Written by Kevin Randrup

Claimed Chloe Choi

Edited by Do Young Kim (Fall 2016)

Introduction to functions in Python and applying them to write shorter and more understandable code.
Assumes that VPython is [http://www.physicsbook.gatech.edu/VPython installed] and you understand the [http://www.physicsbook.gatech.edu/VPython_basics basics] of programming in Python.

==Summary==

Functions at a high level are used to perform a certain task. Functions can accept a number of inputs (also "arguments" or "parameters") and can give back 0 or 1 output values. Furthermore, they make code much easier to understand by encapsulating a task in a single line of code instead of being repeated over and over again.

Functions always start with def, which is short for define, used to define a word with a piece of code, not data.
• Function is like a box of commands
• Functions can take inputs
• The way we pass those inputs is by putting them in parenthesis
Functions can return objects (outputs)

def sayHello():
print "Hello, world"

This is a simple function that defines the variable "sayHello" to return the phrase "Hello, world" (note that the parenthesis will not be printed. Only the ''Hello, world'' portion will be printed).

• Note that python uses indent to consider what is under a command. If print "Hello, world" is not indented, then it will not be a part of the function sayHello().

==Basic of Functions==

===Basic Python function syntax===
Functions can be defined with the keyword "def" followed by the function name and a colon and a parenthesis.
The variable that should be processed by the function goes Inside the parenthesis.

def madlib(name, noun1, noun2, verb):
result = name + " was looking at his " + noun1 + " when a " + noun2 + " " + verb + " nearby."
print result

This is a simple function that creates a madlib with the 4 variables that are given as inputs.

For example, if Sally was inputed as the name, lunch as noun1, pterodactyl as noun2, and flew as the verb, the the function will return the phrase ''Sally was looking at his lunch when a pterodactyl flew nearby''

<code>
>>> madlib("Sally", "lunch", "pterodactyl", "flew")

Sally was looking at his lunch when a pterodactyl flew nearby.
>>>
</code>

===Returning the output given by a function===

When a function takes an input and gives an output such as <code>print</code> in the madlib example, you cannot refer back to the output phrase <code>Sally was looking at his lunch when a pterodactyl flew nearby.</code>
If you would like to use this phrase again, then a function called <code>return</code> is needed.
When <code>return</code> is added at the end of a function, the output is passed back to the caller, which means that you can refer to the output now.

def madlib(name, noun1, noun2, verb):
result = name + " was looking at his " + noun1 + " when a " + noun2 + " " + verb + " nearby."
print result
return result

With this new code, if you need to refer to the sentence <code>Sally was looking at his lunch when a pterodactyl flew nearby.</code>, you can do so by by typing in <code>result</code>.

===Example function===

# This function adds three numbers together and gives back the result using the return keyword
def addNumbers(a, b, c):
return a + b + c

===Conditional Statement===
Lets say you have the following code:

# This function prints every letter in the input
def printLetters(string):
for letter in string:
print letter

What if your ''evil'' TA asks you to only print the vowel letters given in the input?
This means that you need to add a condition to your function where the <code>print</code> function only works if the letter is a vowel.
This can be done by using '''conditional statement'''.

Condtional statement starts with <code>if</code>.

def justvowels(string):
for letter in string:
if letter in "aeiou":
print letter

Instead of printing every letter in the string, this code will only print '''if''' the letter in question can be found in the list "a,e,i,o,u"

==Writing your own Functions==
Writing a function for a task instead of copy and pasting code makes your program much easier to understand and debug.

===Using the function===

We can apply functions to make our code shorter and easier to understand. Here is some example code that we will shorten with functions.

# Calculate the eletric field in two different places
charge = 1.6e-19
origin = vector(0,0,0)

point1 = vector(-10, 5, 15)
point2 = vector(20, -5, 12)

field1 = 9e9 * charge * point1.norm() / point1.mag2
field2 = 9e9 * charge * point2.norm() / point2.mag2

We can use a function in the last 2 lines to reduce the duplicated code to the following

field1 = electricField(charge, point1)
field2 = electricField(charge, point2)

==Frequently Used Functions==

VPython already has a few predefined functions for your ease. The following functions are available for working with vectors.
To better illustrate these functions, let's say we have a vector called exVector.

exVector = vector(-10, 2 ,5)

===mag()===
# Calculates the magnitude of a vector
magExVector = mag(exVector)
print magExVector # will print 11.357

===mag2(A)===
# Calculates the magnitude squared of a vector
mag2ExVector = mag2(exVector)
print magExVector # will print 129

===norm(A)===
# Calculates the unit vector of a vector
unitExVector = norm(exVector)
print unitExVector # will print <-0.88, 0.17, 0.44>

===dot(A, B)===
# Calculates the scalar dot product between the two vectors
# exVector2 = vector(4, -2 ,8)
dotExVector = dot(exVector, exVector2)
print dotExVector # will print 4

===cross(A, B)===
# Calculates the vector cross product between two vectors
# exVector2 = vector(4, -2 ,8)
crossExVector = cross(exVector, exVector2)
print crossExVector # will print <26, 100, 12>

==Example of using these common functions==

<math>\overrightarrow{E}=\frac{1}{4πε_0} \frac{q}{|\overrightarrow{r}|^2} \hat r</math>

The best way to illustrate this is with an example.

For this example, we will create a function to calculate the electric field created by a point charge.

As a reminder, this is the formula we are modeling:

<math>\overrightarrow{E}=\frac{1}{4πε_0} \frac{q}{|\overrightarrow{r}|^2} \hat r</math>

First, we ask what do we need to calculate the electric field?
* <math>q</math> - the charge of the source
* <math>\overrightarrow{r}</math> - the distance from the source to the electric field

<math>q</math> and <math>\overrightarrow{r}</math> will be the two arguments to our function which will look like this:

# Calculates the electric field at a given vector away from the charge
def electricField(q, r):
electricConstant = 9e9
E = electricConstant * q * r.norm() / r.mag2
return E

It is also possible to skip defining the <code>electricConstant * q * r.norm() / r.mag2</code> function and just return it.
def electricField(q, r):
electricConstant = 9e9
return electricConstant * q * r.norm() / r.mag2

== See also ==

===Prerequisites===
[http://www.physicsbook.gatech.edu/VPython VPython]

[http://www.physicsbook.gatech.edu/VPython_basics VPython Basics]

===Further reading===

[http://www.physicsbook.gatech.edu/VPython_Common_Errors_and_Troubleshooting VPython Common Errors and Troubleshooting]

[http://www.physicsbook.gatech.edu/VPython_Lists VPython Lists]

[https://docs.python.org/2/library/functions.html Python standard function library].

==References==

[[Category: Modeling with VPython]]

VPython Functions

2016-11-28T01:24:12Z

Dkim469: /* Basic Python function syntax */