Sequence Nesting

CSCI 1913 – Introduction to Algorithms, Data Structures, and Program Development
Adriana Picoral

Nesting

In python lists and tuples can hold lists, tuples, and strings.

people = [["Ana", 34, "B"],
          ["Peter", 23, "A"]]

How to retrieve first name "Ana"?
How to retrieve first name "Peter"?
How to retrieve the first letter of each first name?
How to retrieve integer 23?

Nesting

How to retrieve first name "Ana"?

people[0][0]

'Ana'

How to retrieve first name "Peter"?

people[1][0]

'Peter'

Nesting

How to retrieve the first letter of each first name?

people[0][0][0]

'A'

people[1][0][0]

'P'

How to retrieve integer 23?

people[1][1]

Nested for loops

We can often use nested for loops to iterate over nested lists, but we don’t always need nested for loops.

def years_to_months(year):
  return year * 12

def change_age(people):
  for person in people:
    person[1] = years_to_months(person[1])
    
def to_string(people):
  new_string = "Name\tAge\tGrade\n"
  for person in people:
    for item in person:
      new_string += f"{item}\t"
    new_string += "\n"
  return new_string

if __name__ == "__main__":
  
  people = [["Ana", 34, "B"],
            ["Peter", 23, "A"],
            ["Mary", 27, "A"],
            ["John", 21, "C"]]
            
  change_age(people)
  print(people)
  
  print(to_string(people))

[['Ana', 408, 'B'], ['Peter', 276, 'A'], ['Mary', 324, 'A'], ['John', 252, 'C']]
Name    Age Grade
Ana 408 B   
Peter   276 A   
Mary    324 A   
John    252 C

Exercise

Write a Python function called calculate_class_average that takes in as argument a nested list representing student grades and calculates the average of the averages. Name your file grade_average.py.

gradebook = [
    [85, 92, 78, 90], # Student 1's grades
    [88, 76, 95, 82], # Student 2's grades  
    [90, 87, 93, 89], # Student 3's grades
    [72, 68, 84, 79]  # Student 4's grades
]

The function should return the average grade across all students in the class (rounded to 2 decimal places).

assert calculate_class_average(gradebook) == 84.25

Solution

def calculate_class_average(grades):
    total = 0
    for student in grades:
        total += sum(student)/len(student)
    return round(total/len(grades), 2)

Nested object change

What will the code below output?

def create_nested_lists(rows, columns):
  one_row = [0] * columns
  return rows * [one_row]

if __name__ == "__main__":
  my_list = create_nested_lists(3, 5)
  my_list[0][0] = -1
  print(my_list)

Nested object change

What will the code below output?

def create_nested_lists(rows, columns):
  one_row = [0] * columns
  return rows * [one_row]

if __name__ == "__main__":
  my_list = create_nested_lists(3, 5)
  my_list[0][0] = -1
  print(my_list)

[[-1, 0, 0, 0, 0], [-1, 0, 0, 0, 0], [-1, 0, 0, 0, 0]]

Same problem

def create_nested_lists(rows, columns):
  return rows * [[0] * columns]

if __name__ == "__main__":
  my_list = create_nested_lists(3, 5)
  my_list[0][0] = -1
  print(my_list)

[[-1, 0, 0, 0, 0], [-1, 0, 0, 0, 0], [-1, 0, 0, 0, 0]]

Solution for the previous problem

def create_nested_lists(rows, columns):
  new_list = []
  for _ in range(rows):
    new_list.append([0] * columns)
  return new_list

if __name__ == "__main__":
  my_list = create_nested_lists(3, 5)
  my_list[0][0] = -1
  print(my_list)

[[-1, 0, 0, 0, 0], [0, 0, 0, 0, 0], [0, 0, 0, 0, 0]]

Solution for the previous problem

With list comprehension:

def create_nested_lists(rows, columns):
  return [ [0] * columns for _ in range(rows)]

if __name__ == "__main__":
  my_list = create_nested_lists(3, 5)
  my_list[0][0] = -1
  print(my_list)

[[-1, 0, 0, 0, 0], [0, 0, 0, 0, 0], [0, 0, 0, 0, 0]]

Deep copy vs. shallow copy

When copying a list, the nested lists will still refer to the same objects as in the original list.

def create_nested_lists(rows, columns):
  return [ [0] * columns for _ in range(rows)]

if __name__ == "__main__":
  my_list = create_nested_lists(3, 5)
  print(my_list)
  
  another_list = my_list.copy()
  
  another_list[0][0] = -1
  
  print(my_list)

[[0, 0, 0, 0, 0], [0, 0, 0, 0, 0], [0, 0, 0, 0, 0]]
[[-1, 0, 0, 0, 0], [0, 0, 0, 0, 0], [0, 0, 0, 0, 0]]

Deep copy

To recursively copy all objects, you need a deep copy.

import copy

def create_nested_lists(rows, columns):
  return [ [0] * columns for _ in range(rows)]

if __name__ == "__main__":
  my_list = create_nested_lists(3, 5)
  print(my_list)
  
  another_list = copy.deepcopy(my_list)
  
  another_list[0][0] = -1
  
  print(my_list)

[[0, 0, 0, 0, 0], [0, 0, 0, 0, 0], [0, 0, 0, 0, 0]]
[[0, 0, 0, 0, 0], [0, 0, 0, 0, 0], [0, 0, 0, 0, 0]]