The else Statement Used with Loops

Python supports to have an else statement associated with a loop statements.

·         If the else statement is used with a for loop, the else statement is executed when the loop has exhausted iterating the list.

·         If the else statement is used with a while loop, the else statement is executed when the condition becomes false.

Example:

The following example illustrates the combination of an else statement with a for statement that searches for prime numbers from 10 through 20.

#!/usr/bin/python

For num in range (10, 20):      #to iterate between  10  to  20

     For i in range  (2,  num)  : #to iterate on the factors of the number

            If  num% I == 0:           #to determine the first facto

               J = num/I                  #to calculate the second factor

              Print  ‘%d equals  %d * %d ‘  % (num, i, j)

              break             #to move to the next number, the #first FOR

else:                             #else part of the loop

print  num,  ‘is a prime number’

This  will produce following result:

10 equals 2*5

11 is a prime number

12 equals 2*6

13 is a prime number

14 equals 2*7

15 equals 3*5

16 equals 2*8

17 is a prime number

18 equals 2*9

19 is a prime number.

The Continue Statement:

The continue statement in Python returns the control to the beginning of the while loop. The continue statement rejects all the remaining statements in the current iteration of the loop and moves the control back to the top of the loop.

The continue statement can be used in both while and for loops.

Example:

#!/usr/bin/python

For letter in ‘Python’ :               # First Example

            If letter  ==  ‘h’:

                continue

            Print  ‘Current  Letter  : ‘ ,  letter

var  = 10

while  var  > 0:

     var  =  var -1

    if  var  ==  5:

            continue

     print  ‘Current   Variable Value  :’, letter

print  ”Good bye !”

This will produce following result:

Current Letter :  P

Current Letter :  y

Current Letter :  t

Current Letter :  h

Current Letter :  o

Current Letter :  n

Current Variable Value : 10

Current Variable Value :  9

Current Variable Value :  8

Current Variable Value :  7

Current Variable Value :  6

Current Variable Value :  5

Current Variable Value :  4

Current Variable Value :  3

Current Variable Value :  2

Current Variable Value :  1

Good bye!

The break Statement of Python

The break statement in python terminates the current loop and resumes execution at the next statement, just like the traditional break found in C.

The most common use for break is when some external condition is triggered requiring a hasty exit from a loop. The break statement can be used in both while and for loops.

Example:

#!/usr/bin/python

For letter in ‘Python’ :               # First Example

            If letter  ==  ‘h’:

                Break

            Print  ‘Current  Letter  : ‘ ,  letter

var  = 10

while  var  > 0:

     print  ‘Current   Variable Value  :’, letter

     var  =  var -1

    if  var  ==  5:

            break;

print  ”Good bye !”

This will produce following result:

Current Letter :  P

Current Letter :  y

Current Letter :  t

Current Variable Value : 10

Current Variable Value :  9

Current Variable Value :  8

Current Variable Value :  7

Current Variable Value :  6

Good bye!

The else Statement of Python

An else statement can be combined with an if statement. An else statement contains the block of code that executes if the conditional expression in the if expression in the if expression resolves to 0 or a false value.

The else statement is an optional statement and there could be at most only one else statement following if.

The syntax of the if….else statement is:

If expression :

            Statement(s)

else:

            Statement(s)

Example:

# ! /usr/bin/python

var1 = 100

if var1:

            print “1 – Got a true expression value”

            print var1

else

            print “1 – Got a true expression value”

            print var1

var2 = 0

if var2:

            print “2 – Got a true expression value”

            print var2

else

            print “2 – Got a true expression value”

            print var2

print  “Good bye”

This will produce following result:

1 – Got a true expression value

100

2 – Got a true expression value

0

Good bye!

If statement of Python

The If statement of python is similar to that of other languages. The if statement contains a logical expression using which data is compared, and a decision is made based on the result of the comparison.

The syntax of the If statement is:

If expression:

    Statement (s)

Here if statement condition is evaluated first. If condition is true that is, if its value is nonzero then the statement(s) block are executed. Otherwise, the next statement following the statement(s) block is executed.

Note: In python, all the statements indented by the same number of character spaces after a programming construct are considered to be part of a single block of code.  Python uses indentation as its method of grouping statements.

Example:

# ! /usr/bin/python

var1 = 100

If  var1:

            Print “1 – Got a true expression value”’

            Print var1

Var2 = 0

If  var2:

            Print “2 – Got a true expression value”’

            Print var2

Print “Good bye!”

This will produce following result:

1 – Got a true expression value

100

Good bye!

The Nested if…elif…else Construct:

There may be a situation when you want to check for another condition after a condition resolves a condition resolves to true. In such a situation, you can use the nested if construct.

In a nested if construct, you can have an if…elif…else construct inside another if…elif…else construct.

The syntax of the nested if…elif…else construct may be:

If expression1:

            statement(s)

If expression2:

            statement(s)

elif expression3:

            statement(s)

else

            statement(s)

elif expression4:

            statement(s)

else:

            statement(s)

Example:

#!/usr/bin/python

Var = 100

If var < 200

  print “Expression value is less than 200”

if var == 150:

  print “which is 150”

elif var == 100:

   print “Which is 100”

elif  var == 50:

   print  “Which is 50”

elif var < 50:

    print  “Expression value is less than 50”

else:

   print  “could not find true expression ”

print  “Good bye!”

This will produce following result:

Expression value is less than 200

Which is 100

Good bye!

Single Statement Suites:

If the suite of an if clause consists only of a single line, it may go on the same line as the header statement:

Here is an example of a one-line if clause:

If(expression  ==  1)  :  print  “value of expression is 1”