List Methods

List Methods#

Note

Source: Adapted from the C# edition (lists/listsyntax.rst and arrays/onedim.rst). Python list methods replace C# List<T> methods (Add, Remove, Contains, etc.). Built-in functions sorted(), min(), max(), and sum() replace C# LINQ equivalents.

Python lists come with a rich set of methods for adding, removing, searching, and reordering elements.

Adding Elements#

  • append(x) adds x to the end — the most common way to grow a list.

  • insert(i, x) inserts x before index i.

  • extend(other) appends all elements of other to the list.

words = ["up", "down"]
words.append("over")
print(words)

words.insert(1, "under")
print(words)

words.extend(["in", "out"])
print(words)

Output:

['up', 'down', 'over']
['up', 'under', 'down', 'over']
['up', 'under', 'down', 'over', 'in', 'out']

Removing Elements#

  • remove(x) deletes the first occurrence of x; raises ValueError if not found.

  • pop(i) removes and returns the element at index i (default: last element).

  • del lst[i] removes the element at index i without returning it.

nums = [3, 1, 4, 1, 5]
nums.remove(1)     # removes the first 1
print(nums)

last = nums.pop()  # removes and returns 5
print(last, nums)

Output:

[3, 4, 1, 5]
5 [3, 4, 1]

Searching#

  • index(x) returns the index of the first occurrence of x; raises ValueError if not found.

  • count(x) returns how many times x appears.

nums = [3, 1, 4, 1, 5]
print(nums.index(4))   # 2
print(nums.count(1))   # 2

Sorting and Reversing#

  • sort() sorts the list in place (ascending by default).

  • reverse() reverses the list in place.

nums = [3, 1, 4, 1, 5, 9]
nums.sort()
print(nums)

nums.reverse()
print(nums)

Output:

[1, 1, 3, 4, 5, 9]
[9, 5, 4, 3, 1, 1]

Built-in Functions#

These built-in functions work on any iterable and do not modify the original list:

  • sorted(lst) returns a new sorted list.

  • reversed(lst) returns an iterator over the list in reverse order.

  • min(lst), max(lst) return the smallest and largest element.

  • sum(lst) returns the sum of all elements (numbers only).

nums = [3, 1, 4, 1, 5]
print(sorted(nums))    # new list; nums unchanged
print(min(nums), max(nums), sum(nums))

Output:

[1, 1, 3, 4, 5]
1 5 14

Concatenation and Repetition#

The + operator concatenates two lists into a new one; * repeats a list:

a = [1, 2]
b = [3, 4]
print(a + b)
print(a * 3)

Output:

[1, 2, 3, 4]
[1, 2, 1, 2, 1, 2]

Note that + is different from extend: + produces a new list and leaves both operands unchanged, while extend mutates the list it is called on.