loops in python

Python programming language provides following types of loops to handle looping requirements. Python provides three ways for executing the loops. While all the ways provide similar basic functionality, they differ in their syntax and condition checking time.

  1. While Loop:

    In python, while loop is used to execute a block of statements repeatedly until a given a condition is satisfied. And when the condition becomes false, the line immediately after the loop in program is executed.
    Syntax :



    while expression:
        statement(s)
    

    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:

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    # Python program to illustrate
    # while loop
    count = 0
    while (count < 3):    
        count = count + 1
        print("Hello Geek")

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    Output:

    Hello Geek
    Hello Geek
    Hello Geek
    
    • Using else statement with while loops: As discussed above, while loop executes the block until a condition is satisfied. When the condition becomes false, the statement immediately after the loop is executed.
      The else clause is only executed when your while condition becomes false. If you break out of the loop, or if an exception is raised, it won’t be executed.
      If else like this:

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      if condition:
          # execute these statements
      else:
          # execute these statements

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      and while loop like this are similar



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      while condition:
           # execute these statements
      else:
           # execute these statements

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      #Python program to illustrate
      # combining else with while
      count = 0
      while (count < 3):    
          count = count + 1
          print("Hello Geek")
      else:
          print("In Else Block")

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      Output:

      Hello Geek
      Hello Geek
      Hello Geek
      In Else Block
      
    • Single statement while block: Just like the if block, if the while block consists of a single statement the we can declare the entire loop in a single line as shown below:
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      # Python program to illustrate
      # Single statement while block
      count = 0
      while (count == 0): print("Hello Geek")

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      Note: It is suggested not to use this type of loops as it is a never ending infinite loop where the condition is always true and you have to forcefully terminate the compiler.

    See this for an example where while loop is used for iterators. As mentioned in the article, it is not recommended to use while loop for iterators in python.

  2. for in Loop: For loops are used for sequential traversal. For example: traversing a list or string or array etc. In Python, there is no C style for loop, i.e., for (i=0; i<n; i++). There is “for in” loop which is similar to for each loop in other languages. Let us learn how to use for in loop for sequential traversals.

    Syntax:

    for iterator_var in sequence:
        statements(s)
    

    It can be used to iterate over iterators and a range.

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    # Python program to illustrate
    # Iterating over a list
    print("List Iteration")
    l = ["geeks", "for", "geeks"]
    for i in l:
        print(i)
           
    # Iterating over a tuple (immutable)
    print("\nTuple Iteration")
    t = ("geeks", "for", "geeks")
    for i in t:
        print(i)
           
    # Iterating over a String
    print("\nString Iteration")    
    s = "Geeks"
    for i in s :
        print(i)
           
    # Iterating over dictionary
    print("\nDictionary Iteration")   
    d = dict() 
    d['xyz'] = 123
    d['abc'] = 345
    for i in d :
        print("%s  %d" %(i, d[i]))

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    Output:

    List Iteration
    geeks
    for
    geeks
    
    Tuple Iteration
    geeks
    for
    geeks
    
    String Iteration
    G
    e
    e
    k
    s
    
    Dictionary Iteration
    xyz  123
    abc  345
    
  3. Iterating by index of sequences: We can also use the index of elements in the sequence to iterate. The key idea is to first calculate the length of the list and in iterate over the sequence within the range of this length.
    See the below example:

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    # Python program to illustrate
    # Iterating by index
      
    list = ["geeks", "for", "geeks"]
    for index in range(len(list)):
        print list[index]

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    Output:

    geeks
    for
    geeks
    
  4. Using else statement with for loops: We can also combine else statement with for loop like in while loop. But as there is no condition in for loop based on which the execution will terminate so the else block will be executed immediately after for block finishes execution.
    Below example explains how to do this:

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    # Python program to illustrate
    # combining else with for
      
    list = ["geeks", "for", "geeks"]
    for index in range(len(list)):
        print list[index]
    else:
        print "Inside Else Block"

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    Output:



    geeks
    for
    geeks
    Inside Else Block
    
  5. We can use for in loop for user defined iterators. See this for example.

  6. Nested Loops: Python programming language allows to use one loop inside another loop. Following section shows few examples to illustrate the concept.
    Syntax:

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    for iterator_var in sequence:
        for iterator_var in sequence:
            statements(s)
            statements(s)

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    The syntax for a nested while loop statement in Python programming language is as follows:

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    while expression:
        while expression: 
            statement(s)
            statement(s)

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    A final note on loop nesting is that we can put any type of loop inside of any other type of loop. For example a for loop can be inside a while loop or vice versa.

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    # Python program to illustrate 
    # nested for loops in Python
    from __future__ import print_function
    for i in range(1, 5):
        for j in range(i):
             print(i, end=' ')
        print()

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    Output:

    1
    2 2
    3 3 3
    4 4 4 4
  7. Loop Control Statements: Loop control statements change execution from its normal sequence. When execution leaves a scope, all automatic objects that were created in that scope are destroyed. Python supports the following control statements.
    • Continue Statement: It returns the control to the beginning of the loop.
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      # Prints all letters except 'e' and 's'
      for letter in 'geeksforgeeks'
          if letter == 'e' or letter == 's':
               continue
          print 'Current Letter :', letter
          var = 10

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      Output:

      Current Letter : g
      Current Letter : k
      Current Letter : f
      Current Letter : o
      Current Letter : r
      Current Letter : g
      Current Letter : k
      
    • Break Statement: It brings control out of the loop
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      for letter in 'geeksforgeeks'
        
          # break the loop as soon it sees 'e' 
          # or 's'
          if letter == 'e' or letter == 's':
               break
        
      print 'Current Letter :', letter

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      Output:

      Current Letter : e
    • Pass Statement: We use pass statement to write empty loops. Pass is also used for empty control statement, function and classes.
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      # An empty loop
      for letter in 'geeksforgeeks':
          pass
      print 'Last Letter :', letter

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      Output:

      Last Letter : s

Exercise: How to print a list in reverse order (from last to first item) using while and for in loops.

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