C Programming: Decision Making, Switch Statements

DECISION MAKING, SWITCH STATEMENTS

 

1. Conditional and Control statements

2. The if statement

3. The if‒else statement

4. Nested if....else statement

5. The if....else Ladder

6. The switch statement

1. CONDITIONAL AND CONTROL STATEMENTS

A piece of data is called logical, if it conveys the idea of true or false. In real life, logical data are created in answer to a question that needs a yes or no answer.

The basic decision statements in computer are selection structure. The decision is described to computer as a conditional statement that can be answered True or False.

In a program all the instructions are executed sequentially by default, when no repetition of some calculations is necessary. In some situations we may have to change the execution order of statements based on condition or to repeat a set of statements until certain conditions are met. In such situations Conditional and Control statements are very useful.

'C' language provides four general categories of control structures.

i) Sequential structure, in which instructions are executed in sequence.

Example:

i=i+1;

j= j+1;

The above statements are executed one by one.

ii) Selection structure, here the sequence of the instructions are determined by using the result of the condition.

Example:

if(x>y)

    i=i+1;

else

    j = j+1;

If the condition is true, then the statement i=i+1 will be executed otherwise it executes j=j+1;

iii) Iteration structure, in which statements are repeatedly executed. These forms program loops.

Example

for(i=1; i<=5; i++)

{

       i = i+1;

}

Where the statement i=i+1 will be executed 5 times and value of i will change from 1,2,3,4 and 5.

iv) Encapsulation structure, in which the other compound structures are included.

Example : We can include an if statement in a for loop or a for in a if statement.

'C' language provides all the standard control structure that are available in programming languages. These structures are capable of processing any information.

'C' language provides the following conditional (decision making) statements.

if statement

if....else statement

nested if....else statement

if....else Ladder

2. The if statement

The if statement is a decision making statement. It is used to control the flow of execution of the statements and also used to test logically whether the condition is true or false. It is always used in conjunction with condition. This statement is used when a question requires answer.

Syntax:

if(condition is true)

{

       True statements;

}

If the condition is true, then the True statements are executed. The "True statements' may be a single statement or group of statements. If the condition is false then the true statements are not executed, instead the program skip past it. The condition is given by the relational operator like ==, !=, <=, >=, etc.

Example 1: Program to illustrates the use of if statement.

/* Program to check whether the number is less than 25*/

#include <stdio.h>

main()

{

          int i;/* Local definition */

          /* Statement */

          printf("\nEnter the number < 10....");

          scanf("%d", &i);

          if(i<10)

                    printf("\nThe entered number %d is < 10", i);

} /* main */

OUTPUT

Enter the number < 10....5

The entered number 5 is <10

EXPLANATION: Get the value for variable i from input and check if it is lesser than 10 and print message.

If you want to execute multiple statements in the if statement, that statements must be blocked with in braces.

Example 2: Program to interchange values between two variables.

#include <stdio.h>

#include <conio.h>

void main()

{

        int m,n,a; /* Local definitions */

        printf("Enter two numbers: ");

        scanf("%d%d",&m,&n);

        if(m>n) /* if statement checks the condition */

        {

                a=m;

                m=n;

                n=a;

        } /* if */

        printf("The interchanged values are: %d %d",m,n);

} /* main */

OUTPUT

Enter two numbers : 34 28

The interchanged values are: 28 34

EXPLANATION: The program reads values for m and n. If statement checks the condition m>n or not. When condition is true the statement block interchanges the values of two variables using third variable and prints the values.

 

3. The if‒else statement

It is basically two way decision making statement and always used in conjunction with condition. It is used to control the flow of execution and also used to carry out the logical test and then pickup one of the two possible actions depending on the logical test.

It is used to execute some statements when the condition is true and execute some other statements, when the condition is false.

Syntax:

if(condition)

{

      True statements;

}

else

{

         False statements;

}

If there is only one statement in the if block (or) else block, then the braces are optional. But if there is more than one statement, the braces are compulsory.

Example 1: Program to determine the given number is even or odd ?

/* Program to determine given number is even or odd */

#include <stdio.h>

#include <conio.h>

void main()

{

         int num,rem; /* Local definitions */

         printf("Enter your number: ");

         scanf("%i", &num);

         rem=num % 2;

         if(rem==0) /* if statement checks the condition */

         printf("The entered number is even. ");

         else

         printf("The entered number is odd.");

} /* main */

OUTPUT 1

Enter your number: 80

The entered number is even.

OUTPUT 2

Enter your number: 59

The entered number is odd.

EXPLANATION: The program reads the integer variable num through scanf() function. The variable num is modular divided by 2 then assigned remainder value to rem. If statement checks the condition rem=0, when condition is true the program prints even number otherwise prints odd number.

Example 2: Program to find biggest among two numbers

 /* To find biggest among two numbers */

#include <stdio.h>

#include <conio.h>

void main()

{

        int i,j,big; /* Local definitions */

        printf("Enter two values: ");

        scanf("%d %d", &i, &j);

        big = i;

        if(big <j)

        {

                big = j;

        } /* if */

        printf("biggest of two numbers is %d\n", big);

        if(i < j)

        {

                big = j;

        }

        else

        {

                big =I;

        } /* if‒else */

        printf("biggest of two numbers(using else) is %d\n", big);

        getch();

} /* main */

OUTPUT:

Enter two values: 45 78

biggest of two numbers is 78

biggest of two numbers(using else) is 78

EXPLANATION: This program is used to find the biggest in two numbers which are taken from input using if..else statement.

 

4. Nested if....else statement

When a series of if..else statements occur in a program, we can write an entire if..else statement in another if..else statement called nesting, and the statement is called nested if.

Syntax:

if(condition 1)

      if(condition 2)

     {

          True statement2;

     }

    else

    {

          False statement2;

     }

else

{

       False statement 1;

}

 

5. The if....else Ladder

Nested if statements can become quite complex. If there are more than three alternatives and indentation is not consistent, it may be different for you to determine the logical structure of the if statement. In situations, you can use the nested if as the else if ladder.

Syntax:

if (condition 1)

{

       Statement 1;

}

else if (condition 2)

{

       Statement 2;

}

else if (condition 3)

{

        Statement 3;

}

else

{

        default‒statements;

}

Example 1: Program to demonstrate nested if..else statement.

/* Program using nested if..else statements */

#include <stdio.h>

main()

{

       int n; /* Local definition */

       printf("\nEnter a number...");

       scanf("%d", &n);

       if(n= =15)

              printf("\nPlay Foot ball");

       else

       {

                if(n= =10)

                     printf("Play Cricket");

              else

                     printf("don't play");

       } /* if*/

} /* main*/

OUTPUT

Enter a number...10

Play Cricket

EXPLANATION: Read a number n from input, If the entered value is equal to 15 then print "Play football" else once again check If the value of n is 10, then print the statement "Play Cricket" else print "don't play".

If you want to test more than one condition in if statement, the logical operator are used as specified below. These are used to combine the results of two or more conditions.

Example 2: Program to display the types of character using if‒els.

/* Program to display the types of character */

#include <stdio.h>

#include <conio.h>

void main()

{

     char chr; /* Local definitions */

     /* Statements */

     printf("Enter a single character: ");

     scanf("%c", &chr);

     /* nested if statement is used to checks the conditios */

     if((chr>='a' && chr<='z') || (chr >= 'A' && chr <= 'Z'))

          printf("Entered character is an alphabetic. \n");

     else

          if(chr>= '0' && chr <= '9')

               printf("Entered character is an digit. \n");

          else

               printf("Entered character is an special character. \n");

} /* main"/

OUTPUT 1:

Enter a single character: M

Entered character is an alphabetic.

OUTPUT 2:

Enter a single character: 5

Entered character is an digit.

OUTPUT 3:

Enter a single character : &

Entered character is an special character.

EXPLANATION: The above program reads the character from the keyboard. The character chr is tested while the given character is alphabet, or digits or special characters or not using nested if‒else statement according to the condition the result is printed.

 

6. THE SWITCH STATEMENT

The switch statement is used to pickup or execute a particular group of statements from several available group of statements. It allows us to make a decision from the number of choices.

It is a multiway decision statement, it tests the value of given variable or expression against a list of case values and when a match is found, a block of statements associated with that case is executed.

Syntax:

switch(expression)

{

      case constant 1:

            block1;

            break;

      case constant 2:

            block2;

            break;

            .

            .

            default:

                  default block;

                  break;

}

The expression following the keyword switch is any 'C' expression that must yield an integer value. It must be an integer constant like 1,2,3 or an expression that evaluates to an integer. The keyword case is followed by an integer or a character constant, each constant in each case must be different from all the other.

First the integer expression following the keyword switch is evaluated. The value it gives is searched against the constant values that follow the case statements. When a match is found, the program executes the statements following the case. If no match is found with any of the case statements, then the statements following the default are executed.

1. Rules for writing switch() statement

a) The expression in switch statement must be an integer value or a character constant.

b) No real numbers are used in an expression.

c) Each case block and default blocks must be terminated with break statements.

d) The default is optional and can be placed anywhere, but usually placed at end.

f) The case keyword must terminate with colon (:).

f) No two case constants are identical.

g) The case labels must be constants.

h) The switch can be nested.

i) The value of switch is compared with the case constant expression in the order specified, i.e., from top to bottom.

j) In the absence of break statement, all statements that are followed by matched cases are executed.

Example 1: Program to print the given number is odd/even using switch case statement.

/* Prints the given number is odd/even */

#include<stdio.h>

#include <conio.h>

void main()

{

       int i,n; /* Local definitions */

       /*Statements*/

       clrscr();

       printf("Enter a Number: ");

       scanf("%d", &n);

       for(i=1; i<=n; i= i+1)

       {

              switch(i%2)

              {

              case 0:

                     printf("The number %d is even \n", i);

                     break;

              case 1:

                     printf("The number %d is odd \n",i);

                     break;

              } /* switch case */

       } /* for */

getch();

} /* main */

OUTPUT:

Enter a Number: 7

The number 1 is odd

The number 2 is even

The number 3 is odd

The number 4 is even

The number 5 is odd

The number 6 is even

The number 7 is odd

EXPLANATION: This program is used to find out whether the given number is even or odd and it checks for the n number. If the number is divided by 2 then it is even, if the number is not divided by 2 then it is odd number.

Example 2: Program to use the computer as calculator. The user input a code +,‒,,/ and values that are to be computed.

/* Program to use the computer as calculator */

#include <stdio.h>

main()

{

        int a,b,c=0;

        char op;

        printf("CALCULATION CODE");

        printf("\n+ADD\n‒SUB\n*MUL\n/DIV\n");

        printf("Enter Code.....");

        scanf("%c",&op);

        printf("Enter values.....");

        scanf("%d %d",&a, &b);

        switch(op)

        { /* switch open */

                case '+':c=a+b;

                        break;

                case '‒': c=a‒b;

                        break;

                case '*': c=a*b;

                        break;

                case '/': c=a/b;

                        break;

        } /* switch */

        printf("\nResult is.....%d", c);

        getch();

} /* main */

OUTPUT

CALCULATION CODE

+ADD

‒SUB

*MUL

/DIV

Enter Code.....+

Enter values.....20 10

Result is.....30

EXPLANATION: In the program the switch statement consists of four options. The option is entered according to user's choice. The four options are addition, subtraction, multiplication and division. Each option contains several operations. Thus statement is executed according to selected choice. In the program the character variable op reads the character entered by the users. According to the entered character, the switch statement is executed and prints the output.