Python String Indexing, Slicing and Traversal Class 11: Outputs, Immutability and Palindromes

Understand String Indexing, Slicing, Immutability and Traversal with Outputs (CBSE 11)

w = "madam"
if w == w[::-1]:
    print("Palindrome")
else:
    print("Not a palindrome")   # prints Palindrome

String Indexing, Slicing, Immutability and Traversal with Outputs (CBSE 11): key facts

Index ranges

• •positive 0 to len-1
• •negative -1 to -len
• •outside the range gives IndexError

For length 8 valid indices are -8 to 7.

Slice

• •s[start:stop:step]
• •stop is excluded
• •defaults are the two ends
• •slices never raise IndexError

s[:n] is the first n characters.

Common slices

• •s[::-1] reversed
• •s[::2] every second character
• •s[-n:] last n characters

A negative step reads from right to left.

Immutable

s[i] = x gives TypeError | s = s[:i] + x + s[i+1:] builds a new string

Methods return new strings.

How to answer String Indexing, Slicing, Immutability and Traversal with Outputs (CBSE 11) questions

1. 1Write the string with its indices above it, positive and negative.
2. 2Fill in the defaults: start 0 and stop len for a positive step; start -1 and stop before the first character for a negative step.
3. 3List the indices from start towards stop, excluding stop.
4. 4Read the characters at those indices in order.
5. 5For a loop trace, write the value of the loop variable and the string built so far at each pass.

String Indexing, Slicing, Immutability and Traversal with Outputs (CBSE 11) in real life

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

Tricks to remember String Indexing, Slicing, Immutability and Traversal with Outputs (CBSE 11)

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

• Stop is a fence

  The stop index is the first character you do NOT take.

  Use it when: Slices.

• Minus one reverses

  A step of -1 starts at the end and walks to the front.

  Use it when: s[::-1] and palindromes.

• Strings are sealed

  You can read any character but never replace one; make a new string instead.

  Use it when: TypeError questions.

String Indexing, Slicing, Immutability and Traversal with Outputs (CBSE 11): solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

s = "Python"
r = ""
for i in range(len(s) - 1, -1, -2):
    r += s[i]
print(r)
t = "a1b2c3"
d = ""
for ch in t:
    if ch >= "0" and ch <= "9":
        d += ch
print(d)
s[0] = "J"

s = "ABCD"
for i in range(1, len(s) + 1):
    print(s[:i])
for i in range(len(s) - 1, 0, -1):
    print(s[:i])

Where students lose marks in String Indexing, Slicing, Immutability and Traversal with Outputs (CBSE 11)

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

• ✗Mistake: Including the stop character

  Counting the stop index as part of the slice.

  Wrong working

  s = "COMPUTER"; s[2:5] written as "MPUT"

  Right working

  s[2:5] = "MPU"

  ✓Do this: Stop is excluded.

  Usually costs: 1 mark

• ✗Mistake: Trying to modify a character

  Treating a string like a list.

  Wrong working

  s[0] = "J"

  Right working

  s = "J" + s[1:]

  ✓Do this: Create a new string.

  Usually costs: 1 to 2 marks

• ✗Mistake: Adding a number to a string

  Forgetting that + needs two strings.

  Wrong working

  "Roll" + 7

  Right working

  "Roll" + str(7)

  ✓Do this: Convert with str().

  Usually costs: 1 mark

Practice questions on String Indexing, Slicing, Immutability and Traversal with Outputs (CBSE 11)

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 · MCQBoard level

   For s = "HELLO", what is s[1:4]?
   (a) "HELL"
   (b) "ELL"
   (c) "ELLO"
   (d) "EL"

   Hint

   Indices 1, 2, 3.

   Answer

   (b) "ELL"

2. Q2 · 1 mark · Assertion-reasonHarder board level

   Assertion (A): For s = "COMPUTER", the expression s[10:] gives an empty string and does not raise an error.
   Reason (R): Slicing stops silently at the boundaries of the string, while a single out-of-range index raises IndexError.
   (a) Both A and R are true, and R is the correct explanation of A.
   (b) Both A and R are true, but R is not the correct explanation of A.
   (c) A is true, but R is false.
   (d) A is false, but R is true.

   Hint

   Slice versus index.

   Answer

   (a) Both A and R are true, and R is the correct explanation of A.

3. Q3 · 4 marks · Short answerHarder board level

   Give the output:

   s = "Computer Science"
   print(s[3:8], s[-7:], s[::3], len(s))

   (4 marks)

   Hint

   Count the space as index 8.

   Answer

   Indices: C0 o1 m2 p3 u4 t5 e6 r7 space8 S9 c10 i11 e12 n13 c14 e15, so len = 16. s[3:8] = "puter"; s[-7:] = indices 9 to 15 = "Science"; s[::3] takes indices 0, 3, 6, 9, 12, 15 = "CpeSee". Output: puter Science CpeSee 16

4. Q4 · 3 marks · Short answerHarder board level

   Give the output:

   a = "ab"
   b = "cd"
   print(a + b * 2, (a + b) * 2, "b" in a + b, "bc" in a + b)

   (3 marks)

   Hint

   Repetition is done before concatenation.

   Answer

   b * 2 is "cdcd", so a + b * 2 is "abcdcd"; (a + b) * 2 is "abcdabcd"; "b" in "abcd" is True; "bc" in "abcd" is True. Output: abcdcd abcdabcd True True

5. Q5 · 5 marks · Short answerHOTS / competency

   The following program has two errors. Identify them, correct the program and give its output.

   name = "sunita"
   name[0] = "S"
   print(name + 5)

   (5 marks)

   Hint

   One error is about changing, the other about joining.

   Answer

   Error 1: name[0] = "S" raises TypeError because strings are immutable. Error 2: name + 5 raises TypeError because a string cannot be joined to an int. Corrected: name = "S" + name[1:] and print(name + str(5)). Output: Sunita5

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.What is the index of the last character?Show answer

   -1, or len(s) - 1.

2. 2.Is the stop index included in a slice?Show answer

   No.

3. 3.What does s[::-1] give?Show answer

   The reversed string.

4. 4.What error does s[0] = "x" give?Show answer

   TypeError.

5. 5.Does an out-of-range slice raise an error?Show answer

   No; it gives a shorter or empty string.

String Indexing, Slicing, Immutability and Traversal with Outputs (CBSE 11): frequently asked questions