1. Python unit_4 review Tue/Wed, Dec 1-2

2. Four Python data types int: age = 20 float: pi = 3.14159 String:
name = ‘Alice’ zipcode=‘95120’
Boolean:
isCold = True

3. Arithmetic operator + Addition - Subtraction * Multiplication
/ Division
% Modulus
if x % 2 ==  odd number
if x % 2 ==  even number
** Exponent
5**3 = 125

4. Assignment operator a = (a + b)  a += b a = (a – b)  a -= b

5. Data types and operators
Numeric types can use all arithmetic operators
If mixing string and numeric, be careful !!!
print(3+5.5) # ok. Output: 8.5
print(3+’5.5’) # error: unsupported type
print(‘3’+’5.5’) # ok. String operation
# output: 35.5
print( ‘Hi’*5) #ok. Output: HiHiHiHiHi
print(‘Hi’+5) # error.

6. Built-in functions for type conversion
int(x): convert x to a integer
example: int(‘5’)
float(x): convert x to a decimal
example: float(‘2.8’)
str(x): convert x to a string
example: str(5)

7. Type conversion example
print(3+’5’) #error
print(3+int(‘5’)) #ok
print(‘Hi’ + 5) #error
print(‘Hi’ + str(5)) #ok. Output: Hi5

8. Built-in function for keyboard input
Example_1
print(“Please enter your username: “)
x=input()
Example_2
x = input(“Please enter your username: “)

9. Keyboard input (cont’)
input() always returns a string.
If you want the result to be a number, use
int(input()) or
float(input())
x=input(“How many books you will buy: “)
num_books = int(x) #use int() to convert
# string to int
price = num_books*22.99

10. Python while loop while loop_condition : statement_1 statement_2
Note:
Check loop_condition, execute statements.
Repeat above, until loop_condition becomes False.

11. while LOOP sample
count = 1
while count < 11 :
count = count + 1

12. Infinite while loop
c = 0
while c< 10 :
print (“Hello !!!”)

13. If you don’t want an infinite loop
c = 0
while c< 10 :
print (“Hello !!!”)
c = c + 1
Use “break” to break out of a loop.
while True :
if c > 9 :
break

14. range() range(start, stop, step)
A quick way to create a list of integers
The first integer = start
Last integer = stop – 1
Example:
range(4)  [0,1,2,3]
range(1, 5)  [1,2,3,4]
range(2,7,2)  [2,4,6]

15. For loop and range() Print “Hello” 4 times
for x in range(4) : # x will be integer 0,1,2,3 one by one
print(“Hello”)
Print the sum of integers from 2 to 8
sum = 0
for x in range(2,9) : # x will be integer 2,3,4,5,6,7,8
sum = sum + x
print(sum)

16. list data structure Len() : how many elements in the list?
Use index to access data (positive or negative index)
myList = [34, 26, ‘abc’, 56, ‘xy’]
| | | | |
index:
or :
in operator
if ‘abc’ in myList
Len() : how many elements in the list?
+ : merge two lists
myList * 3 : repeat myList 3 times

17. list traverse in order
print(myList)
for x in myList :
print(x)

18. list slice syntax: myList[start : stop : step] ending index = stop - 1
if start is missing, start from beginning
if stop is missing, to the end
if step is missing, step = 1
myList[::]  get all list elements in order
myList[::-1]  get all list elements in reverse order

19. Useful List methods
myList.append(‘something’) # add a new item to the end
myList.sort() # reorder and change the original list
myList.sort(reverse=True) # sort in reverse order
myString = ‘- ’.join(myList) # join all items with
# ‘-’ as separator

20. Display partial list langs = ["C", "C++", “XML", "Python"]
print(langs[2:])
output: “XML”
“Python”
for x in langs[2:] :
print(x + “ is cool”)
Output: XML is cool
Python is cool

21. Modify list member: must use index
Question: how to modify the last two elements to “Java”?
langs = ["C", "C++", "Perl", "Python"]
langs[2] = "Perl“ langs[3] = "Python“
range(2,4) gives an integer list [2,3]
To update the last two elements to “Java”
for x in range(2,4) :
langs[x] = “Java”
output: ['C', 'C++', 'Java', 'Java']

22. String myStr = ‘the ring’ index: 0 1 2 3 4 5 6 7
myStr[start : stop : step]
“in” operator
‘r’ in myStr
“+”: link two strings
“*”: repeat string
print(myStr*3)

23. String looping myStr = ‘the ring’ count = 0 for char in myStr :
if char==‘e’ :
count = count + 1

24. But string is immutable!!!
name = ‘007 Spectre’
You can’t change any letter in a string.
name[2] = ‘8’ # ERROR!!!!!!

25. String replace() Python provides a built-in function
myStr.replace(old, new, count)
old -- The old substring to be replaced.
new --The new substring, which would replace old substring.
count – an optional argument.
Note: for all string methods result, assign it to something.
s1 = myStr.replace(old, new, count)
Or myStr = myStr.replace(old, new, count)

26. Dictionary eng2spa = {} # Create an empty dictionary
eng2spa[key] = value
create a new entry or
Replace the value if the key already exists.
key in eng2spa # output: True or False
Key not in eng2spa # output: True or False
Len(eng2spa)
key_list = list(eng2spa.keys()) # get list of keys
val_list = list(eng2spa.values()) # get list of values

27. Function----def foo() :
def play_music() :
print(“Start playing Hello…”)
def find_route_to_home() :
print(“open google map. Point to home.”)
play_music()
find_route_to_home()

28. def foo(arg) : def play_music(music_name) :
print(“Start playing: “, music_name)
play_music(‘Hello’)
play_music(‘Skyfall’)
play_music(‘Happy’)

29. def foo(arg1, arg2) : def get_avg(x, y) : avg = (x+y)/2
print(“average is: “, avg)
get_avg(5, 8)
get_avg(22, 34)
Note: function result can’t be assigned to a variable !!!
x = get_avg(5, 8) # (error !!!)

30. Solution: def foo(arg1, arg2): return
def get_avg(x, y) :
avg = (x+y)/2
return avg
x = get_avg(5, 8)
y = get_avg(3.4, 2.7)

31. Reminder Show dictionary_proj today: Dec1:2nd/Dec2:3rd
Next Project will be Banking. Coming soon.
Test: Fri or Monday. Open notes, but handwritten notes only.

32. Stanford Splash !!
at36
April 2016.
Class try-on for high schooler

33. Extra question just for fun
Write a function that reverses the order of the words in a string. For instance, the string “To be or not to be, that is a question. “ is changed to “question. A is that be, to not or be To”. Assume that all words are space delimited and treat punctuation the same as letters.
You are given two fuses and a lighter. When lit, each fuse takes exactly one hour to burn from one end to the other. The fuses do not burn at a constant rate, though, and they are not identical. In other words, you may take no assumptions about the relationship between the length and the time it will take to burn. Two equal length of fuse might take different time to burn. Using only the fuses and the lighter, measure a period of exactly 45 minutes.