Lists, Arrays, Records and 2D Structures

Why We Need Structured Data

Storing one value at a time in separate variables works fine for a handful of items, but quickly becomes unmanageable. A school might need to store scores for 30 students — using 30 separate variables such as score1, score2... score30 is impractical and makes looping impossible. Structured data types group multiple values together under a single name so they can be processed efficiently.

One-Dimensional Arrays — Python Lists

In Python, a list is the standard implementation of a one-dimensional array. Items are written inside square brackets, separated by commas, and accessed by their index (position), which starts at 0.

scores = [72, 85, 91, 64, 78]
print(scores[0])    # 72 (first element)
print(scores[2])    # 91 (third element)
print(scores[len(scores) - 1])   # 78 (last element)

Iterating Over a List

The most natural way to process every item in a list is with a for loop. You can iterate over the items directly, or use range(len(list)) if you need the index:

# Direct iteration (when you don't need the index)
for score in scores:
    print(score)

# Index-based iteration (when you need to modify items)
for i in range(len(scores)):
    scores[i] = scores[i] + 5    # Add 5 bonus marks to each

Appending to a List

Python lists are dynamic — you can add new items using the append method (.append()). This adds the item to the end of the list:

names = []
for i in range(3):
    name = input("Enter name: ")
    names.append(name)
print(names)

Starting with an empty list [] and building it up with .append() inside a loop is a common pattern for collecting user input or building results dynamically.

Two-Dimensional Arrays — Nested Lists

A two-dimensional array is implemented in Python as a list of lists — also called a nested list. Think of it as a grid: the outer index selects the row, the inner index selects the column.

# A 3x3 grid of temperatures (rows = days, columns = times of day)
temps = [
    [12, 15, 14],
    [10, 18, 16],
    [9,  20, 17]
]
print(temps[0][1])   # 15 (row 0, column 1 = Monday noon)
print(temps[2][2])   # 17 (row 2, column 2 = Wednesday evening)

grid = [
    [1, 2, 3],
    [4, 5, 6],
    [7, 8, 9]
]

for row in grid:
    line = ""
    for value in row:
        line = line + str(value) + " "
    print(line)

1 2 3
4 5 6
7 8 9

Records — Lists as a Table of Fields

A record groups related fields under a single name. In the PLS, a record is implemented as a list: each field is stored at a fixed position (index) rather than under a name, so the field order must be documented — usually with a comment.

# A student record as a list
# Field order: [name, age, score, passed]
student = ["Priya Sharma", 16, 88.5, True]

print(student[0])    # Priya Sharma
print(student[2])    # 88.5

# Field order for each record: [name, score]
students = [
    ["Ali", 72],
    ["Beth", 91],
    ["Carlos", 65]
]

for student in students:
    if student[1] >= 70:
        print(student[0], "has passed")