Time for action – repeating instructions with loops

We can use the for loop in the following ways:

1. Loop over an ordered sequence, such as a list, and print each item as follows:

   >>> food = ['ham', 'egg', 'spam']
   >>> for snack in food:
   ...     print(snack)
   ...
   ham
   egg
   spam
   

2. And remember that, as always, indentation matters in Python. We loop over a range of values with the built-in range() or xrange() functions. The latter function is slightly more efficient in certain cases. Loop over the numbers 1-9 with a step of 2 as follows:

   >>> for i in range(1, 9, 2):
   ...     print(i)
   ...
   1
   3
   5
   7
   

3. The start and step parameter of the range() function are optional with default values of 1. We can also prematurely end a loop. Loop over the numbers 0-9 and break out of the loop when you reach 3:

   >>> for i in range(9):
   ...     print(i)
   ...     if i == 3:
   ...     print('Three')
   ...     break
   ...
   0
   1
   2
   3
   Three
   

4. The loop stopped at 3 and we did not print the higher numbers. Instead of leaving the loop, we can also get out of the current iteration. Print the numbers 0-4, skipping 3 as follows:

   >>> for i in range(5):
   ...     if i == 3:
   ...             print('Three')
   ...             continue
   ...     print(i)
   ...
   0
   1
   2
   Three
   4
   

5. The last line in the loop was not executed when we reached 3 because of the continue statement. In Python, the for loop can have an else statement attached to it. Add an else clause as follows:

   >>> for i in range(5):
   ...     print(i)
   ... else:
   ...     print(i, 'in else clause')
   ...
   0
   1
   2
   3
   4
   (4, 'in else clause')
   

6. Python executes the code in the else clause last. Python also has a while loop. I do not use it that much because the for loop is more useful in my opinion.

What just happened?

We learned how to repeat instructions in Python with loops. This section included the break and continue statements, which exit and continue looping.