Loops
=====

**The for loop**

In Python the 'for' loop has the following structure:

.. code::

 for (item) in (iterable):
     (body)

The item is a variable name that can be used in the body and iterable can be a string, list, tuple, sets, or dicts.  Here are some examples:

.. code::

 for item in ['one', 'three', 'five']:
     print(i)

 one
 three
 five

.. code::

 for letter in 'Python':
     print(letter)

 P
 y
 t
 o
 n

.. code::

 my_dict = {'Alex': 60, 'Anthony': 24, 'Erika': 21}
 for key in my_dict.keys():
     print(key + ' is', my_dict[key], 'years old.')
 
 Alex is 60 years old.
 Anthony is 24 years old.
 Erika is 21 years old.

In programs it is common to iterate through a sequence of integers.   Recall the range function, 'range(start, stop, step)', is a iterator that generates integers.   It can be used in a loop like this:

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

 0
 1
 2
 3
 4

 vowels = 'aeiou'
 alphabet = 'abcdefghijklmnopqrstuvwxyz'
 consonants = ''
 for let in alphabet:
      if let not in vowels:
      consonants += let
 print('The consonants are:', consonants)

 The consonants are: bcdfghjklmnpqrstvwxyz

**The while loop**

The while loop is used when we want to repeat code until a condition is met.  This the structure of the while loop in Python:

.. code::

 while (condition):
     (body)

Here is an example:

.. code::
 
 lst = ['one', 'two', 'three', 'stop', 'four', 'five']
 i = 0
 item = lst[0]
 while item != 'stop':
     print(item)
     i += 1
     item = lst[i]
 one
 two
 three
 
**Using `break`, `continue`, and `else` with a `while`**

**A `while` loop with a 'break'**

The `break` statement will terminate a while loop.   Here is a common idea.   We want to leave it up to the user to continue "playing" until he wants to stop.   Here is some code that uses the while/break to do this.

.. code::

 while True:
     response = input('I will keep going until you input Y or y.\n')
     if response in 'yY':
         break
     print('You input', response, 'so I will keep going.')
 print('You input', response, 'so I stopped.')

 I will keep going until you input Y or y.
 a
 You input a so I will keep going.
 I will keep going until you input Y or y.
 b
 You input b so I will keep going.
 I will keep going until you input Y or y.
 c
 You input c so I will keep going.
 I will keep going until you input Y or y.
 Y
 You input Y so I stopped.

**A while loop with continue**  

The continue statement will move control of the execution back to the top of the loop.


A while loop with else looks like this:

.. code::

 while (condition):
     (body)
 else:
     (more body)

The `else` statement executes when the condition is false.   One way to use this is to decide whether the while loop was terminated by a break or because the condition failed.

.. code ::

 count = 0
 breaker = 11
 while(count < 10):
     count += 1
     if breaker == count:
         break
 else:
     print('The else has been executed.')
 print('The while loop looped', count,'times.')
 > The else has been executed.
 > The while loop looped 10 times.

Since the condition count < 10 fails the else statement is executes after 10 iterations of the loop.

.. code ::


 count = 0
 breaker = 10
 while(count < 10):
     count += 1
     if breaker == count:
         break
 else:
     print('The else has been executed.')
 print('The while loop looped', count,'times.')

 > The while loop looped 10 times.

With `breaker` set to 10 the loop still iterates 10 times.   But the `else` statement is not executed.  On the last iteration the `if` condition is met before the while condition is checked.   

**Nested Loops**

.. code::

 articles = ['A','The']
 verbs = ['walks', 'talks', 'jumps']

 for article in articles:
     for verb in verbs:
         print(article + ' child ' + verb + '.')
 
 A child walks.
 A child talks.
 A child jumps.
 The child walks.
 The child talks.
 The child jumps.

Here is another example.   On an 8 x 8 chessboard the rows are labeled from 1 to 8 and the columns are labeled for a to h.   Let's generate a list of strings that represent each square on a chess board for example 'e4'.

**Put an image here**

`Chessboard <https://en.wikipedia.org/wiki/Algebraic_notation_(chess)#/media/File:SCD_algebraic_notation.svg>`_

.. code::

 squares = []
 for i in range(8, 0, -1):
     for let in 'abcdefgh':
         print(let + str(i), end = ' ')
     print()

 a8 b8 c8 d8 e8 f8 g8 h8
 a7 b7 c7 d7 e7 f7 g7 h7
 a6 b6 c6 d6 e6 f6 g6 h6
 a5 b5 c5 d5 e5 f5 g5 h5
 a4 b4 c4 d4 e4 f4 g4 h4
 a3 b3 c3 d3 e3 f3 g3 h3
 a2 b2 c2 d2 e2 f2 g2 h2
 a1 b1 c1 d1 e1 f1 g1 h1

The extra parameter `end` specifies how to end the print statement.   The default is a newline.   The `print()` prints a newline.  It is indented just one level so that it will execute each time the inner `for` loop finishes iterating.   This way the rows of the output consist of all the squares for each letter.  We reverse the range() so that the output will look like a chessboard from White's perspective.