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Loops

There are only two options in Python for Iterative Processing.
  

1. while loop

 

2. for loop

 

while loop.

 
When the number of interation are not known upfront, use while loop.

syntax:

while <expression> :
code block

 

while loop Examples:

  
Ex 1:
  


a,b = 0,1

while b < 50:

	print(b)

	a,b = b, a+b 


print("done")  #look at the indentation

  
Ex 2:
  
while True: #infinite loop

#code block

break;

 

 

a = 10

while True:
    a -= 2
    print(a)
    if (a < 1):
        break


	

  

for loop

  
iterable – is any data structure that supports element traversal ex: stings, lists, tuples

iterators, lines of file, keys of dict,generator sets etc.

In Python we can use for loop in two styles.
 
style 1:
  
for element in iterable:
#do something with element

Here element is just a variable, and iterable can be list, tuple, strings etc …

This is useful when, iterating through one iterable object.
 
Example :
  

item_list = ['item 0', 'item 1', 'item 2', 'item 3', 'item 4']

for item in item_list:
	print(item)

 
style 2:
 
for i in range (start, end, step):
#do something with i

range() is a built in function, Return an object that produces a sequence of integers.

Here is i is a varible holds return value of range object.

This style is useful when iterating through multiple iterabe object.
  

item_list = ['item 0', 'item 1', 'item 2', 'item 3', 'item 4']

value_list = ['value 0', 'value 1', 'value 2', 'value 3', 'value 4']


for i in range(0, len(item_list)):

	#using i as index for both lists
	
	print(item_list [i], value_list[i])

	

  
Lets look at some more for loop examples.
  
Example :
 
Ex 1:
  

for num in range(10):
	print(num)
	
for num in range(5,25):
	print(num)

for num in range(5,25,2):
	print(num)
	

  
Using enumerate() built in function with for loop.
 
enumerate(iterable[, start]) -> iterator for index, value of iterable

Return an enumerate object. iterable must be another object that supports iteration.
 
Ex 2:
  


music =('rock','pop','alternative','folk')

for i in range(len(music), 1):

    print('music : %d: %s' %(i, music[i])) 


for i, style in enumerate(music,1):

    print ("music %d : %s " %(i, style)))
	
 

  
Ex 3:
  

fd = open('lines.txt')

for line in fd.readlines():

	print(line)
	

  

Example of 2D arrays.
  
2 dimensional arrays are presented as list of lists in Python.
 
each row is a inner list. suppose we have first row
 
2,5,3,5,7 and second row as 1,2,4,5,8.
 
A 2D array of these two rows represented as below.
 


[[2,5,3,5,7]
[1,2,4,5,8]]

 

2D array traversal using for loop

 
Below example shows, how to access or iterate through 2D arrays.

Ex 4:
  



a = [[2,5,3,5,7], [1,2,4,5,8]]

#method 1

print("#### methods 1 ####:")

for row in a:

    for item in row:

        print(item)

print("#### methods 2 #### :")

for i in range(len(a)):

    for j in range(len(a[i])):

        print(a[i][j])
    


break – Break stops the current loop execution.

continue – skips current execution and moves on to the next cycle.

 
pass – means do nothing.
 

a = 5

if a >= 10:
    pass

else:
    print("'a' is less than 10")