1. Intro to CS Nov 21, 2016

2. Today
For loop and while loop
List and string
Dictionary

3. Looping if you know num_times
use for loop and range()
for x in range(4) :
print(“play song of Skyfall ….”)
Use while loop
c = 0:
while c < 4 :
print(“playing song of Skyfall…”)
c = c + 1

4. Get the sum of numbers steps to compute the sum of integer from 1 to 4
sum = sum + 1
sum = sum + 2
sum = sum + 3
sum = sum + 4
Code
for x in range(5) :
sum = sum + x

5. Looping if you don’t know num_times
response = “”
while reponse != ‘q’ :
response = input(“ One more game? “)
print(“starting next game….”)
Note: this syntax is WRONG!!!
while i in range(1,16,2):
print (i)

6. Useful List methods momList = [‘apple’, ‘orange’, ’bread’]
dadList = [‘shovel’, ‘screw driver’]
momList.append(‘milk’) # add a new item to the end
momList.sort() # reorder and change the original list
momList.sort(reverse=True) # sort in reverse order
myString = ‘- ’.join(momList) # join all items with
# ‘-’ as separator
output: ‘apple-orange-bread’

7. Useful List operators momList = [‘apple’, ‘orange’,’bread’]
dadList = [‘shovel’, ‘screw driver’]
family_list = momList + dadList # combine two lists
print(family_list)
output: ['apple', 'orange', 'bread', 'shovel', 'screw driver']
print(2*momList) # same as momList + momList
output: ['apple', 'orange', 'bread', 'apple', 'orange', 'bread']

8. String myStr = ‘the ring’ index: 0 1 2 3 4 5 6 7
A special case of list. The item is one character.
myStr = ‘the ring’
index:
myStr[1] = ‘h’ myStr[7] = ‘g’
myStr[-1] = ‘g’ myStr[-4] = ‘r’

9. String slice and in operator
myStr = ‘the ring’
string slice
myStr[start : stop : step]
myStr[0:3] = ‘the’ myStr[4:] = ‘ring’
myStr[::] = ‘the ring’ myStr[::-1] = ‘gnir eht’
“in” operator
‘r’ in myStr # output: True

10. More string operators and methods
myStr = ‘the ring’
“+”: link two strings
print(‘Lord of ‘ + myStr)
output: Lord of the ring
“*”: repeat string
print(myStr*3)
output: the ringthe ringthe ring

11. string split() myStr = ‘the ring’
Purpose: break a big string to a list of smaller strings
syntax: myStr.split() # split with space as separator
myStr.split(‘,’) # split with coma as separator
wd_List = myStr.split()
print(wd_list) # output: [‘the’, ‘ring’]
Note: this is used in applications like google translate, or word frequency counting.

12. String looping myStr = ‘the ring’ count how many ‘e’ exists in myStr?
for char in myStr :
if char==‘e’ :
count = count + 1

13. Difference between list and string
momList = [‘apple’, ‘orange’]
List is mutable.
momList[1] = ‘grape’ #Item in a list can be changed.
momList.sort() # list is changed in ascending order

14. Difference between list and string
name = ‘007 Spectre’
String is immutable. You can’t change any letter in a string.
name[2] = ‘8’ # ERROR!!!
A string method, such as name.upper(), will return a new string with uppercase. If you don’t assign the new string to a variable, it is gone. The original one always keeps the same.
So, there is NOT name.sort() !!!!

15. Difference between list and string
s2 = '' for x in s1: if x!='5': s2 += x else: s2 += '6' print(s2)
Difference between list and string
name = ‘007 Spectre’
To change one or more letters in a string: create a new string, copy or add new letters one by one from the old one to the new one.
name2 = ‘'
for x in name:
if x!=‘7':
name2 += x
else:
name2 += ‘8'
print(name2)

16. Dictionary
In Python, a “dictionary” is a data container that stores multiple items of data as a list of key:value pairs
The key:value pairs are unordered
Values are referenced by their associated keys
Key must be unique within a dictionary
Key is typically a string name

17. Dictionary A dictionary in real world English  Spanish ‘one’  ‘uno’
‘two’  ‘dos’
Dictionary in Python: a data structure to associate two things to each other. Called “the key” and “the value”. A dictionary is a collection of key-value pairs.
key: ‘one’ value: ‘uno’
key: ‘two’ value: ‘dos’

18. Dictionary eng2spa = {} # Create an empty dictionary
eng2spa[“one”] = “uno” # add or update entry
print(eng2spa) # output: {'one': 'uno'}
eng2spa[“two”] = “dos” #Add more entries.
eng2spa[“three”] = “tres”
print(eng2spa)
# output: {'one': 'uno‘ , ‘two’ : ’dos’, ‘three’ : ‘tres’}

19. The reason to create dictionary: look up
Look up value by key
eng2spa[‘one’] #output: uno
Find out if a key exists in the dictionary:
‘two’ in eng2spa # output: True
‘five’ in eng2spa # output: False
If you try to look up using a key that doesn’t exist, you get error.
eng2spa[‘HiHiHi’] #KeyError

20. Other functions for dictionary
Len(eng2spa) # number of key-value pairs
# output: 3
Get a list of keys
key_list = list(eng2spa.keys())
for k in key_list:
print(k) #output: [‘one’, ‘two’, ‘three’]
Dictionary.values() returns a list with all values.
val_list = list(eng2spa.values())
for v in val_list :
print(v) #output: [‘uno’, ‘dos’, ‘tres’]

21. Dictionary and list list Use index to access list element.
List is ordered. You can order a list by calling
myList.sort()
Dictionary
Use key to access value.
Dictionary has no concept of order, so sorting a dictionary has no sense. If you print a dictionary a lot of times, you get different result.
But, after you get key_list by calling list(dictionary.keys()), you can sort key_list.

22. Compare & Contrast

23. Empty
myString =""
myList = []
myDict = {}

24. Creation myString = "This is a string. Easy to create"
myList = ['ab',3, 5,99, 'Leland']
myDict = { 4:23, 'mark': , 'mangiare': [1,2,3,'cc'] }

25. Access
myString = "Oh say can you see" letter = myString[4] myList = [3,7, 'ppo', 99, 100] value = myList[ 3] myDict = {23:46, 'pi': , 'linda':'smith' } myDict['pi']
Must be an integer number
Must be a key
All use [ ] to access. Python Beauty!

26. Modify
myString = "Oh sey can you see" myString[4] = 'a' illegal! Strings are immutable. Use myString.replace() myList = ['ab',3, 5,99, 'Leland'] myList[3] = 199 valid! Lists are mutable( only if the element exists ) myDict = { 4:23, 'mark': , 'mangiare': [1,2,3,'cc'] } myDict['mark'] = 'rich' valid! Overwrites current value myDict['albert'] = 'Einstein' valid! Creates a new key:value pair

27. How many myString = "This is a string. Easy to create"
len (myString) number of letters in the string
myList = ['ab',3, 5,99, 'Leland']
len (myList) number of values in the list
myDict = { 4:23, 'mark': , 'mangiare': [1,2,3,'cc'] }
len (myDict) number of key-value pairs in dictionary