Information Technology III

Lecture 02 - Control Structures

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

instcout << "x is

" ; cout << x;

}

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

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

}

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 equivalentif (x == 1)

{

cout << "x is 1";

}

else if (x == 2)

{

cout << "x is 2";

}

else

{

cout << "value of x unknown"; }

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.

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 ENDSreturn 0;

}

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.

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 ENDSreturn 0;

}

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.

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

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

Examplefor (n=10; n>0; n--)

{ cout << n << ", "; if (n==3) {

cout << "countdown aborted!"; break;

} }

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;

}