Loops in Python

 Loops in Python

A loop statement uses to execute a statement or group of statements multiple times as specified by the user.

It’s also called iterations.

Iteration: - Repetitive execution of the same code block.

Two types of iteration:

Definite iteration:-

the number of repetitions is specified explicitly in advance. Ex. For loop

Indefinite iteration:-

the code block executes until some condition is met. Ex. while loop.

 

Python provides 2 types of loops:

• While loop
• For loop

 

1) While loop:

It is used to execute multiple statements or codes repeatedly until the given condition is true.

Use this loop when don’t know the number of times to iterate.

Syntax:

while expression:

   statement(s)

 

The loop iterates while the condition is true.

Example

count = 0

while (i < 11):

   print 'The count is:', i

   i = i + 1

print "count end!"

 

output:- 

The count is: 0

The count is: 1

The count is: 2

The count is: 3

The count is: 4

The count is: 5

The count is: 6

The count is: 7

The count is: 8

The count is: 9

The count is: 10

count end

 

Single Statement Suites

Similar to the if statement syntax, while clause consists only of a single statement.

Syntax:

while expression: statement(s)

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2) For loop:

For loop execute a block of statements or code several times until the given condition becomes false.

It is iterating over a sequence (either a list, a tuple, a dictionary, a set, or a string).

The syntax

for iterating_var in sequence:

   statements(s)

example :- 
for letter in 'Python':     # First Example
   print 'Current Letter :', letter
 
fruits = ['banana', 'apple',  'mango']
for fruit in fruits:        # Second Example
   print 'Current fruit :', fruit
 
print "Good bye!"

output: 
Current Letter : P
Current Letter : y
Current Letter : t
Current Letter : h
Current Letter : o
Current Letter : n
Current fruit : banana
Current fruit : apple
Current fruit : mango
Good bye!

Example:
language = [‘Python’, ‘Java’, ‘Ruby’]

 for lang in language:
      print(“Current language is: “, lang)

Output:
Current language is: Python
Current language is: Java
Current language is: Ruby
===========================================

 For loop using range () function:
Range () function is used to generate a sequence of numbers.

for x in "banana":
  print(x)

output:- 
b
a
n
a
n
a
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The break Statement
With the break statement
we can stop the loop before it has looped through all the items:

Example
# Exit the loop when x is "banana":
fruits = ["apple", "banana", "cherry"]

for x in fruits:
  print(x)
  if x == "banana":
    break

Output: 
apple
banana

==============================================
The continue Statement
With the continue statement
we can stop the current iteration of the loop, and continue with the next:

Example
Do not print banana:

fruits = ["apple", "banana", "cherry"]
for x in fruits:
  if x == "banana":
    continue
  print(x)

output: 
apple
banana
cherry
===================================================

The range() Function
range() function use for specified number of times a set of code define.
This function returns a sequence of numbers, starting from 0 by default,
and increments by 1 (by default), and ends at a specified number.

Example
#Using the range()
function:
for x in range(6):
  print("Using the range() function",x)
#Using the start
parameter:
for x in range(2, 6):
  print(x)
#Increment the
sequence with 3 (default is 1):
for x in range(2,30, 3):
  print("Increment the sequence with 3 (default is 1):",x)

output:-
Using the range() function 0
Using the range() function 1
Using the range() function 2
Using the range() function 3
Using the range() function 4
Using the range() function 5
2
3
4
5
Increment the sequence with 3 (default is 1): 2
Increment the sequence with 3 (default is 1): 5
Increment the sequence with 3 (default is 1): 8
Increment the sequence with 3 (default is 1): 11
Increment the sequence with 3 (default is 1): 14
Increment the sequence with 3 (default is 1): 17
Increment the sequence with 3 (default is 1): 20
Increment the sequence with 3 (default is 1): 23
Increment the sequence with 3 (default is 1): 26
Increment the sequence with 3 (default is 1): 29

Lists as an iterable
collection = ['hey', 5, 'd']

for x in collection:
    print(x)

Loop over Lists of lists

list_of_lists = [ [1, 2, 3], [4, 5, 6], [7, 8, 9]]

for list in list_of_lists:
    for x in list:
        print(x)
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Nested Loops
A nested loop is a loop inside a loop.
The "inner loop" will be executed one time for each iteration of the "outer loop":

Example
Print each adjective for every fruit:

adj = ["red", "big", "tasty"]
fruits = ["apple", "banana", "cherry"]

for x in adj:
  for y in fruits:
    print(x, y)

output:- 
red apple
red banana
red cherry
big apple
big banana
big cherry
tasty apple
tasty banana
tasty cherry
======================================================================
The pass Statement
if a for loop with no content, then use the pass statement to avoid an error.
It is used when a statement is required syntactically but do not need any command or code to execute.
The pass statement is a null operation; nothing happens when it executes. 
The pass is also useful in places where code will eventually go, but has not been written yet
(e.g., in stubs for example) −

Syntax
pass

Example
for letter in 'Python': 
   if letter == 'h':
      pass
      print 'This is pass block'
   print 'Current Letter :', letter
 
print "Good bye!"

output:  −
Current Letter : P
Current Letter : y
Current Letter : t
This is pass block
Current Letter : h
Current Letter : o
Current Letter : n
Good bye!
==========================================