Looping Techniques in Python

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Python supports various looping techniques by certain inbuilt functions, in various sequential containers. These methods are primarily verya useful in competitive programming and also in various project which require a specific technique with loops maintaining the overall structure of code.

Where they are used ?

Different looping techniques are primarily useful in the places where we don’t need to actually manipulate the structure and ordering of overall container, rather only print the elements for a single use instance, no inplace change occurs in the container. This can also be used in instances to save time.

Different looping techniques using Python data structures  are:

    • Using enumerate():  enumerate() is used to loop through the containers printing the index number along with the value present in that particular index.
      # python code to demonstrate working of enumerate()
      for key, value in enumerate(['The', 'Big', 'Bang', 'Theory']):
          print(key, value)

      Output :

      0 The
      1 Big
      2 Bang
      3 Theory
      
    • Using zip(): zip() is used to combine 2 similar containers(list-list or dict-dict) printing the values sequentially. The loop exists only till the smaller container ends.
      # python code to demonstrate working of zip()
      # initializing list
      questions = ['name', 'colour', 'shape']
      answers = ['apple', 'red', 'a circle']
      # using zip() to combine two containers
      # and print values
      for question, answer in zip(questions, answers):
          print('What is your {0}?  I am {1}.'.format(question, answer))

      Output :

      What is your name?  I am apple.
      What is your color?  I am red.
      What is your shape?  I am a circle.
      
    • Using iteritem(): iteritems() is used to loop through the dictionary printing the dictionary key-value pair sequentially.
    • Using items():  items() performs the similar task on dictionary as iteritems() but have certain disadvantages when compared with iteritems().
      • It is very time consuming. Calling it on large dictionaries consumes quite a lot of time.
      • It takes a lot of memory. Sometimes takes double the memory when called on dictionary.
    • Example 1:
      # python code to demonstrate working of iteritems(),items()
      d = { "geeks" : "for", "only" : "geeks" }
      # using iteritems to print the dictionary key-value pair
      print ("The key value pair using iteritems is : ")
      for i,j in d.iteritems():
          print i,j
          
      # using items to print the dictionary key-value pair
      print ("The key value pair using items is : ")
      for i,j in d.items():
          print i,j

      Output:

      The key value pair using iteritems is : 
      geeks for
      only geeks
      The key value pair using items is : 
      geeks for
      only geeks
      
    • Example 2:
      # python code to demonstrate working of items()
      king = {'Akbar': 'The Great', 'Chandragupta': 'The Maurya',
              'Modi' : 'The Changer'}
      # using items to print the dictionary key-value pair
      for key, value in king.items():
          print(key, value)

      Output :

      Akbar The Great
      Modi The Changer
      Chandragupta The Maurya
      
      • Using sorted():  sorted() is used to print the container is sorted order. It doesn’t sort the container, but just prints the container in sorted order for 1 instance. Use of set() can be combined to remove duplicate occurrences.
    • Example 1:
      # python code to demonstrate working of sorted()
      # initializing list
      lis = [ 1 , 3, 5, 6, 2, 1, 3 ]
      # using sorted() to print the list in sorted order
      print ("The list in sorted order is : ")
      for i in sorted(lis) :
          print (i,end=" ")
          
      print ("\r")
          
      # using sorted() and set() to print the list in sorted order
      # use of set() removes duplicates.
      print ("The list in sorted order (without duplicates) is : ")
      for i in sorted(set(lis)) :
          print (i,end=" ")

      Output:

      The list in sorted order is : 
      1 1 2 3 3 5 6 
      The list in sorted order (without duplicates) is : 
      1 2 3 5 6 
      
    • Example 2:
      # python code to demonstrate working of sorted()
      # initializing list
      basket = ['guave', 'orange', 'apple', 'pear',
                'guava', 'banana', 'grape']
      # using sorted() and set() to print the list
      #  in sorted order
      for fruit in sorted(set(basket)):
          print(fruit)

      Output:

      apple
      banana
      grape
      guava
      guave
      orange
      pear
      
      • Using reversed(): reversed() is used to print the values of container in the descending order as declared.
    • Example 1:
      # python code to demonstrate working of reversed()
      # initializing list
      lis = [ 1 , 3, 5, 6, 2, 1, 3 ]
      # using revered() to print the list in reversed order
      print ("The list in reversed order is : ")
      for i in reversed(lis) :
          print (i,end=" ")

      Output:

      The list in reversed order is : 
      3 1 2 6 5 3 1 
      
    • Example 2:
      # python code to demonstrate working of reversed()
      # using reversed() to print in reverse order
      for i in reversed(range(1, 10, 3)):
          print (i)

      Output :

      7
      4
      1
      

Advantage of using above techniques over for, while loop

    • These techniques are quick to use and reduces coding effort. for, while loops needs the entire structure of container to be changed.
    • These Looping techniques  do not require any structural changes to container. They have keywords which present the exact purpose of usage. Whereas, no pre-predictions or guesses can be made in for, while loop i.e not easily understandable the purpose at a glance.
    • Looping technique make the code more concise than using for, while loopings.

 

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source and credits: http://www.geeksforgeeks.org
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