A list is Python's most commonly used data structure — an ordered, changeable collection that can hold items of any type, including a mix of types.
Creating a List
R Code
ages = [20, 22, 19, 25] mixed = ["Rafi", 23, True] # lists can mix types print(ages) print(mixed)
Output
[20, 22, 19, 25] ['Rafi', 23, True]
Indexing — Python Starts at 0
R Code
fruits = ["apple", "banana", "mango", "grape", "kiwi"] print(fruits[0]) # first element print(fruits[-1]) # last element print(fruits[1:4]) # slice: index 1 up to (not including) 4
Output
apple kiwi ['banana', 'mango', 'grape']
⚠ Common Mistake
Python indexing starts at 0, not 1 (unlike R). fruits[1] is the SECOND item, not the first. This is one of the most common early bugs when switching between R and Python.Modifying a List
R Code
fruits = ["apple", "banana", "mango"] fruits.append("kiwi") # add to the end fruits.insert(1, "grape") # insert at a specific position fruits.remove("banana") # remove by value fruits[0] = "apricot" # update by index print(fruits)
Output
['apricot', 'grape', 'mango', 'kiwi']
Useful List Methods
R Code
nums = [23, 9, 45, 12, 9, 30] print(len(nums)) # number of elements print(sum(nums)) # total print(max(nums), min(nums)) print(sorted(nums)) # new sorted list, original unchanged nums.sort(reverse=True) # sorts the list itself, descending print(nums) print(nums.count(9)) # how many times 9 appears
Output
6 128 45 9 [9, 9, 12, 23, 30, 45] [45, 30, 23, 12, 9, 9] 2
Copying a List Safely
R Code
original = [1, 2, 3] shallow = original # NOT a copy - same list, two names real_copy = original.copy() # an actual independent copy shallow.append(4) print(original) # changed too! shallow was never a separate list print(real_copy)
Output
[1, 2, 3, 4] [1, 2, 3]
⚠ Common Mistake
new_list = old_list does NOT create a copy - it creates a second name pointing at the exact same list in memory. Modifying one modifies the other. Always use .copy() (or list(old_list)) when you need an independent list.Advanced: List Comprehension
🚀 Advanced Example
A list comprehension builds a new list in a single, readable line - the most 'Pythonic' way to transform or filter a list, and usually faster than an equivalent for loop.R Code
nums = [1, 2, 3, 4, 5, 6] squares = [x**2 for x in nums] evens_only = [x for x in nums if x % 2 == 0] print(squares) print(evens_only)
Output
[1, 4, 9, 16, 25, 36] [2, 4, 6]
Key Points
- Lists are ordered, changeable, and can hold mixed types.
- Indexing starts at 0; negative indices count from the end; slicing uses start:stop (stop is excluded).
- Use .copy() to get an independent copy - plain assignment just creates a second name for the same list.
- A list comprehension [expr for item in list if condition] is the idiomatic way to transform or filter a list.