Lists

In The Data Pile the numbers arrived on paper: a month of recycling weights, and a request for one sentence about them. Every group started with day1 = , day2 = , day3 = , and every group gave up. Twenty variables cannot be looped over, cannot be counted, and cannot be totalled without twenty lines of arithmetic. Data that arrives together should live together.

Many values, one name

minutes = [45, 0, 60, 30, 0, 90, 25]

That is one variable holding seven values in a definite order. In the curriculum’s vocabulary it is a one-dimensional array; in Python it is a list. The values inside are its elements, and the position of each one is its index.

Positions start at zero

print(minutes[0])    # 45 — the first element
print(minutes[6])    # 25 — the seventh and last
print(minutes[-1])   # 25 — the last, counted backwards
print(len(minutes))  # 7 — how many elements there are

The bounds of this list are 0 and 6. Ask for anything outside them and Python stops:

IndexError: list index out of range

minutes[7] is the classic version of that error, because seven elements feels like it should end at seven. It ends at six. Say “len minus one” out loud a few times and it stops catching you.

Growing and changing a list

Lists are not fixed once created — which is the main thing that makes them worth using.

minutes[1] = 20        # replace the element at position 1
minutes.append(15)     # add a new element to the end

sorted_marks = [46, 63, 78, 91]
sorted_marks.insert(2, 70)   # put 70 at position 2, shifting the rest
sorted_marks.remove(63)      # delete the first 63 found
print(sorted_marks)

[46, 70, 78, 91]

insert into a sorted list and remove from the middle are two of the small algorithms the course expects you to be able to design and explain — including the awkward cases. What should remove do when the value is not there? (It raises ValueError.) Where does a new value go when it ties with one already in the list? Decide on purpose, and write the decision down.

Walking the list to find something

Two ways to loop, and the difference matters:

for value in minutes:      # gives you each element
    print(value)
 
for index in range(len(minutes)):   # gives you each position
    print(f"Day {index + 1}: {minutes[index]} minutes")

Use the first when you only care about the values. Use the second when you need to know where you are — which is exactly what a search needs:

marks = [78, 91, 46, 63]
target = 46
position = -1
 
for index in range(len(marks)):
    if marks[index] == target:
        position = index
 
print(position)

2

That is a linear search: check every element, remember where the match was. Starting position at -1 is a convention meaning “not found yet”, so the answer is still meaningful when nothing matches.

A hand trace of the "highest so far" pattern

highest = minutes[0]
for value in minutes:
    if value > highest:
        highest = value

With minutes = [45, 0, 60, 30, 0, 90, 25]:

Pass	value	Is it bigger?	highest after
start	—	—	45
1	45	no	45
2	0	no	45
3	60	yes	60
4	30	no	60
5	0	no	60
6	90	yes	90
7	25	no	90

Starting from minutes[0] rather than from 0 matters: a list of negative numbers would beat a starting value of zero every time and report 0 as the highest — a bug that only shows up on data you did not test with.

Practise all of it in Lists Practice, then read a complete program that turns a list of marks into something a teacher can act on in Working with 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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B3.1

design simple algorithms (e.g., add data to a sorted array, delete a datum from the middle of an array) according to specifications;

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