Alternative or Branching Statement in python

 Alternative or Branching Statement

 Introduction

                     Programs may contain set of statements. Th ese statements are the executable segments that yield the result. In general, statements are executed sequentially, that is the statements are executed one aft er another. Th ere may be situations in our real life programming where we need to skip a segment or set of statements and execute another segment based on the test of a condition. Th is is called alternative or branching. Also, we may need to execute a set of statements multiple times, called iteration or looping. In this chapter we are to focus on the various control structures in Python, their syntax and learn how to develop the programs using them.

Programming in Python

In Python, programs can be written in two ways namely Interactive mode and Script mode. The Interactive mode allows us to write codes in Python command prompt (>>>) whereas in script mode programs can be written and stored as separate file with the extension .py and executed. Script mode is used to create and edit python source file.

Python provides the following types of alternative or branching statements

          i.    Simple if statement 
          ii.   if..else statement 
          iii.  if..elif statement

                                           In our day-to-day life we need to take various decisions and choose an alternate path to achieve our goal. May be we would have taken an alternate route to reach our destination when we find the usual road by which we travel is blocked. This type of decision making is what we are to learn through alternative or branching statement. Checking whether the given number is positive or negative, even or odd can all be done using alternative or branching statement.

 i. Simple if statement 

                                               Simple if is the simplest of all decision making statements. Condition should be in the form of relational or logical expression.

                          Syntax:

                                       if<condition>:

                                                             statements-block1

In the above syntax if the condition is true statements - block 1 will be executed.

Example:

           # Program to check the age and print whether eligible for voting

           x=int (input("Enter your age :")) 

           if x>=18:

            print ("You are eligible for voting") 

Output 1:

             Enter your age :34 

             You are eligible for voting 

Output 2:

              Enter your age :16

              >>>    

                             As you can see in the second execution no output will be printed, only the Python prompt will be displayed because the program does not check the alternative process when the condition is failed.

ii.   if..else statement

            The if .. else statement provides control to check the true block as well as the false block. Following is the syntax of ‘if..else’ statement.

 Syntax:

            if <condition>:

                      statements-block 1

              else: statements-block 2

if..else statement thus provides two possibilities and the condition determines which BLOCK is to be executed.

Example:

           a = int(input("Enter any number :"))

           if a%2==0: print (a, " is an even number") 

           else: print (a, " is an odd number")

Output 1:

            Enter any number :56 56 is an even number 

Output 2:

            Enter any number :67 67 is an odd number

>>>

                                 An alternate method to rewrite the above program is also available in Python. The complete if..else can also written as:

  Syntax:
              variable = variable1 if condition else variable 2 

 iii.  if..elif statement

                                When we need to construct a chain of if statement(s) then ‘elif ’ clause can be used instead of ‘else’.

Syntax:

            if<condition-1> :

                    statements-block 1

            elif<condition-2>:

                     statements-block 2

             else: statements-block n

                             In the syntax of if..elif..else mentioned above, condition-1 is tested if it is true then statements-block1 is executed, otherwise the control checks condition-2, if it is true statementsblock2 is executed and even if it fails statements-block n mentioned in else part is executed.

                             ‘elif ’ clause combines if..else-if..else statements to one if..elif…else. ‘elif ’ can be considered to be abbreviation of ‘else if ’. In an ‘if ’ statement there is no limit of ‘elif ’ clause that can be used, but an ‘else’ clause if used should be placed at the end.

Example:

            m1=int (input(“Enter mark in first subject : ”))

           m2=int (input(“Enter mark in second subject : ”)) 

           avg= (m1+m2)/2 

           if avg>=80: 

           print (“Grade : A”) 

           elif avg>=70 and avg<80: 

           print (“Grade : B”) elif avg>=60 and avg<70: 

           print (“Grade : C”) elif avg>=50 and avg<60: 

           print (“Grade : D”) else: print (“Grade : E”) 

Output 1:

            Enter mark in first subject : 34 

           Enter mark in second subject : 78 

           Grade : D 

Output 2 : 

           Enter mark in first subject : 67

           Enter mark in second subject : 73

           Grade:B

NOTE!

                The two blocks of code in our example of if-statement are both indented four spaces, which is a typical amount of indentation for Python. In most other programming languages, indentation is used only to help make the code look pretty. But in Python, it is required to indicate to which block of code the statement belongs to.