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)