HNDE Lecture note..Labuduwa Galle.. upload by sika

1. Information Technology
III
Lecture 02 - Control Structures

2.  It is used to execute an instruction or block of instructions only if
a condition is fulfilled
if (condition) statement
 If this condition is true, statement is executed
 If it is false, statement is ignored (not executed)
 the program continues next instruction after the conditional
structure
 Example
if (x == 100)
cout << "x is 100" ;
Conditional structure: if and else
if (x == 100)
{ // block of inst
cout << "x is " ;
cout << x;
}

3. If – else Structure
 if (condition) statement1 else statement2
 Example 1:
if (x == 100)
cout << "x is 100" ; // true
else
cout << "x is not 100" ; // false
 Example 2
if (x > 0) // outer if
cout << "x is positive" ;
else if (x < 0) // inner if ( false of outer if)
cout << "x is negative" ;
else cout << "x is 0" ; // false of inner if

4. The selective Structure: switch
 Its objective is to check several possible constant values for an
expression
 Its form is the following:
switch (expression) {
case constant1:
block of instructions 1
break;
case constant2:
block of instructions 2
break;
. . .
default: default block of instructions
}

5. Example
 switch example
switch (x) {
case 1:
cout << "x is 1";
break;
case 2:
cout << "x is 2";
break;
default:
cout << "value of x
unknown";
}
 if-else equivalent
if (x == 1)
{
cout << "x is 1";
}
else if (x == 2)
{
cout << "x is 2";
}
else
{
cout << "value of x unknown";
}

6. Iteration structures (loops)
 Loops have as purpose to repeat a statement a certain number
of times or while a condition is fulfilled
1. while loop
while (expression) statement
 its functionality is simply to repeat statement while the
condition set in expression is true.

7. Example
#include <iostream>
using namespace std;
int main ()
{
int n;
cout << "Enter the starting number > " ;
cin >> n;
while (n>0) // LOOP STARTS
{
cout << n << ", " ;
--n;
} // LOOP ENDS
return 0;
}

8. 2. do-while loop
 Format
do statement/s while (condition);
 Its functionality is repeat the statements
 condition in the do-while is evaluated after the execution of
statement/s instead of before
 granting at least one execution of statement even if condition is
never fulfilled.

9. Example
#include <iostream.h>
Using namespace std;
int main ()
{
unsigned long n;
do // LOOP START
{
cout << "Enter number (0 to end): ";
cin >> n;
cout << "You entered: " << n << "n";
} while (n != 0); // LOOP ENDS
return 0;
}

10. 3. For loop
 Its format is:
 for (initialization; condition; increase) statement;
 Its main function is to repeat statement while condition remains
true
 For provides places to specify an initialization instruction and an
increase instruction
 So this loop is specially designed to perform a repetitive action
with a counter.

11. Example
#include <iostream.h>
Using namespace std;
int main ()
{
for (int n=10; n>0; n--)
{
cout << n << ", ";
}
cout << “LOOP ENDS !";
return 0;
}
initialization; condition; increase

12. The break instruction
 Using break we can leave a loop even if the condition for its end is not fulfilled
 It can be used to end an infinite loop, or to force it to end before its natural end
 Example
for (n=10; n>0; n--)
{
cout << n << ", ";
if (n==3)
{
cout << "countdown aborted!";
break;
}
}

13. The continue instruction
 Continue instruction causes the program to skip the rest of
the loop in the present iteration
for (int n=10; n>0; n--)
{
if (n==5) continue;
cout << n << ", ";
}
cout << “it continues!";
return 0;
}