While Loops

3. Control Flow: Loops

While Loops

The #\color{#8959A8} {\mathtt{\text{while}}}# loop is the other syntactic structure that supports repeated execution. The #\color{#8959A8} {\mathtt{\text{while}}}# loop differs from the #\color{#8959A8} {\mathtt{\text{for}}}# loop in what drives the repetition, for #\color{#8959A8} {\mathtt{\text{for}}}# loops it's iterables, while for #\color{#8959A8} {\mathtt{\text{while}}}# loops it's expressions. A #\color{#8959A8} {\mathtt{\text{while}}}# loop will continue looping while its corresponding expression remains true, when the expression evaluates to false the loop is stopped.

The while loop

\begin{split}
\color{#8959A8} {\mathtt{whil}} & \color{#8959A8}{\mathtt{e}} \textit{ expression} \mathtt{:}\\
& \textit{statement}
\end{split}

Here, expression is any valid Python expression that can be evaluated in boolean context, in other words, expressions that can be interpreted as either #\color{#F5871F}{\mathtt{\text{True}}}# or #\color{#F5871F}{\mathtt{\text{False}}}#. The indented statement refers to the body of the loop, the code that will be executed every iteration.

For example, here we see a simple #\color{#8959A8} {\mathtt{\text{while}}}# loop that counts to #\color{#F5871F}{\mathtt{\text{10}}}#. Naturally, values within the expression are also changed within the loop, if this wasn't the case the loop would run infinitely.

count = 0

while count < 3:
count = count + 1
print(count)

1
2
3

break and continue

Similar to #\color{#8959A8} {\mathtt{\text{for}}}# loops, #\color{#8959A8} {\mathtt{\text{while}}}# loops also support the #\color{#8959A8} {\mathtt{\text{break}}}#, #\color{#8959A8} {\mathtt{\text{continue}}}#, and #\color{#8959A8} {\mathtt{\text{else}}}# keywords for practically the same purposes.

When stop conditions for a #\color{#8959A8} {\mathtt{\text{while}}}# loop get to complex to list in the header, a common strategy is to use an indefinite #\mathtt{\color{#8959A8} {\text{while}} \color{#F5871F}{\text{ True}}}# loop with conditional break statements in the body.

count = 0

while True:
count = count + 1

if count == 10:
break

print(count)

You have probably noticed that #\mathtt{\text{var} \color{#3e999f}{\text{ = }} \text{var} \color{#3e999f}{\text{ + }}\color{
#F5871F}{\text{1}}}# is a very common variable assignment pattern that occurs in loops. It is in fact so common that this particular syntax received its own shorthand notation.

Assignment operators

Shorthand	Expression
#\mathtt{\text{x} \color{#3e999f}{\text{ += }} \text{y}}#	#\mathtt{\text{x} \color{#3e999f}{\text{ = }}\text{x} \color{#3e999f}{\text{ + }} \text{y}}#
#\mathtt{\text{x} \color{#3e999f}{\text{ -= }} \text{y}}#	#\mathtt{\text{x} \color{#3e999f}{\text{ = }}\text{x} \color{#3e999f}{\text{ - }} \text{y}}#
#\mathtt{\text{x} \color{#3e999f}{\text{ *= }} \text{y}}#	#\mathtt{\text{x} \color{#3e999f}{\text{ = }}\text{x} \color{#3e999f}{\text{ * }} \text{y}}#
#\mathtt{\text{x} \color{#3e999f}{\text{ /= }} \text{y}}#	#\mathtt{\text{x} \color{#3e999f}{\text{ = }}\text{x} \color{#3e999f}{\text{ / }} \text{y}}#
#\mathtt{\text{x} \color{#3e999f}{\text{ **= }} \text{y}}#	#\mathtt{\text{x} \color{#3e999f}{\text{ = }}\text{x} \color{#3e999f}{\text{ ** }} \text{y}}#
#\mathtt{\text{x} \color{#3e999f}{\text{ %= }} \text{y}}#	#\mathtt{\text{x} \color{#3e999f}{\text{ = }}\text{x} \color{#3e999f}{\text{ % }} \text{y}}#
#\mathtt{\text{x} \color{#3e999f}{\text{ //= }} \text{y}}#	#\mathtt{\text{x} \color{#3e999f}{\text{ = }}\text{x} \color{#3e999f}{\text{ // }} \text{y}}#

The shorthand notation is supported by all arithmetic operators in Python.

>>> var = 0
>>> var += 1
>>> print(var)