5.1.6 - Arrays

Arrays are called lists in python

arr = [1, 2, 3]
 
print(arr)

Arrays in python can be used as a stack

arr.append(4)
 
arr.pop()

However, we can also insert values in the middle since its not actually a stack, however this is O(n) operation

arr.insert(1, 7) # inserts 7 at index 1

Indexing an array is stil O(1), so is changing the value at the index

arr[0] = 0

Initialising arrays

To initialise an array of size n with default value of 1

n = 5
 
arr = [1] * n # [1, 1, 1, 1, 1]

Indexing arrays

arr[-1] is actually not out of bounds but the last value

arr = [1, 2, 3]
 
print(arr[-1]) # 3

To read the second last value ⇒ arr[-2]

Array slicing

To get a sublist of an array (last index is non-inclusive)

arr = [1, 2, 3, 4, 5]
 
print(arr[1:3]) # [2, 3]

Unpacking

Taking out all the contents of the array and assigning them

a, b, c = [1, 2, 3]

But we must make sure that the number of variables on the LHS is equal to the number of variables in the array

Looping through arrays

nums = [1, 2, 3]
 
# Using index
for i in range(len(nums)):
	print(nums[i])
 
# Without index
for n in nums:
	print(n)
 
# With index and value
for i, n in enumerate(nums):
	print(i, n)

Zip

If we want to loop through multiple arrays simultaneously

nums1 = [1, 3, 5]
 
nums2 = [2, 4, 6]
 
for n1, n2 in zip(nums1, nums2):
 
print(n1, n2)

Reversing an array

nums = [1, 2, 3]
 
nums.reverse()

Sorting

arr.sort()
 
arr.sort(reverse=True)

Sorting a list of strings using the default sort will sort it in alphabetical order

arr.sort() #alphabetical
 
arr.sort(key=lambda x: len(x)) # Custom sort by the length of the string

List comprehension

arr = [i for i in range(5)] # [0, 1, 2, 3, 4]
 
arr = [i+i for i in range(5)] # [0, 2, 4, 6, 8]

2D lists

arr = [[0] * 4 for i in range(4)]

However, the not so right way to create a 2D list:

arr = [[0] * 4] * 4

This is because each of the 4 rows will be the same and modifying one row will modify all of the rows. We are not creating 4 unique rows.