Lists (Arrays)¶
Definition¶
A list contains a number of items which are grouped together under one identifier. Each item in the list has a number associated with it. These index numbers start at zero.
Python doesn’t have arrays, so you will need to use lists.
Note
Lists can store different items with data types, but in general you should try to keep each item’s data types the same.
Easy example¶
colors = ['red', 'green', 'blue']
for color in colors:
print(color)
red
green
blue
Syntax¶
variableName = [item1, item2, item3, ...]
Examples¶
Example 2 - Creating a list with items in¶
shoppingList = ['bread', 'milk', 'sugar']
print(shoppingList)
['bread', 'milk', 'sugar']
Example 3 - Finding the length of a list¶
shoppingList = ['bread', 'milk', 'sugar']
print(len(shoppingList))
3
Note
The length is 3. The item indexes are: 0, 1, 2.
Example 4 - Finding an item in a list¶
shoppingList = ['bread', 'milk', 'sugar']
print(shoppingList[1])
milk
Note
The list starts at index 0 (zero), so shoppingList[1] is the second item in the list.
Example 5 - Updating an item in a list¶
shoppingList = ['bread', 'milk', 'sugar']
print(shoppingList)
shoppingList[1] = 'cream'
print(shoppingList)
['bread', 'milk', 'sugar']
['bread', 'cream', 'sugar']
Example 6 - Sort a list¶
unsortedList = [3, 4, 2, 1, 5]
sortedList = sorted(unsortedList)
print(sortedList)
[1, 2, 3, 4, 5]
Example 7 - Sort a list backwards¶
unsortedList = [3, 4, 2, 1, 5]
sortedList = sorted(unsortedList, reverse=True)
print(sortedList)
[5, 4, 3, 2, 1]
Example 8 - Appending (adding) an item to a list¶
shoppingList = ['bread', 'milk', 'sugar']
shoppingList.append('jam')
print(shoppingList)
['bread', 'milk', 'sugar', 'jam']
Example 9 - Removing an item from a list¶
shoppingList = ['bread', 'milk', 'sugar']
del shoppingList[1]
print(shoppingList)
['bread', 'sugar']
Note
Remember that the list starts at index 0 (zero), so removing item 1 will be the second item in the list.
Example 10 - Seeing if an item is in a list¶
The following program will take a list of shopping items. For milk and jam it will print a customised message based on whether they are needed or not.
shoppingList = ['bread', 'milk', 'sugar']
if 'milk' in shoppingList:
print('We need milk')
else:
print('We don\'t need milk')
if 'jam' in shoppingList:
print('We need jam')
else:
print('We don\'t need jam')
We need milk
We don't need jam
Example 11 - Looping through a list¶
shoppingList = ['bread', 'milk', 'sugar']
for item in shoppingList:
print(item)
bread
milk
sugar
Example 12 - Looping through a list (with range)¶
shoppingList = ['bread', 'milk', 'sugar']
for i in range(len(shoppingList)):
print('Item ' + str(i) + ': ' + shoppingList[i])
Item 0: bread
Item 1: milk
Item 2: sugar
Example 13 - Find the average of a list¶
scores = [72, 78, 83, 56, 89]
total = 0 #initial total which we add to
for score in scores:
total = total + score
print('current total: ' + str(total))
print('final total: ' + str(total))
current total: 72
current total: 150
current total: 233
current total: 289
current total: 378
final total: 378
Example 14 - Building a 2D list¶
tictactoeBoard = [ ['-','-','-'], ['-', '-', '-'], ['-', '-', '-'] ]
print(tictactoeBoard)
[['-', '-', '-'], ['-', '-', '-'], ['-', '-', '-']]
Note
A 2D list (similar to a 2D array) is a list which contains lists. You can think of it as a table.
Example 15 - Accessing and updating an item in a 2D list¶
tictactoeBoard = [ ['-','-','-'], ['-', '-', '-'], ['-', '-', '-'] ]
tictactoeBoard[1][1] = 'x'
tictactoeBoard[1][2] = '0'
for row in tictactoeBoard:
print(row)
[['-', '-', '-'], ['-', 'x', '0'], ['-', '-', '-']]
Example 16 - Looping through a 2D list¶
testScores = [ [53,56,52,53], [82,83,91,87], [63,65,66,62] ]
total = 0
numberOfItems = 0
for scores in testScores:
for individualScore in scores:
total = total + individualScore
numberOfItems = numberOfItems + 1
average = total / numberOfItems
print('Average score: ' + str(average))
Average score: 67.75
Key points¶
Note
A 2D list can be used to form a simple table for a database. Each of the rows would be a record. Example 16 above, for instance, shows four test scores for three different students.
Note
While 2D lists can be used to make tables that store records for a simple database, in general we use SQL, which is far more powerful.
