CPSC150A
Scientific Computing

I Know Where Your Cat Lives

• I Know Where Your Cat Lives

2D Lists

• We already know that lists can contain other lists

  a_list = [[1], [2, 3], [4, 5, 6]]

• 2D lists are lists containing lists with additional restrictions
• All sub lists are the same size and all elements are the same type

• So the above lists is not a 2D list, but the following is

  a_list = [[1, 2, 3], [4, 5, 6]]

• There are two sublists, both of length 3 and both only containing ints
• In memory, a 2D list looks more like the way we defined it above

• But working with them can be simplified if we think of them as a grid, or matrix

  a_list = [[1, 2, 3],
            [4, 5, 6]]

• The grid consists of rows and columns

• So to access an element of the 2D you must specify the row and column of the element

  row = 1
  col = 2
  print(a_list[row][col])

Rows

• Because each sub list is a row, can get an entire row of a 2D list with array access

  a_row = a_list[1]

• Can also get the number of columns by getting the length of a row

  num_cols = len(a_list[0])

• And can use loops to iterate over a row of the list

  row = 1
  for col in range(len(a_list[0])):
      print(a_list[row][col])

Columns

• Because each sub-list is a row, the number of rows is the length of the array

  num_rows = len(a_list)

• Iterating over a column similar to iterating over a row

  col = 1
  for row in range(len(a_list)):
      print(a_list[row][col])

• Getting a column is a little trickier, you can’t get a column with a single array access operation, instead need to iterate

  a_col = []
  col = 1
  for row in range(len(a_list)):
      a_col.append(a_list[row][col])

Activity