Python Flow Control

Your program can take different paths based on the data it works with

There are a couple ways to make decisions but the most common way is to use variations of the if statement

An if statement evaluates a Boolean expression

A Boolean expression evaluates to True or False

If the expression evaluates to True, the indented block of code following the if statement is executed

Otherwise, it is not

By using elif and else clauses, you can further customize its behavior

First variation: a single if clause

>>> age = 18

>>> if age >= 18:

...���� print 'you can vote'

you can vote

This �works,� in that �you can vote� prints successfully given that age has been assigned 18

But if age were assigned a value less than 18, nothing would print

>>> age = 17

>>> if age >= 18: # for age==17, this evaluates� to False

...���� print 'you can vote' # and this line does not exectute

(No printed output appears)

You probably would like to know what happens if the voter�s age is less than 18

An if�else structure works great for binary (yes/no, true/false) types of decision making

The following example uses an else clause in addition to if

An else clause does not take a conditional expression

An else clause only executes when the expression in the if clause evaluates to False

>>> age = 17

>>> # if age is assigned 17, the following evaluates to False

>>> if age >= 18:

...���� # and this line does not execute

...���� print 'you can vote'

... else:

...���� # but this line does!������������������

...���� print 'you cannot vote'

...

you cannot vote

Sometimes a binary decision won�t do�you need if�elif�else

In this variation, we start with an if clause, followed by any number of elif clauses, followed optionally by a single else clause

All of the if and elif clauses evaluate Boolean expressions

elif is short for �else if�

The indented code following the first expression that evaluates to True is executed

The rest of the clauses are ignored

The final else is only executed if all the previous expressions evaluate to False

This example shows how to use this type of decision making structure (try this out in an editor�the command line is too painful):

if age >= 16 and age < 17:

�� print 'you can drive with a permit'

elif age >= 17 and age <= 90:

�� print 'you can drive with a license'

elif age > 90:

�� print 'please think twice about driving'

else:

�� print 'you are not qualified to drive'

Loops

All useful programming languages implement one or more types of loops

Loops let you repeatedly perform a block of code

There are many reasons why you would want to do this

for loop

A for loop is useful when the starting and ending conditions for looping are well known

Here is a simple example:

for x in range(0, 5):

�� print x

Let�s look at the pieces:

for	x	in	range(0,5):
keyword	variable	keyword	method defining start and end

When the loop runs, x takes on a value in the range 0 to 5, incrementing x by 1 through the list

The first number in the range() function defines the starting assignment for x (or whatever variable name we choose)

The second number is the ending condition�it tells the loop to stop when x equals or exceed this condition

We can use the value of x for printing, calculations�you name it

Here is a slightly more complicated example that prints a table of squares:

print 'x', 'x squared'

for x in range(0, 5):

�� print x, x**2

And something yet more complicated�print a degrees to radians table

Here we use range() with 3 arguments

The third is the step�the increment between values of our variable

We use it to specify the spacing between degree increments in the table

Try changing it to other values

import numpy� # make functions in the numpy module available here

������������� # NOTE: the math module contains similar methods

print 'deg', '\t', 'rad' # '\t' prints a tab character

for theta in range(0, 360, 45): # the third argument in range is the step

�� if len(str(theta)) < 3: # use 2 tabs after 1 and 2 digit deg numbers

���� tabs = '\t\t'

�� else:

���� tabs = '\t'� # use 1 tab after numbers with 3 or more digits

�� print theta, tabs, numpy.deg2rad(theta) # convert degrees to radians

Real Python programmers don�t use for loops!

Instead, they replace it (when possible) with something called a list comprehension

Try this:

print range(0, 5)

This is the list that your for loop used invisibly

Now try this:

print [x**2 for x in range(0, 5)]

A list comprehension creates another list, in this case, one in which each original x has been squared

List comprehensions are extremely common in Python programming because they generally run faster than equivalent for loops

We will look at list comprehensions in another lecture

while loop

Although they exist in Python, while loops are used much less than for loops

Here is our degrees/radians table again:

theta = 0 �� # our looping variable

print 'deg', '\t', 'rad' # '\t' prints a tab character

while theta < 360:� # our loop ending condition

�� if len(str(theta)) < 3: # use 2 tabs after 1 and 2 digit deg numbers

���� tabs = '\t\t'

�� else:

���� tabs = '\t'

�� print theta, tabs, numpy.deg2rad(theta)

�� theta += 45��� # remember to increment theta!

Too many places to make mistakes�you can usually do the same thing, easier, with a for loop

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