Control structures

Control Structures
In JAVA
-M Vishnuvardhan,
Dept. of Computer Science,
SSBN Degree College, ATP

SSBN Degree College, ATP M Vishnuvardhan
Introduction
Program contains a set of instructions . Normally the computer
executes these instructions in the sequence in which they appear, one
by one. This condition is called sequence accomplishment. In order to
modify this flow control structures are used.
A Control structure is structure which is used to modify the flow of the
program.
Control structures are classified in to two types
1.Branching Structures.
2.Looping Structures.

Branching Structures
Branching structures are take a decision, basing on the
decision it executes a particular block of code only
once.
In java we have
1.2- way branching Structure Eg: if-else
2.Multi-way Branching Eg: switch

2-Way Branching
If-else is a two-way branching structure in java. Here a
decision is made if it is true then a set of statements are
executed other another set of statements are executed
Flow Chart:-
Test
Condition
True Block False Block
Next
Statement

Branching Structures
Syntax: if(test condition)
{
//True block
}
else
{
//else block
}
Test condition must always be a relation expression in
java (unlike C where it can be a numeral quantity)

Various forms of if-else
1. Simple if
2. Nested if
3. Else if ladder
Else-if ladder is also called as multi-way branching
structure.

Multi Way Branching
Switch is a multi-way branching structure in java.
Unlike if-else a switch statement doesn't have condition
instead it allows a variable to be tested for equality
against a list of values. Each value is called a case, and
the variable being switched on is checked for each case.

Multi Way Branching

Multi Way Branching
Syntax:
switch(expr)
{
case value1: block1 break;
case value2: block2 break;
case value n: blockn break;
default: default block1 break;
}
next statement;
expr must always yield integer/ char quantity only
from jdk1.5.0 Strings are also supported in switch

Rules for switch
1. The variable used in a switch statement can only be integers, convertible
integers (byte, short, char), strings and enums.
2. You can have any number of case statements within a switch. Each case is
followed by the value to be compared to and a colon.
3. The value for a case must be the same data type as the variable in the
switch and it must be a constant or a literal.
4. When the variable being switched on is equal to a case, the statements
following that case will execute until a break statement is reached.
5. When a break statement is reached, the switch terminates, and the flow of
control jumps to the next line following the switch statement.
6. Not every case needs to contain a break. If no break appears, the flow of
control will fall through to subsequent cases until a break is reached.
7. A switch statement can have an optional default case, which must appear
at the end of the switch. The default case can be used for performing a
task when none of the cases is true. No break is needed in the default case.

Looping Structures
There may be a situation when you need to execute a block of
code several number of times. A loop statement allows us to
execute a statement or group of statements multiple times.
Looping Structures are classified in to two types
1.Entry Control loop
Here test condition is checked before entering the loop body
Eg:- while, for
1.Exit Control loop
Here test condition is checked after executing the loop body
Eg:- do-while

Entry Control vs Exit Control
Body of Loop
Condition
Next Statement
True
False
Body of Loop
Condition
Next Statement
False
True

Looping Structures
When writing looping structures programmer should take really
care because they may tend to infinite loops.
The following points to be remembered while writing loops
1.Create and initialize a loop control variable
2.Form a condition using the loop control variable
3.Increment or decrement the loop control variable

While Loop
Syntax:
while(textCondition)
{
=====
===== //body of loop
=====
}
Eg: int c=1;
while(c<=5)
{
System.out.println(“Java “);
c=c+1;
}

for Loop
Syntax:
for(initialization; condition; increment)
{
=====
=====
}
Eg: for(int c=1;c<=5;c++)
{
}

Nested for loops
If a for loop is placed inside another for loop then it s called as nested
for loops
Eg:
for(int i = 1; i <= 5; ++i)
{
for(int j = 1; j <= 5; ++j)
{
System.out.print(“Java”);
}
System.out.println(“Program");
}
For every iteration of outter for loop the inner for loop runs m times
i.e., for n iterations of outer loop inner loops works for nxm times

Labelled loops
Java allows to keep labels for the loops. A label is a valid identifier
follwed by colon. Labelled loops are used for exiting multiple loops at
once with the help of break or continue
Syntax labelName:
Eg:
outter:
for(int i = 1; i <= 5; ++i)
{
inner:
for(int j = 1; j <= 5; ++j)
{
System.out.print(“Java”);
}
System.out.println(“Program");
}

Enhanced for Loop (for each loop)
Enhanced for loop is generally used to process arrays collections.
It is introduced in JDK 1.5.0
Features:
It starts with the keyword for like a normal for-loop.
Instead of declaring and initializing a loop counter variable,
you declare a variable that is the same type as the base type of
the array, followed by a colon, which is then followed by the
array name.
In the loop body, you can use the loop variable you created
rather than using an indexed array element.
It’s commonly used to iterate over an array or a Collections
class (eg, ArrayList)

Enhanced for Loop (for each loop)
Syntax:
for(datatype var : arrayName)
{
=====
=====
}
Eg: int a[]={10,20,30,40,50};
for(int element : a)
{
System.out.println(element);
}
int a[]={10,20,30,40,50};
for(int i=0;i<5;i++))
{
System.out.println(a[i]);
}
(OR)

Do While Loop
Syntax:
do
{
=====
=====
} while(textCondition);
Eg: int c=1;
do
{
c=c+1;
} while(c<=5);

Control Statements
Java provides 3 control statements
1. break 2. continue 3. return
Break statement is generally used to break loop or switch
statement. It breaks the current flow of the program at specified
condition. In case of nested loops it breaks only the loop in which
it is placed.
Syntax : break; public class BreakExample
{
public static void main(String[] args)
{
for(int i=1;i<=10;i++)
{
if(i==5)
break;
System.out.println(i);
}
}
}

Control Statements
public class BreakExample2
{
public static void main(String[] args)
{
for(int i=1;i<=3;i++){
for(int j=1;j<=3;j++){
if(i==2&&j==2){
break;
}
System.out.println(i+" "+j);
}
}
}

Questions

Control structures

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