1;
}
else
{
if<exp2>
{
statement2;
}
}
 |
 |
 |
LOOP CONTROL STRUCTURE
If we
want to perform certain action for no of times or we want to execute same
statement or a group of statement repeatedly then we can use different type of
loop structure available in C. Basically there are 3 types of loop structure
available in C
(1)
While loop
(2)
Do..while
(3)
For loop
While statement:
 |
The while statement is used to carry out looping operations. The general form of the statements
The
enclose statements within two braces will be executed repeatedly as long as the
expression evaluated as true. When the expression is evaluate a false or when
condition will be false then it will come out from the loop and stop the
execution of that loop. Initialization statement initialize some memory variable with some
value. Increment or decrement operator increase or decrease the value of
operator is use by expression. This statement can be simple of compound, though
it is typically a compound statement, it must include some features, which
eventually offers the value of expression, thus provides a stopping condition
for the loop.
The
practice, the included expression is usually a logical expression that is
either true or false. (remember that true corresponds to non-zero value and
false corresponds to zero value). Thus the statement will continue to execute
as long as the logical expression is true.
The part
of while statement, which contains the statement, is called the body of the
loop. Body of the loop may have one or more statements. The braces are needed
only if the body contains two or more statements. However, it is a good
practice to use braces even if the body has only one statement.
Suppose
we want to display the consecutive digits 0,1,2,3…9 with one digit on the each
line. This can be accomplished with the following program.
 |
Initially,
digit is assigned a value of 0. The while loop then displays the current value
of digit, increases its value by 1 and then repeats the cycle, until the value
of the loop will be repeated by 10 times, resulting in 10 consecutive lines of
output. Each line will contain a successive integer value, beginning with and
ending with 9. Thus when the program to run, the following output will be
generated :
0 1 2 3 4 5 6 7 8 9
Do-While statement :
Sometimes,
however, it is desirable to have a loop with the test for continuation at the
end or each pass. This can be accomplished by means of the do-while statement.
 |
The general form of do-while statement is
The
enclosed statement within 2 braces will
be executed repeatedly as the value of expression is not zero. Notice that
statement will always be executed at least once. Since the test for repetition
does not occur until the end of the first pass through the loop. The statement
can be either simple or compound, though most applications will require it to
be a compound statement. It must include some feature which eventually alters
the value of expression so that the looping action can terminate. Since the
expression is evaluated at the bottom of the loop, the do…while loop construct
provides an exit… controlled loop and therefore the body of the loop is always
executed at least once.
In
practice, expression is usually a logical expression, which is either true(with
a non zero value) or a false(with a value of 0). The included statement will be
repeated (i.e. another pass will be made through the loop) if the logical
expression is true.
For most
applications it is more natural to test for continuation of a loop at the
beginning rather than at the end of the loop for this reason, the do…while
statement is use less frequently than while statement. Suppose that we want to
display consecutive 0,1,2,3,…9 with one digit on each line. This can be
accomplished with the following program.
|
|
#include <stdio.h>
main()
{
int digit
= 0;
do
{
printf(“%d”, digit++);
}while(digit<=9);
}
The
digit is initially assigned a value of 0. The do while loop displays the
current value of digit. Increases it value by 1, and then tests to see if the
current value of digit exceeds 0. If so loop terminates; otherwise the loop
continues, using the new value of digit. Note that the last is carried out at
the end of each pass through the loop. The net effect is that the loop repeated
10 times, resulting in 10 successive lines of output. Thus, when
the
program is run the following output is generated :
0 1 2
3 4 5 6 7
8 9
FOR statement :
The for
statement is another entry controller that provides a more concise loop control
structure. The general form of the for loop is :
|
|
For(initialization,
test condition, increment)
{
statement 1;
statement 2;
}
The
execution of the for statement is as follows :
1.
Initialization of the control variables is done first,
using assignments statement such as I=0 and count=0. The variables I and count
are known as loop controls.
2.
The value of the control variables is tested using the
test condition. The test condition is relational expression, such as I>0 or
I<10 that determines when the loop is terminated and the execution continues
with statement that immediately follows the loop.
3.
When the body of the loop is executed, the control is
transferred back to the for statement either evaluating the last statement is
the loop. Now, the control variable is incremented using and assignment
statement such as I=I+1 and if the new value of the control is satisfied under
test condition, then the body of the loop is executed. This process continues
till the value of the control variable fails to satisfy the test-condition.
(a)
Consider the following segment of a program :
 |
Consider the following segment of a program :
This for
loop is executed 10 times and prints the digits 0 to 9 in one line. The three sections within parentheses must be
separated by a semicolons. Note that there is no semicolon at eh end of the
increment section x++.
{
Printf(“%d”,x0
Printf(“/n”);
}
 |
The for statement allows for negative increments. For example, the loop discussed above can be written as follows:
This loop
is also executed 10 times. But the output would be from 9 to 0 instead of 0 to
9. Note that, braces are optional when the body of the loop contains only one
statement.
Additional features of for loop :
The for
loop in C has several capabilities that are not found in the other loop
constructs. For example, more than one variable can be initialized at a time in
the for statement.
{
p=m/n;
printf(“%d%d%d
\n”,n,m,p);
}
 |
This is
perfectly valid. The multiple arguments in the increment section are separated
by commas.
The third
feature is that the test condition may have nay compound relation and the
testing need not be limited only to the loop control variable. Consider the
example below :
 |
The loop
uses a compound test condition with the control variable; and external variable
sum. The loop is executed as long as both the conditions I<20 and sum<100
are true. The sum is evaluated inside the loop.
It is
also permissible to use expressions in the assignment statements of
initialization and increment sections, For example, a statement of the type :
For(z=(m+n)/2;
x<0;x=x/2)
is
perfectly valid.
Another
unique aspect of the for loop is that one or more sections can be omitted, if
necessary.
 |
Consider the following statements :
Both the
initialization and increment sections are omitted in the for statement. The
initialization has been done before the for statement and the control variable
is incremented inside the loop. In such cases the sections are left blank.
However, the semicolons separating the sections must remain. If the test
condition is not present, the for statement sets up an infinite loop. Such
loops can be broken using break or go-to statements in the loop. We can set up
time delay loop using the null statement as follow:
For(j=1000;j>0;j--)
{}
The loop
executed 1000 times without producing any output; it simply causes a time
delay. Notice that the body of the loop contains only a semicolon, known as
null statement. This can also be written as :
For(I=1000;
I>0; I--);
This
implies that C compiler will not give an error message. If we place a semicolon
by mistake at the end of a for statement. The semicolon will be considered as a
null statement and the program may produce some nonsense.
Loops can
be nested (i.e. embedded) one within another. The inner and outer loops need not be generated by the
same type of control structure. It is essential that one loop be completely
embedded within the other. There can be no overlap. Also each loop must be
controlled by a different index.
BREAK STATEMENT:
The break
statement is used to terminate loops or to exit a switch. The break statement
will break or terminate the inner-most loop. It can be used within a while, a
do-while, a for or a switch statement. The break statement is written simply as
break; Without any embedded expressions or statement. For example,
 |
The
output will be 1,2,3,4 and then break will terminate this loop and stop the
execution of the for loop.
CONTINUE STATEMENT
The
continue statement is used to skip or to bypass some step or iteration of
looping structure. It does not terminate the loop but just skip or bypass the
particular sequence of the loop structure. It is simply written as continue;.
 |
The
output of the above program will be 1,2,3,4,6,7,8,9,10. The 5th
iteration of the loop will be skipped as we have define the continue for that
iteration. So it will not print the value ‘5’.
SWITCH-CASE Statement :
The
switch statement causes a particular group of statements to be chosen from
several available groups. The selection is based upon the current value of an
expression that is included within a switch statement. The general form of
switch-case statement is :
 |
Usually
each of these expression will be written as either an integer constant or a
character constant. Each individual statement following the case labels may be
either simple or complex. Where expression1 1, expression 2,………,expression m
represent constant, integer valued expression .When switch statement is executed
the expression is evaluated and control is transferred directly to the group of
statements whose case labels value matches the value of the expression. If none
of the case-label values matches the value of the expression, then non of the
groups within the switch statement w8ll be selected in this case control is
transferred directly to the statement that follows the switch statement.
Example :
 |
Thus RED
will be displayed if choice represents either r or R, WHITE will be display
choice represents either w or W and BLUE will be displayed if choice represents
either b or B. Nothing will be displayed if any other character has been
assigned to choice. Notice that each group of statements has two case labels,
to account for either upper or lower case. Also, note that each of the first
two group ends with the break statement. The break statement causes control to
be transferred out of the switch statement thus preventing more than one group
of statement from being executed.
One of
the labeled groups of statements within the switch statement may be labeled
default. This group will be selected if none of the case labels matches the
value of expression. The default group may appear anywhere within the switch
statement. It need not necessary be placed at the end. If none the case labels
matches the value of the expression and the default group is present, then the
statement will take no action.
 |
Here is a variation of switch statement :
The switch statement now contains a
default group, which generates an error message if none of the case labels
matches the original expression. Each switch statement now contains a default
group, which generates an error message if none of the case labels matches the
original expressions.
Each of
the first three groups of statements now has only one case labels. Multiple
case labels are not necessary in the example, since the library function
toupper causes all incoming characters to be converted to the upper case. Hence
choice will always be assigned an upper case character. In a special sense, the
switch statement may be thought of as an alternative to the use of nested
if-else statements, though it can only replace those if else statements that
tests for equality. In such situations using the switch statement is generally
much more convenient.
Notice
that each group of statement ends with a break statement in order to transfer
control out of the switch statement. The break statement is required within
each of the first three groups in order to prevent the succeeding groups of
statement from executing. This last group does not require a break statement
since control will automatically be transferred out of the switch statement
after the last group has been executed. This last break statement is included,
however has a matter of good programming practice. So that it will be present,
if another group of statement is added later.
THE GOTO STATEMENT :
The goto
statement is used to alter the normal sequence of program execution by
transferring control to some other part of the program. In its general for the
goto statement is written as goto label; Where label is an identifier used to
label the target statement to which control will be transferred.
Control
may be transferred to any other statement within the program. The target statement
must be labeled and the label must be followed by a color. Thus the target
statement will appear as label : statement. Each labeled statement within the
program must have unique label, i.e. no two statement can have same label.
The common application of goto are :
1.
Branching around statement or groups of statements
under conditions.
2.
Jumping to the end of a loop under certain conditions,
thus by passing the reminder of the loop during the loop during the current
pass.
3.
Jumping completely out of a loop under certain
conditions, thus terminating the execution of the loop.
Branching
ground statement can be accomplished with the if-else statement, jumping to the
end of a loop can be carried out with the continue statement, and jumping out
of a loop easily completed using the break statement. The uses of these
structured features are preferable to the used of the goto statement because
the use of goto tends to encourage logic that skips all over the program.
Occasional
situations do arise, however, in which the goto statement can be useful.
Consider, for example, a situation in which it is necessary to jump out of a
doubly nested loop if a certain conditions are detected. This can be
accomplished with two if break statements. One within each loop, though this is
awaked. A batter solution in this particular situation might make user of the
goto statement to transfer out of both loops at once.
0 comments:
Post a Comment