Programs not only store
data but also manipulate data in terms of consolidation, rearranging, modifying
data. To perform all these tasks, programs need tools for performing repetitive
actions or for making decisions.
FLOW OF CONTROL
As we know, a C/C++
program starts its execution from the first statement of main() function and
continues till the end. But what if we need to send control back a few
statements or if we want to repeat a sequence of statements a given number of
times? Most of the problems in real life has two outcomes, either a yes (i.e.
true) or a no (i.e. false). How to let our program take a decision?
Well all these problems
are solved using special tools which are called the flow of control
manipulators.
But before we learn
about them, we must learn what is a statement and what is a block of
statements.
Statements
Statements are the
instructions given to the computer to perform any kind of action, be it data
movements, be it making decisions or be it repeating actions. Statements are
the smallest executable unit of a C/C++ program. Statements are terminated with
a semicolon (;). The simplest statement is the empty or NULL statement. It is
simply a semicolon (;). A NULL statement is useful in those instances where the
syntax of the language requires the presence of a statement but the logic of
the program does not (As in loops and their bodies).
A compound statement in
C++ is a sequence of statements enclosed by a pair of curly brackets ({}). For
instance,
{
statement
1;
statement
2;
statement
3; // & so on
}
A compound statement is
called a block. A block is regarded as a single statement and can
be used anywhere in a program where a single statement can be used.
Now we will see in
brief, all the tools in C++ to control the normal flow of execution.
The if statement of C/C++
An if statement
is used to test a particular condition; if that condition is true then only the
control will move to the statement following if, otherwise that
statement is ignored (Here the statement following if may be a single or
a compound statement).
Obviously, any
condition can yield two possible results, either true or false. If the condition
in if part is false then the statement in the else clause gets executed.
The following figure shows a flow chart on how if-else statements
work.
It is important to know
that C/C++ doesn’t make it compulsory to use an else part with if.
The rule is simple. If the condition is true, the immediately following
statement of if gets executed, otherwise the control simply moves to the
next statement. This next statement may be an else or it may be a normal
statement.
Also, if can be
used without else but the converse is not true. Therefore, there cannot
be an else statement without an if. The following codes will make
the concepts clear :
if(condition)
statement
1;
else
statement
2;
Here, if the condition is
true, statement 1 is executed; otherwise statement 2 is executed.
if(condition)
statement
1;
statement
2;
Here, if the condition
is true, statement 1 is executed and then the control moves to statement 2;
otherwise statement 1 is ignored and control directly passes to statement 2.
if(condition)
statement
1;
statement
2;
else
statement
3;
This gives an error. The
control on executing 2 has already come out of if clause. When it encounters
else, it cannot figure out the if part of this else clause and so declares an
error.
Note
:
In
if-else statements, either only the if statement is executed or only the else statement
but never both.
In
all the above examples, statement 1,2 & 3 could be a single statement or a
compound statement or even a NULL statement.
The switch statement of C++
C++ provides a multiple-branch
selection statement known as switch. This selection statement successively
tests the value of an expression against a list of integer or character
constants. When match is found, the statements associated with that constant
are executed.
The syntax for switch
case is :
where exp is any
expression or a variable (only int or char).
The expression is
evaluated and the result is compared sequentially from top to bottom, starting
at case constant 1. The moment the match is found, all the
statement sequences of that case are executed.
Now the importance of break
comes into play. Suppose the exp value matches constant
2 and there is no break statement in that case. Then the
control keeps on executing all the statement sequences of the following cases
until it encounters a break.
The default case is
executed in case none of the constants match exp.
Let us see a few
examples :
int
n; cin>>n;
switch(n)
{
case
1 : cout<< “ONE ”; break;
case
2 : cout<< “TWO”; break;
case
3 : cout<< “THREE”; break;
case
4 : cout<< “FOUR”; break;
default
: cout<< “GREATER THAN FOUR”;
}
Input : 2
Output : TWO
Input : 3
Output : THREE
Input : 6
Output : GREATER THAN FOUR
Now let us see what
happens if we omit break statement.
int
n; cin>>n;
switch(n)
{
case
1 : cout<< “ONE ”;
case
2 : cout<< “TWO ”; break;
case
3 : cout<< “THREE ”;
case
4 : cout<< “FOUR ”;
default
: cout<< “GREATER THAN FOUR”;
}
Input : 1
Output : ONE TWO
Input : 2
Output : TWO
Input : 3
Output :
THREE FOUR GREATER THAN FOUR
Other flow of control
statements in C/C++ are the loops. C/C++ provides 3 looping structures to
execute the same set of statements any desired number of times. These are :
for loop
while loop
do while loop
Click on them to learn
in details about them. (Sorry the link will be restored shortly.)
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