if - else & if - elseif - elseif - ….. else & nested if structures

A more complex form of the selection structures is given by if - else .

Look at the following diagram:

Sequence of statement execution: Block #1 will be executed The condition of the if statement will be checked If the condition is true then block#2 will be executed and then block#4 If the condition is false then block#3 and then block#4 will be executed

Even more complex structures can be obtained by the use of if - elseif - elseif - ….. else

See the following diagram

Sequence of statement execution: Block#0 will be executed Condition#1 of the if statement will be checked If condition#1 is true then block#1 will be executed and then we will jump to block#5 If condition#1 is false then condition#2 will be checked If condition#2 is true then block#2 will be executed and then we will jump again to block#5 If condition#2 is false then condition#3 will be checked And so..on and so..on If all conditions are false then block#4 will be executed and then block#5

An interactive example: enter a value for x in the box and watch the output of the following segmant of a program:

( the blue shade indicates the active section of the structure)

x =

The code segmant:

The output:

a = 0.0;

b = 0.0;

cout << “\n\n all variables are initialized to zero;

if ( x <= 0.0 )

{

a = 1.0;

b = 1.1;

cout << “ \n\n you entered a negative x “;

}

else if ( x > 0.0 && x <= 1.0 )

{

a = 2.0;

b = 2.2;

cout << “ \n\n you are in the range (0.0,1.0]”;

}

else if ( x > 1.0 && x <= 10.0 )

{

a = 3.0;

b = 3.3;

cout << “ \n\n one to ten is wide range “ ;

}

else

{

a = 100.0;

b = 100.0;

cout << “ \n\n unrealistic value for x “;

}

cout << “ \n\n a = “ << a << “ and b= “ << b;

Even more complex if structures: Nested ifs

Since each block of instructions in an if structure can be any type of C++ code, it is possible to have other if structures as part of some of the blocks of the original if structure. The following diagram shows such a nested if structure:

© 2001 J.Y. Kazakia. All rights reserved.