First day of Python!

1. ACM init()
Day 1: April 15, 2015
1
Monty

2. Welcome!
Welcome to ACM init(), a Computer Science class for
beginners
init() is a several week long class intended to introduce you to
a language called Python

3. About init()
• No prior programming
experience needed! In fact, it
is assumed you have none
• Hosted by the Association for
Computing Machinery (ACM)
• Meets once a week, 6-8pm.
Days TBA
• You will learn how to code in a
language called Python

4. Resources for you
• ACM's website: uclaacm.com
• Slides posted at: slideshare.net/uclaacm
• ACM Facebook group: https://www.facebook.com/
groups/uclaacm/
• init() Facebook group: https://www.facebook.com/
groups/uclaacminit/
• My email: omalleyk@ucla.edu

5. Why Computer Science?
• Computer Science is a fast
growing field with a surplus of
high-paying jobs
• Learning a programming
language will enhance your
resume and make you more
desirable to companies (or
grad schools!)
• Learning how to code will
better your problem-solving
skills and help you in all of
your classes
Source: http://www.ovrdrv.com/blog/making-computer-science-cool/

6. The numbers speak for
themselves...
Source: http://code.org/promote

7. TIOBE Index

8. TIOBE Index
The chart on the previous page demonstrates the value of
Python. It is the 8th most popular programming language in
the world right now!
!
The TIOBE Index ranks the popularity of the top 100
programming languages based on number of job
advertisements.

9. Why learn Python?
• Python is easy to read and
use, and is considered to be
one of the best first languages
to learn
• Python developers are in high
demand
• Python is useful-- many
different types of applications
use Python
• Most important-- its logo is
UCLA's colors!

10. Python demo- factorial
• Factorial: written as #!
• For example, 5! is 5*4*3*2*1 = 120
• I wrote a simple program called fact.rb that will
compute the factorial for us
• Let's try it out! Compute the factorial of 6.

11. Factorial

12. A bit about computers
Computers think in 0s and 1s. Everything that you input into
a computer gets stored in this manner. For example:
You don't need to know how this conversion works. However, the concept is
important because it is the reason why we need programming languages.
Human-readable programming languages go through a process that
converts them to bytes, which can be understood by the computer.

13. How do we make our code
work?
We can't just type code into a word document and hope it
will magically do something. We need a tool that will help
the computer understand our code.

14. koding.com

15. Enter your email and choose a username

16. This is what you should see once you log in

17. Running your first Python program: type print "Hello world!"
into the top box

18. Now, save the file. Select the option Save As.

19. Name the file 'hello.py,' then click Save! Always use '.py' for
Python files.

20. Now we'll run it! Type python hello.py in the bottom window.

21. Press enter, and the program will run!
That's all it takes! All Python programs will be run using
python filename.py

22. Now you know how to run
Python programs. Let's
learn how to write them!

23. Data types
• There are three main data types in Python: numbers,
booleans, and strings.
• Numbers: Exactly what it sounds like. (example: 1, 400,
45.7)
• Booleans: Something that is either True or False (note the
capitals).
• String: A word or phrase. (example: "Hello World!")

24. Data types
It's important to know the difference between data types
because they are written and used differently.
5 = number
"5" = string
!
True = boolean
"True" = string

25. Variables
We can store these data types in variables so we can come
back and use them again later.
Take a look:
Now if we want to use any of these stored values
again all we have to do is type the variable name!

26. Variables
Some things to notice:
!
A number is just a regular number like 3 or 42.
!
A boolean can only be True or False.
!
A string must have quotes around it. Example: "This is my
string."

27. Variables
Looking at this example again, see that I called my variables
my_num, my_string, etc. There are several naming
conventions to observe with variables.
!
• Only start a variable with a lowercase letter.
• No spaces in variable names. For multiple words, use '_'.
• Don't use weird symbols like # or @.
• Ok: this_is_a_variable = 3
• Bad: @@hi guys! = 3

28. Quick topic: printing
We'll often want to print to the computer screen. The command
for this is print

29. More about printing
What if we try this?
Surprise- it won't work. We can fix it!
To access a variable within a string, use a %s inside the " " and
put the variable name as % (variable_name) after.

30. Strings are special
Remember, a string is a word or phrase.
It's always written in " "

31. String pitfalls
Make sure to always use double quotes " "
!
If you use single quotes ' ' for strings it will work, but then if you
ever try to use a contraction things will go wrong!

32. String methods
Python has some special built-in functions that we can use for
strings.
!
They are:
!
len()!
lower()!
upper()!
!

33. len()
len() gets the length/ number of characters in a string.
Note that it is lowercase.
!
Put the string you are trying to find the length of in the
parenthesis.
The length is 35 if you were wondering.

34. lower()
lower() converts a string to all lowercase.
Note that lower() is used by typing string.lower()
This will not work with len()

35. upper()
upper() converts a string to uppercase (surprise!)
upper() is also called using string.upper()

36. Math
There are six main math operations we are going to go over
today:

37. + - / *
• + is addition
• - is subtraction
• * is multiplication
• / is division
• These work the same way
your calculator does -- no
surprises here

38. ** and %
• ** (exponent) raises one
number to the power of
another. For example, 2**3
would be 8 because 2*2*2=8
• % (modulus) returns the
remainder of division. For
example, 25%7 would be 4
because 25/7 is 3r4.

39. Comments
What if you write a really long program then don't look at it
for a year? What if you're working on a project with other
people and you have to share your code?
!
We need a way to explain what is going on in the programs
we write.
!
The solution? Comments.

40. How to make a comment
Comments are a tool we have to communicate what our
code is intended to do.
!
To put a comment in your code all you have to do is type
# your comment here!
!
Anything that comes after '#' and is on the same line will be
ignored when the code is being run.

41. Comment example

42. This is from a HW assignment of mine.
The green lines are comments.
Use them!
!
Note: this is not Python

43. What we covered
• A little bit about Python
• How to use Koding.com
• Data types: numbers, string, and booleans
• Variables and naming conventions
• Printing
• String methods
• Math operators
• Comments

44. A cool example: hangman.py

45. from random import choice
!!!!!def find(lst, a):
! result = []
! for i, x in enumerate(lst):
! if x == a:
! result.append(i)
! return result
!!!!!words = ['rhythms', 'tree', 'constant', 'manager', 'training', 'hotel', 'destroy']
!word = choice(words)
!count = 0
!chances = 4
!letterlist = list(word)
!dashlist = []
!for l in letterlist:
! dashlist.append('-')
!print 'Welcome to HangmannnGuess the: ' + str(len(letterlist)) + ' letter word.n'
!print ' '.join(dashlist) + 'nn'
!while count <= chances:
! chancesleft = chances - count
! let = raw_input("Enter a letter: ")
! if let in letterlist:
! indexes = find(letterlist, let)
! for i in indexes:
! dashlist[i] = let
! print 'n' + ' '.join(dashlist) + 'n' + 'Good guess!n'
! else:
! print 'Wrong! Try again!'
! count += 1
! print 'Chances left: ' + str(chancesleft) + 'n'
! combined = ''.join(dashlist)
! if combined == word:
! print 'You win! The word was: ' + word
! break
! if count > chances:
! print 'Game over. You lose! The word was: ' + word
! break
Here is the code for hangman.py
!
You don't need to understand this
right now, but it's a cool example
of what you can do.