09-1d-arrays.py
python-for-j277 / data-structures  ·  Lesson 09
Lesson 09 — Data structures

1-D Arrays

Store a whole list of values under one name.

An array — called a list in Python — stores many values under a single name. A one-dimensional array is a simple line of items.

Making and reading an array

read.py
scores = [12, 8, 15, 9]
print(scores)
print(scores[0])
print(scores[2])
Output
[12, 8, 15, 9]
12
15
How it works
  • Square brackets create a list of values.
  • Each item has a position number called an index.
  • Indexing starts at 0, so scores[0] is the first item and scores[2] is the third.

Changing and measuring an array

change.py
scores = [12, 8, 15, 9]
scores[1] = 20
print(scores)
print(len(scores))
Output
[12, 20, 15, 9]
4
How it works
  • scores[1] = 20 replaces the item at index 1.
  • len() tells you how many items the list holds.

Adding items

append.py
shopping = ["bread", "milk"]
shopping.append("eggs")
print(shopping)
Output
['bread', 'milk', 'eggs']
How it works
  • .append() adds a new item to the end of the list.
  • The list grows by one each time you append.

Looping through an array

loop.py
scores = [12, 8, 15, 9]
total = 0
for score in scores:
    total = total + score
print("Total:", total)
print("Average:", total / len(scores))
Output
Total: 44
Average: 11.0
How it works
  • A for-each loop visits every value in the list.
  • We add each one to total.
  • Dividing total by len(scores) gives the average.
Mind the last indexValid indexes run from 0 to len − 1. A list of 4 items has indexes 0, 1, 2, 3. Asking for scores[4] causes an error.
One name, many valuesAn array stores many values under one name; reach any of them with an index in square brackets.

What you have learned

  • A list (array) holds many values in square brackets.
  • Index positions start at 0.
  • len() counts the items; .append() adds one to the end.
  • Change an item with name[index] = value.
  • A for-each loop is the easy way to visit every item.