In
practice, we have a number of situations where we may have to change the order
of execution of statements based on certain conditions, or repeat a group of
statements until certain specified conditions are met. This involves a kind of
decision making.
If the label is before the statement goto label; a loop will be formed and some statements will be executed repeatedly. Such a jump is known as a backward jump.
C
language possesses such decision-making capabilities by supporting the
following statements:
Ø if Statement
Ø switch Statement
Ø Conditional
Operator Statement
Ø goto Statement
IF STATEMENT
§ General form:
if (this condition is true)
execute this statement;
§ Syntax:
if (test-expression)
{
statement-block;
}
statement-x;
§ The keyword ‘if’
tells the compiler that what follows is a decision control statement.
§ The condition
following the keyword ‘if’ is always enclosed within a pair of parenthesis.
§ If the condition
is true, then the statement is executed. If the statement is not true, then the
statement is not executed.
§ To express a
condition we use relational operators.
< <= > >= = = !=
§ Flowchart:
IF-ELSE STATEMENT
§ General form:
if (this condition is true)
execute this statement;
else
execute this statement;
§ Syntax:
if (test-expression)
{
true-block statement;
}
else
{
false-block statement;
}
statement-x;
§ If the condition
is true, then the statement(s) immediately following the ‘if’ statement are
executed, otherwise the false-block statements are executed.
§ Flowchart:
NESTED IF-ELSE STATEMENT
§ When a series of
decisions are involved, we may have to use more than one if…else statement in
nested form.
§ General form:
if (condition-1)
{
if (condition-2)
{
stmt-1;
}
else
{
stmt-2;
}
}
else
{
stmt-3;
}
stmt-x;
§ If condition-1
is false, stmt-3 will be executed, otherwise it continues to perform the second
test.
§ If condition-2
is true, stmt-1 will be executed otherwise stmt-2 will be evaluated and then
the control is transferred to stmt-x.
§ Flowchart:
ELSE-IF LADDER
§ A multipath
decision is a chain of if’s in which the statement associated with each else is
an if.
§ General form:
if (condition-1)
stmt-1;
else if (condition-2)
stmt-2;
else if (condition-3)
stmt-3;
else if (condition-4)
stmt-4;
else
default-stmt;
stmt-x;
§ This construct
is known as the else-if ladder. The conditions are evaluated from the top,
downwards.
§ As soon as a
true condition is found, the statement associated with it is executed and the
control is transferred to the stmt-x (skipping the rest of the ladder).
§ When all the
n-conditions become false, then the final else containing the default-stmt will
be executed.
§ Flowchart:
| RULES FOR INDENTATION
When using
control structures, a statement often controls many other statements that
follow it. In such situations it is good practice to use indentation to show that the
indented statements are dependent on the preceding controlling statement.
Some guidelines that could be followed while using indentation are listed
below:
v Indent statements that are dependent on
the previous statements; provide at least three spaces of indentation.
v Align vertically else clause with their
matching if clause.
v Use braces on separate lines to identify
a block of statements.
v Indent the statements in the block by at
least three spaces to the right of the braces.
v Align the opening and closing braces.
v Use appropriate comments to signify the
beginning and end of blocks.
v Indent the nested statements as per the
above rules.
v Code only one clause or statements on
each line.
|
§ When we have to
select one of the many alternatives, we can use an if-statement to control the
selection. However, the complexity of such a program increases as the number of
alternatives increase.
§ C has a built-in
multiway decision statement known as switch.
The switch statement tests the value of a given variable (or expression)
against a list of case values and when a match is found, a block of statements
associated with that case is executed.
§ General form:
switch
(expression)
{
case value-1:
block-1;
break;
case value-2:
block-2;
break;
…..
…..
default:
default-block;
break;
}
next-stmt;
where:
i.
expression is an integer
expression or character.
ii.
value-1, value-2, … are
constants or constant expressions and are known as case labels.
NOTE: Float values
are not accepted.
NOTE: Each of these
values should be unique within a switch statement.
NOTE: Case labels
ends with a colon(:)
iii.
block-1,
block-2,….
Are statement lists and may contain zero or more statements.
iv.
The
break statement at the end of each block signals the end of a particular case
and causes an exit from the switch statement, transferring the control to the
next-stmt following the switch.
§ When the switch
is executed, the value of the expression is successfully compared against the
values value-1, value-2,… If a case is found whose value matches with the value
of the expression, then the block of statements that follow are executed.
§ The default is
an optional case. (It can be placed anywhere but usually is placed at the end).
§ It is permitted
to nest switch statement.
§ Flowchart:
Switch
versus if-else ladder
Switch works faster than an equivalent
if-else ladder.
This
is because, the comp iler generates a jump table for a switch during
compilation. As a result, during the execution it simply refers the jump table
to decide which case should be executed, rather than actually checking which
case is satisfied.
If-else’s
are slower because the conditions in them are evaluated at execution time.
NOTE: In case of questions having
range, if-else is a better option than switch statement.
GOTO STATEMENT
§ The C language
supports the goto statement to branch unconditionally from one point to another
in the program.
§ The goto
statement requires a label in order to identify the place where the branch is
to be made.
A label is any
valid variable name, and must be followed by a colon. The label is placed
immediately before the statement where the control is to be transferred.
§ General form:
§ The label : can
be anywhere in the program either before or after the goto label statement.
§ During the
running of the program when a statement like:
goto
begin;
is met, the flow
of control will jump to the statement immediately following the label begin:.
This happens unconditionally.
NOTE that a goto breaks the normal
sequential execution of the program.
If the label is before the statement goto label; a loop will be formed and some statements will be executed repeatedly. Such a jump is known as a backward jump.
On the other hand, if the label: is
placed after the goto label; some statements will be skipped and the jump is
known as a forward jump.
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