Lecture-25 2d List_create 2024-25

COMP101 Introduction to Programming 2024-25

Lecture 25: 2-D Lists

Overview

• Focus on creating 2-dimensional (2-D) lists.

• Various methods of initialization and their implications.

List Dimensions

• Definition: A 2-D list has multiple rows and columns.

• Example: A list dimension of 4 x 7 has 4 rows and 7 columns (total of 28 elements).

• Structure representation:

  • Rows and columns indexed in a grid format:

  0	1	2	3	4	5	6
  0,0	0,1	0,2	0,3	0,4	0,5	0,6
  1,0	1,1	1,2	1,3	1,4	1,5	1,6
  2,0	2,1	2,2	2,3	2,4	2,5	2,6
  3,0	3,1	3,2	3,3	3,4	3,5	3,6

• Key Terms:

  • Dimension: Size of the 2-D list.

  • Element: A specific cell accessed via two indexes.

  • Addressing: Accessing elements in 'row-column order'.

    • First element: [0,0]

    • Last element: [3,6]

  • Homogeneous: All elements must hold the same data type.

Creating a 2-D List

• Methods:

  1. Static Creation: Values defined by the programmer.

  2. Automatic Creation: Values generated by the software.

     • Techniques:

       • List comprehension (efficient for large lists).

       • Generator (suitable for very large lists).

       • Dynamic (not recommended due to side effects).

Different Types of 2-D Lists

• Types:

  • Null List: All elements initialized to zero (most common).

  • Unary List: All elements initialized to one.

  • Dense List: Most elements carry a value.

  • Sparse List: Majority of elements are zero.

  • Diagonal List: Only diagonal elements have the same value.

Static Creation of Lists

Creating a Null List

• Example of a 2x2 null list:

  null_list_1 = [[0], [0]]
  num_list_2 = [[0], [0]]
  num_list_both = [num_list_1, num_list_2]
  print(num_list_both)  # Outputs: [[0,0], [0,0]]

• Revised better way:

  arrayNull = [[0, 0], [0, 0]]
  print(arrayNull)  # Outputs: [[0,0],[0,0]]

Additional Creation Examples

• Example for a 3x4 Null List:

  list_3x4_null = [[0, 0, 0, 0], [0, 0, 0, 0], [0, 0, 0, 0]]
  print(list_3x4_null)  # Outputs: [[0,0,0,0],[0,0,0,0],[0,0,0,0]]

Populating Lists

Populated from 1-D Lists

• Example:

  num_list_1 = [1, 2, 3]
  num_list_2 = [4, 5, 6]
  num_list_both = [num_list_1, num_list_2]
  print(num_list_both)  # Outputs: [[1, 2, 3], [4, 5, 6]]

Creating Specific Dimensions

• Example for a 2x3 List:

  list_2x3 = [[1, 2, 3], [4, 5, 6]]
  print(list_2x3)  # Outputs: [[1, 2, 3], [4, 5, 6]]

Advanced List Populating Methods

Using List Comprehension

• Creates a null list:

  num_of_rows = 4
  num_of_cols = 4
  list_comp = [[0 for row in range(num_of_rows)] for col in range(num_of_cols)]
  print(list_comp)  # Outputs: [[0,0,0,0],[0,0,0,0],[0,0,0,0],[0,0,0,0]]

Using Generators for Null Lists

• Example:

  list_gen = [[0] * num_of_cols for i in range(num_of_rows)]
  print(list_gen)  # Outputs a 4x4 list of nulls.

Diagonal Population of Lists

• Populating diagonal elements:

  list_diag_4x4 = [[0 for row in range(4)] for col in range(4)]
  for i in range(4):
      list_diag_4x4[i][i] = 1
  for row in list_diag_4x4:
      print(row)  # Outputs the diagonal populated list.

Caution Against Dynamic Methods

• Example (which should be avoided):

  list_null = [[0] * col] * row

• This creates references, thus modifying one element affects all; this leads to unexpected results.