Working with Lists

Mr. Ferreira teaches two Grade 11 classes and has a fixed habit before reports go home: he reads down the mark list looking for the students he should talk to first. It takes twenty minutes per class and he sometimes misses one. This program does the reading; he still does the talking.

The program

# Mark check-in helper for Mr. Ferreira. It works with marks only —
# no names, no student numbers. He matches the numbers to people
# himself, from his own class list.
 
marks = [78, 91, 46, 63, 88, 52, 39, 70, 84, 59]
check_in_below = 60
 
total = 0
highest = marks[0]
lowest = marks[0]
 
for mark in marks:
    total = total + mark
    if mark > highest:
        highest = mark
    if mark < lowest:
        lowest = mark
 
average = total / len(marks)
 
low_marks = []
for mark in marks:
    if mark < check_in_below:
        low_marks.append(mark)
 
print(f"Marks entered: {len(marks)}")
print(f"Average:       {average:.1f}")
print(f"Highest:       {highest}")
print(f"Lowest:        {lowest}")
print(f"Below {check_in_below}:      {len(low_marks)} of them: {low_marks}")

Marks entered: 10
Average:       67.0
Highest:       91
Lowest:        39
Below 60:      4 of them: [46, 52, 39, 59]

How it works

One list holds ten values under one name, so the program can ask questions of all of them at once. len(marks) is how many there are — which means nothing breaks when Mr. Ferreira pastes in a class of twenty-eight.

The first loop does three jobs in one pass: it accumulates total, and it keeps highest and lowest up to date. Both records start at marks[0] rather than at 0, because a starting value of zero could never be beaten by a low mark, and the program would confidently report a lowest of 0 that nobody earned.

The second loop builds a new list. low_marks starts empty and append adds to it, so after the loop it holds exactly the marks below the threshold — and printing a list prints its brackets and commas for free.

Two loops rather than one is a deliberate choice here. You could do both jobs in a single pass, and it would run imperceptibly faster; two short loops that each do one nameable thing are easier to read, to test, and to explain to Mr. Ferreira. Speed matters when it matters; clarity matters every day.

Why there are no names in this program

A file of marks with names attached is a file that must never be emailed, left on a shared drive, or copied to a memory stick — and the moment it exists, somebody will do one of those things. Marks alone are not personal information, so the program was designed without names on purpose. That is the smallest-data question from Files and Persistence, answered before a line was written.

Change it

1. One line. Set check_in_below to 70. The last line becomes Below 70: 5 of them: [46, 63, 52, 39, 59] — the threshold appears in the output because it was a variable, not a number typed into a print.
2. A few lines. Add a count of marks at 80 or above, using the same pattern as low_marks but with a counter instead of a list. For this data the answer is 3.
3. A real change. Replace the hard-coded list with typed entry: an empty marks = [], then a while loop that asks for a mark, adds it with marks.append(int(entry)) while entry.isdigit(), and stops when the user types done. Everything below the list keeps working untouched — which is the clearest evidence you will get that the rest of the program was written against marks, not against those particular ten numbers.

Practise the patterns in Lists Practice, and read the ideas in Lists.

Curriculum connection

A1.5

describe the structure of one-dimensional arrays and related concepts, including elements, indexes, and bounds;

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A1.6

write programs that declare, initialize, modify, and access one-dimensional arrays.

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A2.3

write algorithms with nested structures (e.g., to count elements in an array, calculate a total, find highest or lowest value, or perform a linear search).

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