Arrays

The static arrays used in other languages like C and C++ are not available in Python. Instead, you can use dynamic ones, known by the list type:

>>> l = [1, 2, 3]
>>> len(l)
3

>>> for num in l:
...     print(num)
...
1
2
3
>>> l.append(4)  # [1, 2, 3, 4]

>>> l = [1, 2, "three"]  # it can also contain mixed types

Operations

Let's check the complexity of the most common arrays' operations [1]:

Operation	Time Complexity	Space Complexity	Syntax
Size	$O (1)$	$O (1)$	`len(l)`
Get i-th	$O (1)$	$O (1)$	`l[i]`
Set i-th	$O (1)$	$O (1)$	`l[i] = val`
Copy	$O (n)$	$O (n)$	`[val for val in l]`
Append	$O (1)$	$O (1)$	`l.append(val)`
Pop last	$O (1)$	$O (1)$	`l.pop()`
Pop i-th	$O (n)$	$O (n)$	`l.pop(i)`
Delete value	$O (n)$	$O (n)$	`l.remove(val)`
Iterate	$O (n)$	$O (1)$	`for val in l: print(val)`
Search	$O (n)$	$O (1)$	`val in l`
Minimum	$O (n)$	$O (1)$	`min(l)`
Maximum	$O (n)$	$O (1)$	`max(l)`

Best Practices

Iterate over the list instead of using the index

# Good
for num in nums:
    print(num)

# Not-so-good
for i in range(len(nums)):
    print(nums[i])

Use `enumerate`

# Good
for i, num in enumerate(nums):
    print(i, num)

# Not-so-good
for i in range(len(nums)):
    print(i, nums[i])

Use `enumerate` and `start` argument

# Good
for i, num in enumerate(nums, start=1):
    print(i, num)

# Not-so-good
for i, num in enumerate(nums):
    print(i+1, num)

Use `zip` to iterate over multiple lists

# Good
for num_a, num_b, num_c in zip(lst_a, lst_b, lst_c):
    print(num_a, num_b, num_c)

# Not-so-good
assert len(lst_a) == len(lst_b) == len(lst_c)
for i in range(len(lst_a)):
    print(lst_a[i], lst_b[i], lst_c[i])

Use `reversed` for backward iteration

# Good
for num in reversed(nums):
    print(num)

# Not-so-good
for i in range(len(nums) - 1, -1, -1):
    print(nums[i])

References

[1]: https://wiki.python.org/moin/TimeComplexity

Keyboard shortcuts

algo-ds.com