Examples

• 17

  Break and Continue in Loops

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  17

  break statement

  When a break statement executes inside a loop, control flow "breaks" out of the loop immediately:

  i = 0
  while i < 7:
      print(i)
      if i == 4:
          print("Breaking from loop")
          break
      i += 1
  

  The loop conditional will not be evaluated after the break statement is executed. Note that break statements are only allowed inside loops, syntactically. A break statement inside a function cannot be used to terminate loops that called that function.

  Executing the following prints every digit until number 4 when the break statement is met and the loop stops:

  0
  1
  2
  3
  4
  Breaking from loop
  

  break statements can also be used inside for loops, the other looping construct provided by Python:

  for i in (0, 1, 2, 3, 4):
      print(i)
      if i == 2:
          break
  

  Executing this loop now prints:

  0
  1
  2
  

  Note that 3 and 4 are not printed since the loop has ended.

  If a loop has an else clause, it does not execute when the loop is terminated through a break statement.

  continue statement

  A continue statement will skip to the next iteration of the loop bypassing the rest of the current block but continuing the loop. As with break, continue can only appear inside loops:

  for i in (0, 1, 2, 3, 4, 5):
      if i == 2 or i == 4:
          continue
      print(i)
  
  0
  1
  3
  5
  

  Note that 2 and 4 aren't printed, this is because continue goes to the next iteration instead of continuing on to print(i) when i == 2 or i == 4.

  Nested Loops

  break and continue only operate on a single level of loop. The following example will only break out of the inner for loop, not the outer while loop:

  while True:
      for i in range(1,5):
          if i == 2:
              break    # Will only break out of the inner loop!
  

  Python doesn't have the ability to break out of multiple levels of loop at once -- if this behavior is desired, refactoring one or more loops into a function and replacing break with return may be the way to go.

  Use return from within a function as a break

  The return statement exits from a function, without executing the code that comes after it.

  If you have a loop inside a function, using return from inside that loop is equivalent to having a break as the rest of the code of the loop is not executed (note that any code after the loop is not executed either):

  def break_loop():
      for i in range(1, 5):
          if (i == 2):
              return(i)
          print(i)
      return(5)
  

  If you have nested loops, the return statement will break all loops:

  def break_all():
      for j in range(1, 5):
          for i in range(1,4):
              if i*j == 6:
                  return(i)
              print(i*j)
  

  will output:

  1 # 1*1
  2 # 1*2
  3 # 1*3
  4 # 1*4
  2 # 2*1
  4 # 2*2
  # return because 2*3 = 6, the remaining iterations of both loops are not executed
  

  edited Dec 20 '16 at 8:43

  

  

  

  

  

  +8
• 10

  For loops

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  10

  for loops iterate over a collection of items, such as list or dict, and run a block of code with each element from the collection.

  for i in [0, 1, 2, 3, 4]:
      print(i)
  

  The above for loop iterates over a list of numbers.

  Each iteration sets the value of i to the next element of the list. So first it will be 0, then 1, then 2, etc. The output will be as follow:

  0  
  1
  2
  3
  4
  

  range is a function that returns a series of numbers under an iterable form, thus it can be used in for loops:

  for i in range(5):
      print(i)
  

  gives the exact same result as the first for loop. Note that 5 is not printed as the range here is the first five numbers counting from 0.

  Iterable objects and iterators

  for loop can iterate on any iterable object which is an object which defines a __getitem__ or a __iter__ function. The __iter__ function returns an iterator, which is an object with a next function that is used to access the next element of the iterable.

  edited Sep 21 '16 at 23:53

  

  

  

  

  

  +2
• 6

  Iterating over lists

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  6

  To iterate through a list you can use for:

  for x in ['one', 'two', 'three', 'four']:
      print(x)
  

  This will print out the elements of the list:

  one
  two
  three
  four
  

  Generated numbers using range is also gives a list as an output.

  for x in xrange(1, 6):
      print(x)
  

  Result will not be in a list.

  1
  2
  3
  4
  5
  

  If you want to loop though both the elements of a list and have an index for the elements as well, you can use python's excellent enumerate function:

  for index, item in enumerate(['one', 'two', 'three', 'four']):
      print(index, '::', item)
  

  enumerate will generate tuples, which are unpacked into index (an integer) and item (the actual value from the list). The above loop will print

  (0, '::', 'one')
  (1, '::', 'two')
  (2, '::', 'three')
  (3, '::', 'four')
  

  Iterate over a list with value manipulation using map and lambda, i.e. apply lambda function on each element in the list:

  x = map(lambda e :  e.upper(), ['one', 'two', 'three', 'four'])
  print(x)
  

  Output:

  ['ONE', 'TWO', 'THREE', 'FOUR'] # python 2.x
  

  NB: in Python 3.x map returns an iterator instead of a list so you in case you need a list you have to cast the result print(list(x)).

  edited Dec 20 '16 at 8:43

  

  

  

  

  

  +8
• 5

  Loops with an "else" clause

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  5

  The for and while compound statements (loops) can optionally have an else clause (in practice, this usage is fairly rare).

  The else clause only executes after a for loop terminates by iterating to completion, or after a while loop terminates by its conditional expression becoming false.

  for i in range(3):
      print(i)
  else:
      print('done')
  
  i = 0
  while i < 3:
      print(i)
      i += 1
  else:
      print('done')
  

  output:

   0
   1
   2
   done
  

  The else clause does not execute if the loop terminates some other way (through a break statement or by raising an exception):

  for i in range(2):
      print(i)
      if i == 1:
          break
  else:
      print('done')
  

  output:

  0
  1
  

  Most other programming languages lack this optional else clause of loops. The use of the keyword else in particular is often considered confusing.

  The original concept for such a clause dates back to Donald Knuth and the meaning of the else keyword becomes clear if we rewrite a loop in terms of if statements and goto statements from earlier days before structured programming or from a lower-level assembly language.

  For example:

  while loop_condition():
      ...
      if break_condition():
          break
      ...
  

  is equivalent to:

  # pseudocode
  
  <<start>>:
  if loop_condition():
      ...
      if break_condition():
          goto <<end>>
      ...
      goto <<start>>
  
  <<end>>:
  

  These remain equivalent if we attach an else clause to each of them.

  For example:

  while loop_condition():
      ...
      if break_condition():
          break
      ...
  else:
      print('done')
  

  is equivalent to:

  # pseudocode
  
  <<start>>:
  if loop_condition():
      ...
      if break_condition():
          goto <<end>>
      ...
      goto <<start>>
  else:
      print('done')
  
  <<end>>:
  

  A for loop with an else clause can be understood the same way. Conceptually, there is a loop condition that remains True as long as the iterable object or sequence still has some remaining elements.

  edited Jul 28 '16 at 19:50

  

  

  
• 3

  Iterating over dictionaries

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  3

  Considering the following dictionary:

   d = {"a": 1, "b": 2, "c": 3}
  

  ---

  To iterate through its keys, you can use:

  for key in d:
      print(key)
  

  Output:

  "a"
  "b"
  "c"
  

  This is equivalent to:

  for key in d.keys():
      print(key)
  

  or in Python 2:

  for key in d.iterkeys():
      print(key)
  

  ---

  To iterate through its values, use:

  for value in d.values():
      print(value)
  

  Output:

  1
  2
  3
  

  ---

  To iterate through its keys and values, use:

  for key, value in d.items():
      print(key, "::", value)
  

  Output:

  a :: 1
  b :: 2
  c :: 3
  

  ---

  Note that in Python 2, .keys(), .values() and .items() return a list object. If you simply need to iterate trough the result, you can use the equivalent .iterkeys(), .itervalues() and .iteritems().

  Note also that in Python 3, Order of items printed in the above manner does not follow any order.

  edited Jul 28 '16 at 6:44

  

  

  

  

  
• 2

  Iterating different portion of a list with different step size

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  2

  Suppose we have declared a list named lst:

  lst = ['One', 'Two', 'Three', 'Four', 'Five', 'Six', 'Seven', 'Eight', 'Nine', 'Ten'];
  

  We can iterate the whole list like this:

  for x in lst:
      #do something with x
  

  Or like this:

  for x in range(len(lst)):
      #do something with lst[x]
  

  Now, if we want to iterate Four [index 3] to Eight [index 7], then we have to iterate through index[0-based] like below:

  for x in range(3,8): 
      #do something with lst[x]
      #lst[x] would be 'Four', 'Five', 'Six', 'Seven' and 'Eight'
  

  Or alternatively use a slice of lst as below:

  for x in lst[3:8]:
      # x will be 'Four', 'Five', 'Six', 'Seven' and 'Eight' as before.
  

  If we have to do something with the values Two [index 1] to Seven [index 6] skipping one after taking one each time, then we could do the following:

  for x in range(1,7,2): 
      #do something with lst[x]
      #lst[x] would be 'Two', 'Four' and 'Six'
  

  edited 2 days ago

  

  
• 2

  The Pass Statement

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  2

  pass is a null statement for when a statement is required by Python syntax (such as within the body of a for or while loop), but no action is required or desired by the programmer. This can be useful as a placeholder for code that is yet to be written.

  for x in range(10):
      pass #we don't want to do anything, or are not ready to do anything here, so we'll pass
  

  In this example, nothing will happen. The for loop will complete without error, but no commands or code will be actioned. pass allows us to run our code successfully without having all commands and action fully implemented.

  Similarly, pass can be used in while loops, as well as in selections and function definitions etc.

  while x == y:
      pass
  

  created Jul 23 '16 at 0:57

  
• 2

  While Loop

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  2

  A while loop will cause the loop statements to be executed until the loop condition is falsey. The following code will execute the loop statements a total of 4 times.

  i = 0 
  while i < 4:
      #loop statements
      i = i + 1
  

  While the above loop can easily be translated into a more elegant for loop, while loops are useful for checking if some condition has been met. The following loop will continue to execute until myObject is ready.

  myObject = anObject()
  while myObject.isNotReady():
      myObject.tryToGetReady()
  

  while loops can also run without a condition by using numbers (complex or real) or True:

  import cmath
  
  complex_num = cmath.sqrt(-1)
  while complex_num:      # You can also replace complex_num with any number, True or a value of any type
      print(complex_num)   # Prints 1j forever
  

  If the condition is always true the while loop will run forever (infinite loop) if it is not terminated by a break or return statement or an exception.

  while True:
      print "Infinite loop"
  # Infinite loop
  # Infinite loop
  # Infinite loop
  # ...
  

  edited Nov 5 '16 at 13:23