Thursday, 17 October 2013

Switch Statement in Programming

Switch Statement and its Use

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Switch Statement
Sometimes, we have multiple conditions and take some action according to each condition. For example, in the payroll of a company, there are many conditions to deduct tax from the salary of an employee. If the salary is less than Rs. 10000, there is no deduction. But if it falls in the slab Rs. 10000 – 20000, then the income tax is deducted. If it exceeds the limit of Rs. 20000, some additional tax will be deducted. So the appropriate deduction is made according to the category or slab of the salary. We can also understand this from the example of grades of a student. If the student has grade ‘A’ we print ‘Excellent’ and 'Very good', 'good', 'poor' and 'fail' for grades B, C, D, and F respectively. Now we have to see how this multi-condition situation can be applied in a program. We have a tool for decision making i.e. 'if statement'. We can use 'if statement' to decide what description for a grade should be displayed. So we check the grade in if statement and display the appropriate description. We have five categories of grades-- A, B, C, D, and F. We have to write five if statements to check all the five possibilities (probabilities) of grade. So we write this in our program as under-
if ( grade == ‘A’ )
cout << “Excellent” ;
if ( grade == ‘B’ )
cout << “Very Good” ;
 if ( grade == ‘C’ )
cout << “Good” ;
if ( grade == ‘D’ )
cout << “Poor” ;
if ( grade == ‘F’ )
 cout << “Fail” ;
These statements are correct and perform the required task. In the following code we are using nested if statement.
If ( grade == ‘A’ )
cout << “Excellent” ;
else if ( grade == ‘B’ )
            cout << “Very Good” ;
else if ( grade == ‘C’ )
cout << “Good” ;
else if ( grade == ‘D’ )
cout << “Poor” ;
else if ( grade == ‘F’ )
            cout << “Fail” ;
In the above example, we see that there are two approaches for a multi way decision. In the first approach, we use as many if statements as needed. This is an expensive approach. The second is the use of nested if statements. The second is little more-efficient than the first one. In the 'nested if statements' the nested else is not executed if the first if condition is true and the control goes out of the if block. The C language provides us a stand-alone construct to handle these instances. This construct is switch structure. The switch structure is a multiple-selection construct that is used in such cases (multi way decisions) to make the code more efficient and easy to read and understand. The syntax of switch statement is as follows.
switch ( variable/expression )
case constant1 : statementLlist1 ;
case constant2 : statementLlist2 ;
case constantN : statementListN ;
default : statementList ;
In the switch statement, there should be an integer variable (also include char) or an expression which must evaluate an integer type (whole numbers only, the decimal numbers 2.5, 14.3 etc are not allowed). We can’t use compound conditions (i.e. the conditions that use logical operators && or ||) in switch statement and in case statements. The constants also must be integer constants (which include char). We can’t use a variable name with the case key word. The default statement is optional. If there is no case which matches the value of the switch statement, then the statements of default are executed. The switch statement takes the value of the variable, if there is an expression then it evaluates the expression and after that looks for its value among the case constants. If the value is found among the constants listed in cases, the statements in that statementList are executed. Otherwise, it does nothing. However if there is a default (which is optional), the statements of default are executed.
Thus our previous grade example will be written in switch statement as below.
switch ( grade )
 case ‘A’ : cout << “Excellent” ;
case ‘B’ : cout << “Very Good” ;
case ‘C’ : cout << “Good” ;
case ‘D’ : cout << “Poor” ;
case ‘F’ : cout << “Fail” ;
We know that C language is 'case sensitive'. In this language, ‘A’ is different from ‘a’. Every character has a numeric value which is stored by the computer.. The numeric value of a character is known as ASCII code of the character. The ASCII code of small letters (a, b, c etc ) are different from ASCII code of capital letters (A, B, C etc). We can use characters in switch statement as the characters are represented as whole numbers inside the computers. Now we will see how the use of ' the letter a' instead of 'A' can affect our program. We want our program to be user- friendly. We don’t want to restrict the user to enter the grade in capital letters only. So we have to handle both small and capital letters in our program. Here comes the limitations of switch statement. We can’t say in our statement like case ‘A’ or ‘a’ : statements ; We have to make two separate cases so we write
 case ‘A” :
 case ‘a’ :
In the switch statement, the cases fall through the case which is true. All the statements after that case will be executed right down to the end of the switch statement. This is very important to understand it. Let's suppose that the user enters grade ‘B’. Now the case ‘A’ is skipped. Next case ‘B’ matches and statement cout << “Very Good” ; is executed. After that, all the statements will be executed. So cout << “Good” ; cout << “Poor” ;and cout << “Fail” ; will be executed after one another. We don’t want this to happen. We want that when a case matches, then after executing its statement, the control should jump out of switch statement leaving the other cases. For this purpose we use key work break.

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