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In a predict and run task you do not write code from scratch. You start from a small piece of working code, work out what you think it will do before you run it, and then run it to check whether you were right.

Three students take a spelling test every week, and a program stores their results in a 2D array called scores. Each row is one student: their name first, then their marks in four tests.

scores = [
    ["Bob", 5, 9, 10, 7],
    ["Mary", 4, 10, 3, 6],
    ["James", 7, 6, 4, 8]
]

Every example below starts from this array.

Predict

For every example, study the code carefully and write down a prediction before you touch the Run button. Your prediction should use the key terms 2D array, row, column and index. It helps to draw the array as a table first and number its rows and columns, starting from 0. You can type your prediction straight into the code as a comment, on a line starting with #.

Only once you have written your prediction should you run the code and compare the result with what you expected.

Example 1

scores = [
    ["Bob", 5, 9, 10, 7],
    ["Mary", 4, 10, 3, 6],
    ["James", 7, 6, 4, 8]
]

print(scores[0][0])

Predict the one line this prints. Which row and which column does [0][0] point at?

Example 2

scores = [
    ["Bob", 5, 9, 10, 7],
    ["Mary", 4, 10, 3, 6],
    ["James", 7, 6, 4, 8]
]

print(scores[1][3])

Predict the number this prints. Count carefully: the name takes up column 0, so which test’s mark is in column 3, and whose?

Example 3

scores = [
    ["Bob", 5, 9, 10, 7],
    ["Mary", 4, 10, 3, 6],
    ["James", 7, 6, 4, 8]
]

print(scores[1][0] + " " + str(scores[1][4]))

This one joins two items and a space with +. One of the items is a number, so it is wrapped in str() first. Predict the output.

Answer

Mary 6

scores[1][0] is the text "Mary" and scores[1][4] is the whole number 6. A 2D array can hold text and numbers side by side, and each item keeps its own data type, so the number is wrapped in str() before it is joined onto text with +.

Example 4

scores = [
    ["Bob", 5, 9, 10, 7],
    ["Mary", 4, 10, 3, 6],
    ["James", 7, 6, 4, 8]
]

scores[2][3] = 9
scores[0][3] = 6
scores[1][0] = "Alice"

print(scores)

Before you run it, write out the whole array as it will look after lines 7 to 9 have run, one row per line, the same way it is written at the top. Then run it to check: print(scores) prints the whole array on one line.

Answer

scores = [
    ["Bob", 5, 9, 6, 7],
    ["Alice", 4, 10, 3, 6],
    ["James", 7, 6, 9, 8]
]

James’s third test changes from 4 to 9, Bob’s from 10 to 6, and Mary’s name is replaced by Alice. Each assignment overwrites exactly one item, and the array keeps its three rows of five.

Printed, the whole array comes out on one line:

[['Bob', 5, 9, 6, 7], ['Alice', 4, 10, 3, 6], ['James', 7, 6, 9, 8]]

Help! My code doesn't work!

Make sure that you check for the following things:

The row index comes first and the column index second.
Both indices start at zero, and the name is in column 0, so the first test is in column 1.
Numbers are wrapped in str() before they are joined onto text with a +.