List Operations

List + operator

• The + operator can be used to concatenate lists

Example

start = [1, 2]
end = [3, 4]
complete = start + end
print(complete)

Appending

• The + operator may be used for appending
• Single items must be represented as a list of length 1

Example

mylist = [1,2,3]

# The following will raise an exception
mylist = mylist + 4

print(mylist)

Exercise

Create a program that builds a list of 10 randomly generated numbers

Solution

import random

nums = []

for _ in range(10):
    nums += [random.randint(1,100)]

print(nums)

List * operator

• The * operator can be used to duplicate lists

Example

mylist = [1, 2, 3]
longlist = mylist * 20
print(longlist)

Slicing

• Lists support slicing as used with strings
• Omitted values will be treated as the start and end of the list

Example

mylist = [1, 2, 3, 4, 5, 6]

print(mylist[1:4])
print(mylist[:4])
print(mylist[1:])

Slices in assignment

• Slices can be used in the left hand side of an assignment

Example

mylist = [1, 2, 3, 4, 5, 6]

mylist[1:4] = [7, 8, 9]

print(mylist)

Copying Lists

• Because lists are mutable, it can be useful to create copies
• The copy method can be used for this
• Slicing syntax ([:]) can also be used

Example

mylist = [1,2,3]
original = mylist.copy()

mylist = mylist + [4]

print(mylist, original)

append

• The append method can be used to add an item to the end of a list
• If lists are passed to append, a nested list will be created

Example

mylist = [1,2,3]

mylist.append(4)

print(mylist)

extend

• The extend method can add many elements to a list
• The argument passed in should be a list

Example

mylist = [1,2,3]

mylist.extend([4, 5, 6])

print(mylist)

sort

• The sort method can be used to sort list elements in place
• The list will be modified by this operation
• sort always returns None

Example

mylist = [4, 1, 3, 2]

mylist.sort()

print(mylist)

In-place modification

• It can be tempting to assign the result of sort back to the original list
• This is not desirable, as it will replace the value with None

Example

mylist = [4, 1, 3, 2]

mylist = mylist.sort()

print(mylist)

Exercise

Create a Python program that prints the three largest numbers in a list

Deleting Elements

• pop will remove and return an element at a position (or the last element by default)
• del can be used to remove the element without returning it (can use slices)
• remove will remove the element by value

pop Example

mylist = [1, 2, 3]

removed = mylist.pop(1)

print(mylist, removed)

del Example

mylist = [1, 2, 3]

del mylist[1]

print(mylist)

remove Example

mylist = [1, 2, 3]

mylist.remove(1)

print(mylist)

Exercise

Create a Python program to remove all single-digit numbers from a list