Lecture 7

• tuples
• lists

• introduce new compound data types
  • dictionaries

• So far, we can store using separate lists for every info

names = ['Ali', 'Ahmet', 'Fatma', 'Kezban']
grades = ['B', 'A+', 'A', 'A']
courses = [101, 102, 101, 201]

• a separate list for each item
• each list must have the same length
• info stored across lists at same index, each index refers to a different person

names = ['Ali', 'Ahmet', 'Fatma', 'Kezban']
grades = ['B', 'A+', 'A', 'A']
courses = [101, 102, 101, 201]

def get_grade(student):
  i = names.index(student)
  grade = grades[i]
  course = courses[i]
  return (course, grade)
  

print(get_grade("Fatma"))

## (101, 'A')

• messy if have a lot of different info to keep track of
• must maintain many lists and pass them as arguments
• must always index using integers
• must remember to change multiple lists

• Objects of type dict (short for dictionary) are like lists
  • except that we index them using keys
• Think of a dictionary as a set of key/value pairs
• Literals of type dict are enclosed in curly braces
• Each element is written as a key followed by a colon followed by a value

my_dict = {"burkay":41, "ahmet": 23, "ayşe":34}
print(my_dict["ahmet"])

## 23

print(my_dict["ayşe"])

## 34

• The key or the value can be anything

months = {1:"January", 2:"February", 3:"March", 4:"April"}
married = {"burkay":True, "ahmet":False, "ayşe":False}

print("The third month is", months[3])

## The third month is March

for i in married:
  if married[i]:
    print(i, "is married.")
  else:
    print(i, "is not married.")

## burkay is married.
## ahmet is not married.
## ayşe is not married.

• If the key is not found:

married = {"burkay":True, "ahmet":False, "ayşe":False}

print(married["hasan"])

## KeyError: 'hasan'

• Dictionaries are mutable.
  • Add, change or delete key/value pairs.

• To add a new key/value pair, just define it:

married["hasan"] = True
print(married)

## {'burkay': True, 'ahmet': False, 'ayşe': False, 'hasan': True}

• To test existence of a key, use in:

"burkay" in married

## True

"fatma" in married

## False

• To delete a key, use del function:

print(married)

## {'burkay': True, 'ahmet': False, 'ayşe': False, 'hasan': True}

del(married["burkay"])
print(married)

## {'ahmet': False, 'ayşe': False, 'hasan': True}

• You can change an existing value

print(married)

## {'ahmet': False, 'ayşe': False, 'hasan': True}

married["hasan"] = False
print(married)

## {'ahmet': False, 'ayşe': False, 'hasan': False}

• Use the dictionary.get(key, val) function to safely check the value of a key
  • If key exists, returns dictionary[key]
  • Else, returns val

testDict = {1:"a", 2:"b", 3:"c", 4:"d"}

testDict.get(2, "does not exist")

## 'b'

testDict.get(9, "does not exist")

## 'does not exist'

• dictionary[i] does not give you the i^th item in the dictionary
• It returns the value corresponding to key i
  • And, it can be anywhere

squares = {5:25, 4:16, 3:9, 2:4, 1:1}
squares[1]

## 1

squares[5]

## 25

• Write a function that counts the number of letters in a string.

• Write a function that counts the number of letters in a string.

• strategy

  • use a dictionary to keep record of letter counts
  • iterate over the string letter by letter
    • add or update letter count

• Write a function that counts the number of letters in a string.

testString = "this is a long string used for testing purposes"

def letterCount(s):
  d = {}
  for c in s:
    if c in d:
      d[c] += 1   # Same as d[c] = d[c] + 1
    else:
      d[c] = 1
  return d
  
print(letterCount(testString))

## {'t': 4, 'h': 1, 'i': 4, 's': 7, ' ': 8, 'a': 1, 'l': 1, 'o': 3, 'n': 3, 'g': 3, 'r': 3, 'u': 2, 'e': 3, 'd': 1, 'f': 1, 'p': 2}

• You can get all (key, value) pairs as a list of tuples using ’items()`.
• This allows you to convert a dictionary to a list.

d = letterCount(testString)
d2list = list(d.items())
print(d2list)

## [('t', 4), ('h', 1), ('i', 4), ('s', 7), (' ', 8), ('a', 1), ('l', 1), ('o', 3), ('n', 3), ('g', 3), ('r', 3), ('u', 2), ('e', 3), ('d', 1), ('f', 1), ('p', 2)]

print("Value for key", d2list[3][0], "is", d2list[3][1], ".")

## Value for key s is 7 .

• You can get a list of all keys in a dictionary:

d = letterCount(testString)
d.keys()

## dict_keys(['t', 'h', 'i', 's', ' ', 'a', 'l', 'o', 'n', 'g', 'r', 'u', 'e', 'd', 'f', 'p'])

• this returns a dict_keys object which is iterable:

for key in d.keys():
  print(key)

## t
## h
## i
## s
##  
## a
## l
## o
## n
## g
## r
## u
## e
## d
## f
## p

• You can also convert a dict_keys to a list:

keys_list = list(d.keys())
print(keys_list)

## ['t', 'h', 'i', 's', ' ', 'a', 'l', 'o', 'n', 'g', 'r', 'u', 'e', 'd', 'f', 'p']

• You can get a list of all values in a dictionary:

d.values()

## dict_values([4, 1, 4, 7, 8, 1, 1, 3, 3, 3, 3, 2, 3, 1, 1, 2])

• this returns a dict_values object which is iterable:

for value in d.values():
  print(value)

## 4
## 1
## 4
## 7
## 8
## 1
## 1
## 3
## 3
## 3
## 3
## 2
## 3
## 1
## 1
## 2

• You can also convert a dict_values to a list:

values_list = list(d.values())
print(values_list)

## [4, 1, 4, 7, 8, 1, 1, 3, 3, 3, 3, 2, 3, 1, 1, 2]

• values
  • any type (immutable and mutable)
  • can be duplicates
  • dictionary values can be lists, even other dictionaries!
• keys
  • must be unique
  • immutable type (int, float, string, tuple, bool)
  • actually need an object that is hashable, but think of it as immutable as all immutable types are hashable in Python
  • careful with float type as a key
• no order to keys or values!

d = {4:{1:0}, (1,3):"twelve", 'const':[3.14,2.7,8.44]}