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Flow of Control | C++ - C++ Selection statements | 11th Computer Science : Chapter 10 : Flow of Control

Chapter: 11th Computer Science : Chapter 10 : Flow of Control

C++ Selection statements

In a program a decision causes a one time jump to a different part of a program.

Selection statements

 

In a program a decision causes a one time jump to a different part of a program. Decisions in C++ are made in several ways, most importantly with if .. else … statement which chooses between two alternatives. Another decision statement , switch creates branches for multiple alternatives sections of code, depending on the value of a single variable.

 

if statement

 

The if statement evaluates a condition, if the condition is true then a true-block (a statement or set of statements) is executed, otherwise the true-block is skipped.The general syntax of the if statement is:

if (expression)

true-block;

statement-x;

In the above syntax, if is a keyword that should contain expression or condition which is enclosed within parentheses. If the expression is true (nonzero) then the true-block is executed and followed by statement-x are also executed, otherwise, the control passes to statement-x. The true-block may consists of a single statement, a compound statement or empty statement. The control flow of if statement and the corresponding flow chart is shown below.


Illustration 10.1 C++ program to calculate total expenses using if statement

#include<iostream>

using namespace std;

int main()

{

      int qty, dis=0;

      float rate, tot;

      cout<<"\nEnter the quantity ";

     cin>>qty;

      cout<<"\nEnter the rate ";

      cin>>rate;

      if ( qty> 500)

      dis=10;

      tot = (qty * rate) - ( qty * rate * dis / 100);

      cout<<"The total expenses is "<< tot;

      return 0;

}

 

Output

First Run

Enter the quantity 550

Enter the rate 10

The total expenses is 4950

Second Run

Enter the quantity 450

Enter the rate 10

The total expenses is 4500

In the first execution of the program, the test condition evaluates to true, since qty>

Therefore, the variable dis which is initialized to 0 at the time of declaration, now gets a new value 10. The total expenses is calculated using a new dis value.

In the second execution of the program, the test condition evaluates to false, since qty> 500. Thus, the variable dis which is initialized to 0 at the time of declaration, remains 0. Hence, the expression after the minus sign evaluates to 0. So, the total expenses is calculated without discount.

 

Illustration 10.2 C++ program to check whether a person is eligible to vote using if statement

#include <iostream>

using namespace std;

int main()

{

      int age;

      cout<< "\n Enter your age: ";

      cin>> age; if(age>=18)

      cout<< "\n You are eligible for voting ....";

      cout<< "This statement is always executed.";

      return 0;

}


 

Output

Enter your age: 23

You are eligible for voting….

This statement is always executed.

 

Illustration 10.3 C++ program to calculate bonus using if statement

#include<iostream>

using namespace std;

int main()

{

int bonus,yr_of_ser;

cout<<"\nEnter your year of service ";

cin>>yr_of_ser;

      if ( yr_of_ser> 3 )

      {

      bonus=2000;

      cout<<"\n Your bonus is " <<bonus;

      }

cout<<"\nCongratulations...";

return 0;

}

Output

Enter your year of service 5

Your bonus is 2000

Congratulations...

 

if-else statement

 

In the above examples of if, you have seen so for allow you to execute a set of statement is a condition evaluates to true. What if there is another course of action to be followed if the condition evaluates to false. There is another form of if that allows for this kind of either or condition by providing an else clause. The syntax of the if-else statement is given below:

if ( expression)

{

      True-block;

}

else

{

      False-block;

}

Statement-x

In if-else statement, first the expression or condition is evaluated either true of false. If the result is true, then the statements inside true-block is executed and false-block is skipped. If the result is false, then the statement inside the false-block is executed i.e., the true-block is skipped.


 

Illustration 10.4 C++ program to find whether the given number is even number or odd number using if-else statement

#include <iostream>

using namespace std;

int main()

{

      int num, rem;

      cout<< "\n Enter a number: ";

      cin>>num;

      rem = num % 2;

      if (rem==0)

           cout<< "\n The given number" <<num<< " is Even";

      else

           cout<< "\n The given number "<<num<< " is Odd";

      return 0;

}

Output

Enter number: 10

The given number 10 is Even

In the above program, the remainder of the given number is stored in rem. If the value of rem is zero, the given number is inferred as an even number otherwise, it is inferred as on odd number.


Nested if

 

An if statement contains another if statement is called nested if. The nested can have one of the following three forms.

1. If nested inside if part

2. If nested inside else part

3. If nested inside both if part and else part

The syntax of the nested if:


If nested inside if part

if (expression-1)

{

      if (expression)

      {

           True_Part_Statements;

      }

      else

      {

           False_Part_Statements;

      }

}

else

      body of else part;

 

If nested inside else part

if (expression-1)

{

      body of else part;

}

else

{

      if (expression)

      {

           True_Part_Statements;

      }

      else

      {

           False_Part_Statements;

      }

}

If nested inside both if part and else part

if (expression)

{

      if (expression)

      {

           True_Part_Statements;

      }

      else

      {

           False_Part_Statements;

      }

}

else

{

      if (expression)

      {

      True_Part_Statements;

      }

      else

      {

      False_Part_Statements;

      }

}

In the first syntax of the nested if mentioned above the expression-1 is evaluated and the expression result is false then control passes to statement-m. Otherwise, expression-2 is evaluated,if the condition is true, then Nested-True-block is executed, next statement-n is also executed. Otherwise Nested-False-Block, statement-n and statement-m are executed.

The working procedure of the above said if..else structures are given as flowchart below:



 

Illustration 10.5 – C++ program to calculate commission according to grade using nested if statement

#include <iostream>

using namespace std;

int main()

{

      int sales, commission;

      char grade;

      cout << "\n Enter Sales amount: ";

      cin >> sales;

      cout << "\n Enter Grade: ";

      cin >> grade;

      if (sales > 5000)

      {

           commission = sales * 0.10;

           cout << "\n Commission: " << commission;

      }

      else

      {

           commission = sales * 0.05;

           cout << "\n Commission: " << commission;

      }

      cout << "\n Good Job ..... ";

      return 0;

}

Output:

Enter Sales amount: 6000

Enter Grade: A

Commission: 600

Good Job .....


if -else-if ladder

 

The if-else ladder is a multi-path decision making statement. In this type of statement 'if' is followed by one or more else if statements and finally end with an else statement.

The syntax of if-else ladder:

if (expression 1)

{

      Statemet-1

}

else

      if( expression 2)

      {

           Statemet-2

      }

      else

           if ( expression 3)

           {

           Statemet-3

           }

           else

           {

                   Statement-4

           }

When the respective expression becomes true, the statement associated with block is executed, and the rest of the ladder is bypassed. If none of the conditions is true, then the final else statement will be executed.


 

Illustration 10.6 C++ program to find your grade using if-else ladder.

#include <iostream>

using namespace std;

int main ()

{

int marks;

cout<<" Enter the Marks :";

cin>>marks;

if( marks >= 60 )

      cout<< "Your grade is 1st class !!" <<endl;

           else if( marks >= 50 && marks < 60)

                   cout<< "your grade is 2nd class !!" <<endl;

                             else if( marks >= 40 && marks < 50)

                                      cout<< "your grade is 3rd class !!" <<endl;

else

      cout<< "You are fail !!" <<endl;

return 0;

}

Output

Enter the Marks :60

Your grade is 1st class !!

When the marks are greater than or equal to 60, the message "Your grade is 1st class !!" is displayed and the rest of the ladder is bypassed. When the marks are between 50 and 59, the message "Your grade is 2nd class !!" is displayed, and the other ladder is bypassed. When the mark between 40 to 49, the message "Your grade is 3nd class !!" is displayed, otherwise, the message "You are fail !!" is displayed.

 

The ?: Alternative to if- else

 

The conditional operator (or Ternary operator) is an alternative for ‘if else statement’. The conditional operator that consists of two symbols (?:). It takes three arguments. The control flow of conditional operator is shown below

 

The syntax of the conditional operator is:

expression 1? expression 2 : expression 3


In the above syntax, the expression 1 is a condition which is evaluated, if the condition is true (Non-zero), then the control is transferred to expression 2, otherwise, the control passes to expression 3.

 

Illustration 10.7 – C++ program to find greatest of two numbers using conditional operator

#include <iostream>

using namespace std;

int main()

{

      int a, b, largest;

      cout << "\n Enter any two numbers: ";

      cin >> a >> b;

      largest = (a>b)? a : b;

      cout << "\n Largest number : " << largest;

      return 0;

}

Output:

Enter any two numbers: 12 98

Largest number : 98

 

Switch statement

 

The switch statement is a multi-way branch statement. It provides an easy way to dispatch execution to different parts of code based on the value of the expression. The switch statement replaces multiple if-else sequence.

The syntax of the switch statement is;

switch(expression)

{

      case constant 1:

           statement(s);

           break;

      case constant 2:

           statement(s);

           break;

      .

      .

      .

      .

      default:

           statement(s);

}

In the above syntax, the expression is evaluated and if its value matches against the constant value specified in one of the case statements, that respective set of statementsare executed. Otherwise, the statements under the default option are executed. The workflow of switch statement and flow chart are shown below.


Rules:

1. The expression provided in the switch should result in a constant value otherwise it would not be valid.

2. Duplicate case values are not allowed.

3. The default statement is optional.

4. The break statement is used inside the switch to terminate a statement sequence. When a break statement is reached, the switch terminates, and the flow of control jumps to the next line following the switch statement.

5. The break statement is optional. If omitted, execution will continue on into the next case. The flow of control will fall through to subsequent cases until a break is reached.

6. Nesting of switch statements is also allowed.

 

Illustration 10.8 – C++ program to demonstrate switch statement

#include <iostream>

using namespace std;

int main()

{

      int num;

      cout << "\n Enter week day number: ";

      cin >> num;

      switch (num)

      {

           case 1 : cout << "\n Sunday"; break;

           case 2 : cout << "\n Monday"; break;

           case 3 : cout << "\n Tuesday"; break;

           case 4 : cout << "\n Wednessday"; break;

           case 5 : cout << "\n Thursday"; break;

           case 6 : cout << "\n Friday"; break;

           case 7 : cout << "\n Saturday"; break;

           default: cout << "\n Wrong input....";

      }

}

Output:

Enter week day number: 6

Friday

 

Illustration 10.9 – C++ program to demonstrate switch statement

#include <iostream>

using namespace std;

int main()

{

      char grade;

      cout << "\n Enter Grade: ";

      cin >> grade;

switch(grade)

      {

      case 'A' : cout << "\n Excellent...";

      break;

      case 'B' :

      case 'C' : cout << "\n Welldone ...";

      break;

      case 'D' : cout << "\n You passed ...";

      break;

      case 'E' : cout << "\n Better try again ...";

      break;

      default : cout << "\n Invalid Grade ...";

}

cout << "\n Your grade is " << grade;

return 0;

}

Output:

Enter Grade: C

Welldone ...

Your grade is C

 

Switch vs if-else

 

“if-else” and “switch” both are selection statements. The selection statements, transfer the flow of the program to the particular block of statements based upon whether the condition is “true” or “false”. However, there are some differences in their operations. These are given below:

 

Key Differences Between if-else and switch

 

1. Expression inside if statement decide whether to execute the statements inside if block or under else block. On the other hand, expression inside switch statement decide which case to execute.

2. An if-else statement uses multiple statements for multiple choices. On other hand, switch statement uses single expression for multiple choices.

3. If-esle statement checks for equality as well as for logical expression. On the other hand, switch checks only for equality.

4. The if statement evaluates integer, character, pointer or floating-point type or Boolean type. On the other hand, switch statement evaluates only character or a integer data type.

5. Sequence of execution is like either statement under if block will execute or statements under else block statement will execute. On the other hand the expression in switch statement decide which case to execute and if do not apply a break statement after each case it will execute till the end of switch statement.

6. If expression inside if turn out to be false, statement inside else block will be executed. If expression inside switch statement turn out to be false then default statements are executed.

7. It is difficult to edit if-else statements as it is tedious to trace where the correction is required. On the other hand, it is easy to edit switch statements as they are easy to trace.

The if statement is more flexible than switch statement.


Some important things to know about switch

There are some important things to know about switch statement. They are

1. A switch statement can only work for quality of comparisons.

2. No two case labels in the same switch can have identical values.

3. If character constants are used in the switch statement, they are automatically converted to their equivalent ASCII codes.

4. The switch statement is more efficient choice than if in a situation that supports the nature of the switch operation.

Tips: The switch statement is more efficient than if-else statement.

 

Nested switch

 

When a switch is a part of the statement sequence of another switch, then it is called as nested switch statement. The inner switch and the outer switch constant may or may not be the same.

The syntax of the nested switch statement is;

switch (expression)

{

      case constant 1:

      statement(s);

      break;

           switch(expression)

           {

           case constant 1:

                   statement(s);

                             break;

           case constant 2:

                   statement(s);

                   break;

                   .

                   .

                   .

           default :

           statement(s);

      }

case constant 2:

statement(s);

break;

.

.

.

default :

statement(s);

}

The below program illustrates nested switch statement example. The outer switch checks for zero or non-zero and the inner switch checks for odd or even.

Illustration 10.10 C++ program to check for zero or non-zero and odd or even using  nested switch statement

#include <iostream>

using namespace std;

int main()

{

int a = 8;

cout<<"The Number is : " <<a <<endl;

switch (a)

      {

      case 0 :

      cout<<"The number is zero" <<endl;

      break;

      default:

      cout<<"The number is a non-zero integer" <<endl;

      int b = a % 2;

      switch (b)

      {

      case 0:

      cout<<"The number is even" <<endl;

      break;

      case 1:

      cout<<"The number is odd" <<endl;

      break;

      }

      }

return 0;

}

Output

The Number is : 8

The number is a non-zero integer

The number is even


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