Lists Class 11: List Operations, Methods, Nested Lists and List Programs in Python

What Lists is about

Weightage: This chapter is part of Unit II, Computational Thinking and Programming - 1, which carries 45 of the 70 theory marks in the school annual exam together with Introduction to Problem Solving, Getting Started with Python, Flow of Control, Strings, Tuples and Dictionaries and Introduction to Python Modules.Source: CBSE Computer Science (083) syllabus 2026-27, Class XI distribution of marks, Unit II Computational Thinking and Programming - 1

Key concepts

Creating lists and indexing

NCERT section NCERT 9.1

A list is an ordered sequence of items separated by commas inside square brackets: marks = [78, 91, 66]. Items can be of different types (['Asha', 16, 72.5, True]), and a list can contain another list. [] is the empty list. list() converts another sequence into a list: list('abc') = ['a', 'b', 'c'], list(range(3)) = [0, 1, 2].

Items are accessed by index exactly as in strings: 0 to n - 1 from the left and -1 to -n from the right; an index outside the range raises IndexError. len(L) gives the number of items (a nested list counts as one item).

Worked example

L = [10, 'ten', [1, 0], 10.0]. Give len(L), L[1], L[-2], L[2][1], L[1][0].

Try it first, then show the solutionSolution

1. Four items (the inner list is one item): len = 4.
2. L[1] = 'ten'; L[-2] = [1, 0]; L[2][1] = 0.
3. L[1][0] indexes the string 'ten': 't'.

Answer: 4, 'ten', [1, 0], 0, 't'

Lists are mutable

NCERT section NCERT 9.1.2

Unlike strings and tuples, lists are mutable: items can be changed, added and removed in place.

• change one item: L[1] = 99
• change a slice: L[0:2] = ['a', 'b', 'c'] (the slice may be replaced by a list of a different length)
• delete: del L[0] or del L[1:3]

Because the same list object is changed, any other name referring to it sees the change (see copying).

Worked example

Give the output:

L = [5, 10, 15, 20, 25]
L[1] = 11
L[2:4] = [0]
print(L)
del L[-1]
print(L, len(L))

Try it first, then show the solutionSolution

1. After L[1] = 11: [5, 11, 15, 20, 25].
2. Replacing the slice of two items [15, 20] with one item [0]: [5, 11, 0, 25].
3. del L[-1] removes 25.

Answer: [5, 11, 0, 25]
[5, 11, 0] 3

List operations: +, *, in, slicing

NCERT section NCERT 9.2

• Concatenation L1 + L2 makes a new list with the items of both; both operands must be lists ([1] + 2 is a TypeError).
• Repetition L * n repeats the items: [0] * 3 = [0, 0, 0].
• Membership x in L checks whether x is an item of L (not part of an item): 2 in [1, 2, 3] is True; 2 in [[1, 2]] is False.
• Slicing L[start:stop:step] gives a new list, stop excluded, exactly as for strings; L[::-1] is a reversed copy and L[:] is a full copy.

Lists can be compared with == (same items in the same order).

Worked example

A = [1, 2, 3], B = ['x', 'y']. Give A + B, B * 2, 'y' in B, [2, 3] in A, A[::-1], (A + B)[2:4].

Try it first, then show the solutionSolution

1. A + B = [1, 2, 3, 'x', 'y']; B * 2 = ['x', 'y', 'x', 'y'].
2. 'y' is an item: True. [2, 3] is not an item of A (it is a slice): False.
3. A[::-1] = [3, 2, 1]; (A + B)[2:4] = [3, 'x'].

Answer: [1, 2, 3, 'x', 'y'], ['x', 'y', 'x', 'y'], True, False, [3, 2, 1], [3, 'x']

Traversing a list

NCERT section NCERT 9.3

Visit each item with a for loop:

• by item: for x in L: ... (cannot change the list's items through x)
• by index: for i in range(len(L)): ... L[i] (needed to change items in place or to know positions)
• with a while loop: i = 0; while i < len(L): ...; i += 1

To change every item, use the index form: for i in range(len(L)): L[i] = L[i] * 2.

Worked example

Give the output and explain the difference:

L = [1, 2, 3]
for x in L:
    x = x * 10
print(L)
for i in range(len(L)):
    L[i] = L[i] * 10
print(L)

Try it first, then show the solutionSolution

1. In the first loop x is a separate name; changing x does not change the list.
2. In the second loop L[i] = ... changes the list items in place.

Answer: [1, 2, 3]
[10, 20, 30]

Methods that add and remove items

NCERT section NCERT 9.4, Table 9.1

These change the list in place and (except pop) return None.

• append(x): adds x as one item at the end. L.append([4, 5]) adds a single nested list.
• extend(iterable): adds each item of a list (or other sequence) at the end. L.extend([4, 5]) adds 4 and 5; L.extend('ab') adds 'a' and 'b'.
• insert(i, x): puts x before index i (so x ends up at index i). An index beyond the end simply appends.
• remove(x): deletes the first occurrence of the value x; ValueError if x is absent.
• pop([i]): removes and returns the item at index i (default: the last item); IndexError on an empty list or a bad index.
• del L[i] is a statement that deletes by position and returns nothing.

Worked example

Give the output:

L = [3, 6, 9]
L.append([12])
L.extend([15, 18])
L.insert(1, 4)
print(L)
print(L.pop(), L.pop(0))
L.remove(9)
print(L)

Try it first, then show the solutionSolution

1. append adds [12] as one item: [3, 6, 9, [12]].
2. extend adds 15 and 18: [3, 6, 9, [12], 15, 18]; insert 4 at index 1: [3, 4, 6, 9, [12], 15, 18].
3. pop() returns 18; pop(0) returns 3. remove(9) deletes the value 9.

Answer: [3, 4, 6, 9, [12], 15, 18]
18 3
[4, 6, [12], 15]

Counting, searching, ordering and built-in functions

NCERT section NCERT 9.4, Table 9.1

• count(x): number of times x occurs.
• index(x): index of the first occurrence of x; ValueError if absent.
• reverse(): reverses the list in place; returns None.
• sort(): sorts the list in place in ascending order; sort(reverse=True) for descending; returns None. Items must be comparable (mixing int and str raises TypeError). Strings sort by character codes, so uppercase before lowercase.
• sorted(L): a built-in function that returns a new sorted list and leaves L unchanged; works on any sequence.
• min(L), max(L), sum(L): smallest, largest, total (sum needs numbers).
• len(L): number of items; list(seq): makes a list.

A common trap: L = L.sort() makes L None, because sort() returns None.

Worked example

Give the output:

L = [40, 10, 30, 10, 20]
S = sorted(L)
print(S, L)
L.sort(reverse=True)
print(L, L.count(10), L.index(10))
print(min(L), max(L), sum(L) / len(L))
print(L.reverse())

Try it first, then show the solutionSolution

1. sorted returns a new list; L is unchanged.
2. sort(reverse=True) changes L to [40, 30, 20, 10, 10]; 10 occurs twice; its first index is now 3.
3. min 10, max 40, mean 110 / 5 = 22.0.
4. reverse() changes L but returns None, which is printed.

Answer: [10, 10, 20, 30, 40] [40, 10, 30, 10, 20]
[40, 30, 20, 10, 10] 2 3
10 40 22.0
None

Nested lists

NCERT section NCERT 9.5

A list can contain lists, giving a table-like structure: M = [[1, 2, 3], [4, 5, 6]] has 2 rows and 3 columns. M[i] is row i and M[i][j] is the item in row i, column j. len(M) is the number of rows and len(M[0]) the number of columns. Use nested loops to traverse: the outer loop over rows, the inner over the items of each row.

Worked example

marks = [[78, 85, 90], [66, 72, 58], [91, 88, 95]] holds three students' marks in three subjects. Print each student's total and the class total in subject 2 (index 1).

Try it first, then show the solutionSolution

1. marks = [[78, 85, 90], [66, 72, 58], [91, 88, 95]]
   for row in marks:
       print(sum(row), end=' ')
   print()
   t = 0
   for row in marks:
       t += row[1]
   print(t)

2. Totals: 253, 196, 274. Column 1: 85 + 72 + 88 = 245.

Answer: 253 196 274
245

Copying lists (aliasing)

NCERT section NCERT 9.6

B = A does not copy a list: it makes B a second name (an alias) for the same list, so a change through B also shows through A. To get an independent copy of a simple list use any of:

• B = A[:] (slicing)
• B = list(A)
• B = A.copy()

Then A is B is False, though A == B is True until one of them changes.

Worked example

Give the output:

A = [1, 2, 3]
B = A
C = A[:]
B.append(4)
C.append(5)
print(A, B, C)

Try it first, then show the solutionSolution

1. B is the same list as A, so appending 4 through B changes A.
2. C is a separate copy; appending 5 changes only C.

Answer: [1, 2, 3, 4] [1, 2, 3, 4] [1, 2, 3, 5]

Suggested programs: max, min, mean, linear search, frequency

NCERT section Syllabus suggested programs; NCERT 9.8

The syllabus names these programs. Know both the built-in way and the loop way.

• Maximum/minimum without max()/min(): start with big = L[0], then compare every item and update.
• Mean: sum(L) / len(L), or add in a loop.
• Linear search: compare the key with each item from the first; stop (break) when found and report the position; if the loop ends without a match, report not found. In the worst case it checks every item.
• Frequency of elements: for each distinct item, count its occurrences (L.count(x), or a loop), taking care to report each value once.

Worked example

Write a program that does a linear search for 27 in [12, 45, 27, 8, 27] and reports the first position (counting from 1) and the number of comparisons made.

Try it first, then show the solutionSolution

1. L = [12, 45, 27, 8, 27]
   key = 27
   found = False
   for i in range(len(L)):
       if L[i] == key:
           found = True
           break
   if found:
       print('Found at position', i + 1, 'after', i + 1, 'comparisons')
   else:
       print('Not found after', len(L), 'comparisons')

2. 27 is at index 2, so position 3; three comparisons were made.

Answer: Found at position 3 after 3 comparisons

Lists in real life

Each concept, met in an everyday situation. Read the situation, answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Topics in this chapter

• List Methods: append, extend, insert, remove, pop, sort and Output Tracing (CBSE 11)
• Nested Lists, Copying (Aliasing) and List Programs: Max, Mean, Linear Search and Frequency (CBSE 11)

Lists: syntax and key facts

Add items

append(x): one item at the end; extend(seq): each item at the end; insert(i, x): before index i

All return None.

Remove items

remove(x): first value x; pop(i): by index, returns the item (default last); del L[i]

remove → ValueError if absent; pop → IndexError if empty.

Order

L.sort() / L.reverse(): in place, return None; sorted(L): new list

sort(reverse=True) for descending.

Look-up

count(x), index(x), x in L, len(L), min, max, sum

index gives the first occurrence.

Copy

B = A (alias); B = A[:] or list(A) or A.copy() (copy)

Aliases share changes.

Nested access

M[i][j]: row i, column j; rows = len(M), columns = len(M[0])

Use nested loops to traverse.

How the school exam asks Lists

The question patterns to expect, and the method that works for each.

Tricks to remember Lists

Short memory aids. Say each one aloud once, then use it on a question the same day.

• Append = one bag, extend = empty the bag

  append puts the whole bag in as one item; extend tips out its contents one by one.

  Use it when: append versus extend.

• remove by value, pop by position

  Remove the Value; Pop the Position (and pop gives it back).

  Use it when: remove versus pop.

• In-place methods say None

  append, extend, insert, remove, sort, reverse all return None; only pop hands something back.

  Use it when: Output questions that print a method call.

• sorted = new, sort = same

  The '-ed' version gives you a new list.

  Use it when: sort versus sorted.

• B = A is a nickname

  Giving the list a second name does not make a second list.

  Use it when: Aliasing questions.

Lists: exam-level solved questions

Ordered from exam level to HOTS. Cover the solution, write your own, then compare step by step.

L = ['b', 'a', 'd']
M = L
M += ['c']
L.sort()
print(M)
N = sorted(L, reverse=True)
print(N, L is M, N is L)
print(L.remove('a'), L)

Case-study practice (Section E style)

Read the situation, then answer every part. In a school exam set on the board pattern this is a 4-mark question split into 1 + 1 + 1 + 1 marks.

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Where students lose marks in Lists

The slips examiners see most often, and what to write instead.

• ✗Mistake: Writing L = L.sort()

  sort() sorts in place and returns None, so L becomes None.

  Wrong working

  L = [3, 1, 2]
  L = L.sort()
  print(L) → None

  Right working

  L.sort()
  print(L) → [1, 2, 3] (or L = sorted(L))

  ✓Do this: Call in-place methods on their own line.

  Usually costs: 1 mark

• ✗Mistake: Using append to add several items

  append adds its argument as one item, nesting a list.

  Wrong working

  L = [1]; L.append([2, 3]) → [1, 2, 3]

  Right working

  L.append([2, 3]) → [1, [2, 3]]; L.extend([2, 3]) → [1, 2, 3]

  ✓Do this: Use extend (or +=) for several items.

  Usually costs: 1 mark

• ✗Mistake: Passing an index to remove()

  remove takes a value, not a position.

  Wrong working

  L = [10, 20, 30]; L.remove(0) → removes 10

  Right working

  L.remove(0) → ValueError (0 is not in L); use L.pop(0) or del L[0]

  ✓Do this: Position → pop or del; value → remove.

  Usually costs: 1 mark

• ✗Mistake: Assuming B = A copies the list

  Both names refer to the same list.

  Wrong working

  B = A; B[0] = 9; A is unchanged

  Right working

  B = A; B[0] = 9; A[0] is also 9. Use B = A[:] to copy.

  ✓Do this: Copy with [:], list() or copy().

  Usually costs: 1 mark

• ✗Mistake: Removing items from a list while looping over it

  Items shift left and the loop skips some of them.

  Wrong working

  L = [2, 4, 5, 6]
  for x in L:
  if x % 2 == 0: L.remove(x)
  # L becomes [4, 5]

  Right working

  L = [x for x in L if x % 2 != 0] or build a new list in a loop → [5]

  ✓Do this: Build a new list instead of removing during the loop.

  Usually costs: 1-2 marks

NCERT exercise guide: Lists

What each exercise in the NCERT chapter tests, how to approach it, and a question of the same kind solved step by step. Questions here are written in our own words.

nums = [7, 3, 9, 3]
nums.extend([1, 2])
nums.insert(-1, 8)
print(nums)
print(nums.pop(2), nums.index(3), nums.count(3))
print(nums[::-2])

Practice questions on Lists

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

1. Q1 · 1 mark · MCQExam level

   What will L = [1, 2, 3]; L.append([4]); print(len(L)) print? (a) 3 (b) 4 (c) 5 (d) Error

   Hint

   One item added.

   Answer

   (b) 4

2. Q2 · 1 mark · Very short answerHarder exam level

   Give the output: L = [5, 3, 8]; print(sorted(L), L, L.sort())

   Hint

   Arguments are evaluated left to right.

   Answer

   [3, 5, 8] [3, 5, 8] None. All three arguments are evaluated before anything is printed; L.sort() sorts L in place and returns None, so the second item already shows the sorted list.

3. Q3 · 2 marks · Short answerHarder exam level

   Predict the output:

   L = [10, 20, 30, 40, 50]
   L[1:3] = [25]
   print(L, L[-2:], 30 in L)

   Hint

   Two items replaced by one.

   Answer

   [10, 25, 40, 50] [40, 50] False

4. Q4 · 2 marks · Short answerExam level

   Write the difference between remove() and pop() with one example each.

   Hint

   Value or index? What is returned?

   Answer

   remove(x) deletes the first occurrence of the value x and returns None: [1, 2, 1].remove(1) → [2, 1]. pop(i) deletes the item at index i (default last) and returns it: [1, 2, 3].pop() returns 3, leaving [1, 2].

5. Q5 · 2 marks · Short answerExam level

   Write a program to create a list of the squares of the numbers 1 to 10 and print the sum of those squares that are even.

   Hint

   Even squares come from even numbers.

   Answer

   sq = []
   for i in range(1, 11):
   sq.append(i * i)
   s = 0
   for x in sq:
   if x % 2 == 0:
   s += x
   print(sq, s)
   # s = 4 + 16 + 36 + 64 + 100 = 220

6. Q6 · 3 marks · Short answerHOTS / competency

   Predict the output:

   A = [[1, 2], [3, 4]]
   B = A[:]
   B[0] = [9, 9]
   B[1][0] = 7
   print(A)
   print(B)

   Hint

   A[:] copies the outer list only.

   Answer

   [[1, 2], [7, 4]]
   [[9, 9], [7, 4]]
   (Replacing B[0] does not affect A, but B[1] is the same inner list as A[1], so changing B[1][0] shows in A too.)

7. Q7 · 1 mark · MCQHarder exam level

   Assertion (A): After L2 = L1, changing L2[0] also changes L1[0]. Reason (R): Assignment of a list to another name creates an alias, not a copy. Choose (a) both true, R explains A (b) both true, R does not explain A (c) A true, R false (d) A false, R true.

   Hint

   Same object?

   Answer

   (a)

8. Q8 · 3 marks · Short answerHarder exam level

   Write a linear search program that prints all positions (from 1) where 5 occurs in [5, 1, 5, 7, 5] and prints 'Not found' if it never occurs.

   Hint

   Do not break; keep a flag.

   Answer

   L = [5, 1, 5, 7, 5]
   found = False
   for i in range(len(L)):
   if L[i] == 5:
   print(i + 1, end=' ')
   found = True
   if not found:
   print('Not found')
   Output: 1 3 5

9. Q9 · 3 marks · Short answerHOTS / competency

   Why does this code not remove all the zeros? L = [0, 0, 1, 0]; for x in L: if x == 0: L.remove(x). Give the final L and a correct method.

   Hint

   Items shift while the loop index moves on.

   Answer

   Removing an item shifts the rest left, so the loop skips the next item. Final L is [1, 0]. Correct: build a new list: L = [x for x in L if x != 0], or use while 0 in L: L.remove(0).

10. Q10 · 2 marks · Short answerHarder exam level

    Give the output: print([1, 2] * 2 + [3], max(['apple', 'Banana', 'cherry']), sum([1.5, 2.5]))

    Hint

    Strings compare by character codes.

    Answer

    [1, 2, 1, 2, 3] cherry 4.0

11. Q11 · 3 marks · Short answerHOTS / competency

    Write a program to input 5 marks into a list and print the marks in descending order along with the average of the top three.

    Hint

    sorted(..., reverse=True)[:3].

    Answer

    M = []
    for i in range(5):
    M.append(float(input()))
    D = sorted(M, reverse=True)
    print(D)
    print(sum(D[:3]) / 3)

12. Q12 · 1 mark · Very short answerExam level

    What does list('2027') give, and what is its count('2')?

    Hint

    One item per character.

    Answer

    ['2', '0', '2', '7']; count('2') is 2.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1. 1.Are lists mutable?Show answer

   Yes.

2. 2.What does [0] * 4 give?Show answer

   [0, 0, 0, 0].

3. 3.What does pop() with no argument remove?Show answer

   The last item, which it returns.

4. 4.What does L.index(x) raise if x is absent?Show answer

   ValueError.

5. 5.What does sorted() return?Show answer

   A new sorted list.

6. 6.For M = [[1, 2], [3, 4]], what is M[1][0]?Show answer

   3.

7. 7.Which methods add items to a list?Show answer

   append(), extend() and insert().

8. 8.What is the output of print([1, 2].append(3))?Show answer

   None.

Quick revision checklist

The night before the exam: can you do each of these without looking?

• ☐List: mutable ordered sequence in [ ]; any types; nested allowed.
• ☐+ makes a new list; * repeats; in checks items.
• ☐append(x) one item; extend(seq) many; insert(i, x).
• ☐remove(value), pop(index) returns item, del L[i].
• ☐count, index (ValueError if absent), reverse, sort in place (None).
• ☐sorted(L) new list; min, max, sum, len.
• ☐B = A aliases; copy with A[:], list(A) or A.copy().
• ☐M[i][j]: row i, column j; nested loops to traverse.

Lists: frequently asked questions