Answers

F3.5 Two-dimensional arrays

Strings, lists and records · GCSE · OCR J277 2.2.3, AQA 8525 3.2.6, Edexcel 1CP2 6.3.1 · about 15 min

What this lesson is about

Rows and columns: the depth grid as a list of lists.

• A list of lists
• Visiting every item
• The depth grid
• Finding something in the grid

Questions 6 marks in all

1. [1 mark]What does this program print?

   board = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
   print(board[1][2])

   Answer:

   6

   Row 1 is [4, 5, 6], and column 2 of it is 6.

2. [1 mark]What does this program print?

   board = [[1, 2], [3, 4]]
   total = 0
   for row in board:
       for item in row:
           total = total + item
   print(total)

   Answer:

   10

   The two loops visit every item: 1 + 2 + 3 + 4.

3. [1 mark]A 2D array has 8 rows and 8 columns. How many items does it hold?

   Answer: 64. Rows times columns.

4. [1 mark]The depth grid comes as one flat list of 64 readings, row after row. Which slice is row 2?

   1. Aflat[16:24]
   2. Bflat[2:10]
   3. Cflat[8:16]
   4. Dflat[16:32]
   Answer: A. Row r starts at r * 8 and has 8 items, so row 2 is from 16 up to 24.

5. [1 mark]What does this program print?

   grid = [[0, 0], [0, 0]]
   grid[1][0] = 5
   print(grid)

   Answer:

   [[0, 0], [5, 0]]

   grid[1][0] is row 1, column 0.

6. [1 mark]In OCR's Exam Reference Language, how is row 3, column 4 of grid written?

   1. Agrid[3, 4]
   2. Bgrid[3][4]
   3. Cgrid(3, 4)
   4. Dgrid.3.4
   Answer: A. The Reference Language puts both indexes in one pair of brackets. Python and AQA use two.

The task: nearest column

# the two lines every program starts with: the commands, then the robot
from bugbot import *
connect()

flat = tof_grid()
best = min(flat)
print("nearest:", best)

from bugbot import *
connect()
grid = tof_grid()
best = 999
best_col = 0
for row in (2, 3):                     # the level rows; the rows below see the mat
    for col in range(8):
        d = grid[row * 8 + col]
        if d < best:
            best = d
            best_col = col
print(f'nearest: {best} cm in column {best_col}')