intro to dictionaries (class slides)

CSc 110 - dictionaries

Dictionaries

Data Structure

  • A data-structure is a way of arranging and organizing data in a computer program

  • Python has several useful data-structures built into the language

    • One is a list (already covered)

    • Another, dictionary

Mapping

  • Many data structures allow data to be stored and retrieved using a concept called mapping

  • Mapping is the process of associating one value with another (a key with a value)

    • Sometimes also referred to as Hashing or Associativity

Mapping

  • Lists map keys to values too!

    • Indices of the list are the keys

    • Elements in the list are the values

  • Keys (indices) are used to acess or modify the elements in the list

Mapping and Lists

numbers = [12, 49, -2, 26, 5, 17, -6]
  • What are the keys?
  • What are the values?
  • Which keys map to which values?

Mapping and Lists

numbers = [12, 49, -2, 26, 5, 17, -6]

# Using the key 3 to lookup the associated value of 26
# and then save the value into variable
new = numbers[3]

# Modifying the list so that the key 5 now maps to 77
# instead of 17
numbers[5] = 77

Dictionary

  • Like lists:

    • Associates a set of keys to their corresponding values
    • Each key has exactly 1 associated value

  • Unlike lists:

    • The keys can be types other than ints: strings

Dictionary

Example (mapping strings to integers)

players = { "Lebron James": 38,
            "Steph Curry":  35,
            "Devin Booker":  12 }
            
# Using the key "Lebron James" 
# to lookup the number 38
players["Lebron James"]

# Modifying the number associated with 
# "Devin Booker" from 12 to 26
players["Devin Booker"] = 26

Evaluate the expressions

word_count = {"and": 324, "why": 134, "cannot": 76, "sanded": 13}
word_count["cannot"] = 90
word_count["and"] = 110
word_count["foot"] = "feet"
word_count["and"] += 10

# what will these evaluate to?
word_count["and"] 
word_count["cannot"]
word_count["foot"]

Evaluate the expressions

word_count = {"and": 324, "why": 134, "cannot": 76, "sanded": 13}
word_count["cannot"] = 90
word_count["and"] = 110
word_count["foot"] = "feet"
word_count["and"] += 10

# what will these evaluate to?
word_count["and"]
120
word_count["cannot"]
90
word_count["foot"]
'feet'

Evaluate the expressions

num_to_player = {}   # A valid, but empty dictionary
num_to_player[13] = "Paul George"
num_to_player[3]  = "Chris Paul"
num_to_player[23] = "Lebron James"
num_to_player[13] = "James Harden"

# what will these evaluate to?
num_to_player[23] 
num_to_player[3]
num_to_player[13]
num_to_player

Evaluate the expressions

num_to_player = {}   # A valid, but empty dictionary
num_to_player[13] = "Paul George"
num_to_player[3]  = "Chris Paul"
num_to_player[23] = "Lebron James"
num_to_player[13] = "James Harden"

# what will these evaluate to?
num_to_player[23]
'Lebron James'
num_to_player[3]
'Chris Paul'
num_to_player[13]
'James Harden'
num_to_player
{13: 'James Harden', 3: 'Chris Paul', 23: 'Lebron James'}

Attendance

Attendance Evaluate the expression on Gradescope.

Review: list methods

  • .append(value)
  • .remove(value)
  • .pop(index)

Dictionary operations

scores = {'A': 10, 'B': 25, 'C': 27, 'D': 10, 'E': 5}
 
scores['A+'] = 7     # Adds a key/value pair

scores['B'] = 20     # Changes value associated with a key

scores['C']    # Retrieves a value, given a key

scores.pop('E')      # Removes a key/value pair

The in operator

With strings:

"a" in "aeiou"
True

With lists:

1 in [1, 2, 3, 4, 5]
True

And dictionary keys:

word_count = {"and": 324, "why": 134, "cannot": 76, "Sanded": 13}
"why" in word_count
True

Write a function

  1. Its name is count_vowels
  2. It takes a string argument
  3. It creates a dictionary
  4. It returns the dictionary with the count of every lowercase vowel in string (iterate over the string with a for loop)
print( count_vowels("") ) # {"a": 0, "e": 0, "i": 0, "o": 0, "u": 0}
print( count_vowels("pineapple") ) # {"a": 1, "e": 2, "i": 1, "o": 0, "u": 0}

Write a function – solution

def count_vowels(string):
  counts = {"a": 0, "e": 0, "i": 0, "o": 0, "u": 0}
  for i in range(len(string)):
    char = string[i]
    if char in counts:
      counts[char] += 1
  return counts

def main():
  print( count_vowels("") ) # {"a": 0, "e": 0, "i": 0, "o": 0, "u": 0}
  print( count_vowels("pineapple") ) # {"a": 1, "e": 2, "i": 1, "o": 0, "u": 0}
  
main()
{'a': 0, 'e': 0, 'i': 0, 'o': 0, 'u': 0}
{'a': 1, 'e': 2, 'i': 1, 'o': 0, 'u': 0}

Write a function

  1. Its name is count_chars
  2. It takes a string argument
  3. It creates a dictionary
  4. It returns the dictionary with the count of every characters in string
print( count_chars("") ) # {}
print( count_chars("banana") ) # {"b": 1, "a": 3, "n": 2}

Write a function – solution

def count_chars(string):
  counts = {}
  for i in range(len(string)):
    char = string[i]
    if char in counts:
      counts[char] += 1
    else:
      counts[char] = 1
      
  return counts

def main():
  print( count_chars("") ) # {}
  print( count_chars("banana") ) # {"b": 1, "a": 3, "n": 2}

main()
{}
{'b': 1, 'a': 3, 'n': 2}

Quiz 07

current time

You have 10 minutes to complete the quiz

  • No need for comments, no need for a main(), no test cases
  • Just write your function and what’s inside the function
  • your choice of loop (for or while)

Built-in functions you can use: round(), input(), float(), str(), int(), len() — you don’t have to use all of these