Infinite Loops
==============
Here's the code for a simple ``while`` loop. Run the program first to make sure it
works. Then make one of these changes, and run the program again:
#. Remove or comment out ``num += 1``.
#. Replace the ``+=`` operator with ``-=``.
.. raw:: html
Yikes! What happened? The program just keeps running!
.. index:: ! infinite loop
This is an example of an **infinite loop**, which is a set of code that repeats
forever.
#. By removing ``num += 1`` from the loop body, the value of ``num`` remains 0.
#. By changing the operator to ``num -= 1``, the value of the variable
decreases every iteration.
By making either of these changes, the expression ``num < 21`` will ALWAYS
return ``True``, so the ``while`` loop will NEVER stop.
Coding Infinity
---------------
Simple mistakes in the code create infinite loops, and *every* programmer
accidentally creates one from time to time. Consider it a rite of
passage---when you launch your first, welcome to the club.
.. admonition:: Tip
When this happens to you, holding down ``control-c`` will usually force your
program to stop.
Infinite loops are usually created from small typos or missing statements.
These mistakes set up a situation where the ending condition cannot be reached.
.. admonition:: Example
Here's another infinite while loop. The program is supposed to print a
decreasing total until that total reaches 0.
.. sourcecode:: python
:linenos:
start_value = 26
total = start_value
while start_value > 0:
total -= 5
print(total)
In this case, the update statement is correct (line 5), but the error occurs
in line 4. Instead of using ``total`` in the boolean expression, we used
``start_value``, which never gets updated in the loop.
.. admonition:: Example
Here's an example of an infinite ``for`` loop in Python:
.. sourcecode:: python
:linenos:
# NOPE! Nothing to see here!
Well. That's nice.
Although it *is* possible to create an infinite ``for`` loop in Python, you
need to be very deliberate and think very hard about how to make it happen.
You are unlikely to create one by accident.
Fun Fact
--------
Infinite loops are such an established part of programming that Apple even
made one the address of its California campus.
.. figure:: figures/infinite-loop-street.jpg
:alt: Apple campus, 1 Infinite Loop, Cupertino, CA 95014
1 Infinite Loop, Cupertino, CA
Check Your Understanding
------------------------
.. admonition:: Question
The following code contains an infinite loop. Which is the BEST explanation
for why the loop does not end?
.. sourcecode:: python
:linenos:
num = 10
answer = 1
while num > 0:
answer = answer + num
num += 1
print(answer)
.. raw:: html
num starts at 10 and increases by 1 each time through the loop, so it will always be positive.
answer starts at 1 and increases by num each time, so it will always be positive.
You cannot compare num to 0 in a while loop. You must compare it to another variable.