Iteration (Updating Variables)

Iteration – repetition of a mathematical or computational procedure applied to the result of a previous applications, typically as a means of obtaining successively closer approximations to the solution of the problem.

A common pattern in assignment statements is an assignments statement that updates a variable – where the new value of the variable depends on the old.

>>> x = x + 1

Traceback (most recent call last):

  File "", line 1, in

NameError: name 'x' is not defined

Unfortunately the variable x is not defined so we got an error. The x = x +1 means get the current value of x, add one, and then update x with the new value. In order to successfully update variable x let’s enter the starting value.

>>>value = raw_input(‘Enter a value:’)

Enter a value: 5

>>>x = int(value)

>>>x = x + 1

>>>print x

6

So before you can update a variable, you have to initialize it, usually with simple assignment but in our case we’ve used built-in Python function called raw_input that you’ve already met in our previous lessons.

After initialization of variable now we can update variable by adding 1 and this is called an increment, while subtracting by 1 is called decrement.