How Strings Work Differently Across Languages in DSA Python - Debugging & Practice

Challenge - 5 Problems

❓ Predict Output

intermediate

String Immutability in Python

What is the output of this Python code snippet?

DSA Python

s = "hello"
s[0] = 'H'
print(s)

A SyntaxError

B hello

C Hello

D TypeError

💡 Hint

Strings in Python cannot be changed after creation.

Strings in Python are immutable. Trying to assign a new character to an index causes a TypeError.

❓ Predict Output

intermediate

String Concatenation Performance

What is the output of this Python code?

DSA Python

result = ''
for i in range(3):
    result += str(i)
print(result)

A ['0', '1', '2']

B 0 1 2

C 012

D Error

💡 Hint

The loop appends string versions of numbers to result.

The loop concatenates '0', '1', and '2' to the empty string, resulting in '012'.

❓ Predict Output

advanced

String Interning Behavior

What is the output of this Python code snippet?

DSA Python

a = 'hello'
b = 'hello'
print(a is b)

A True

B False

C Error

D None

💡 Hint

Python may reuse the same string object for identical literals.

Python interns short strings, so 'a' and 'b' point to the same object, making 'a is b' True.

❓ Predict Output

advanced

Unicode String Length

What is the output of this Python code?

DSA Python

s = 'naïve'
print(len(s))

A 5

B 6

C 4

D Error

💡 Hint

Python 3 strings are Unicode and count characters, not bytes.

The string 'naïve' has 5 characters including the 'ï' which counts as one character.

❓ Predict Output

expert

Mutable vs Immutable String-like Types

What is the output of this Python code?

DSA Python

from array import array
s = 'abc'
a = array('u', s)
a[0] = 'z'
print(''.join(a))

A ValueError

B zbc

C abc

D TypeError

💡 Hint

array of type 'u' supports mutable Unicode characters.

The array of Unicode characters is mutable, so changing index 0 to 'z' works and prints 'zbc'.

Practice

1. In Python, what is the type of a string like "hello"?

easy

A. A mutable list of characters

B. An immutable sequence of characters

C. A fixed-size array of bytes

D. A pointer to a memory address

Solution

1. Step 1: Understand Python string type

   Python strings are sequences of characters that cannot be changed after creation.

2. Step 2: Compare with options

   An immutable sequence of characters correctly describes strings as immutable sequences. Options A, C, and D do not match Python's string behavior.

3. Final Answer:

   An immutable sequence of characters -> Option B

4. Quick Check:

   Python strings are immutable sequences [OK]

Hint: Remember: Python strings cannot be changed after creation [OK]

Common Mistakes:

• Thinking strings are mutable like lists
• Confusing strings with byte arrays
• Assuming strings are pointers

2. Which of the following is the correct way to create a multi-line string in Python?

easy

A. Using triple quotes like '''Hello\nWorld'''

B. Using backslash at end of each line like 'Hello\n\World'

C. Using double quotes only like "Hello\nWorld"

D. Using parentheses without quotes like (Hello\nWorld)

Solution

1. Step 1: Identify Python multi-line string syntax

   Python uses triple quotes (''' or """) to create multi-line strings easily.

2. Step 2: Check options

   Using triple quotes like '''Hello\nWorld''' uses triple quotes correctly. Using backslash at end of each line like 'Hello\n\World' is invalid syntax. Using double quotes only like "Hello\nWorld" creates a single-line string with \n as newline character but not multi-line. Using parentheses without quotes like (Hello\nWorld) is invalid syntax.

3. Final Answer:

   Using triple quotes like '''Hello\nWorld''' -> Option A

4. Quick Check:

   Triple quotes create multi-line strings [OK]

Hint: Use triple quotes for multi-line strings in Python [OK]

Common Mistakes:

• Using backslash incorrectly for multi-line
• Confusing escape sequences with multi-line strings
• Omitting quotes around strings

3. What is the output of this Python code?

text = 'abc'
text[1] = 'z'
print(text)

medium

A. IndexError

B. abc

C. abz

D. TypeError

Solution

1. Step 1: Understand string immutability in Python

   Strings cannot be changed by assigning to an index; this causes an error.

2. Step 2: Identify error type

   Assigning to text[1] causes a TypeError: 'str' object does not support item assignment.

3. Final Answer:

   TypeError -> Option D

4. Quick Check:

   Strings are immutable, so assignment causes TypeError [OK]

Hint: Strings cannot be changed by index assignment in Python [OK]

Common Mistakes:

• Expecting string to change like a list
• Confusing TypeError with IndexError
• Ignoring immutability of strings

4. Find the error in this Python code snippet:

my_str = "hello"
my_str[0] = 'H'
print(my_str)

medium

A. TypeError because strings are immutable

B. SyntaxError due to quotes

C. No error, output is 'Hello'

D. NameError due to undefined variable

Solution

1. Step 1: Check string modification attempt

   The code tries to change the first character of a string, which is not allowed.

2. Step 2: Identify error type

   This causes a TypeError because strings in Python cannot be changed after creation.

3. Final Answer:

   TypeError because strings are immutable -> Option A

4. Quick Check:

   Strings are immutable, so assignment causes TypeError [OK]

Hint: Strings cannot be changed by index assignment in Python [OK]

Common Mistakes:

• Thinking strings behave like lists
• Expecting output 'Hello' without error
• Confusing error types

5. Given the list words = ['apple', 'banana', 'cherry'], which Python code correctly creates a new list with the first letter of each word capitalized, considering strings are immutable?

hard

A. words.map(lambda w: w.capitalize())

B. for word in words: word[0] = word[0].upper()

C. [word[0].upper() + word[1:] for word in words]

D. words = words.upper()

Solution

1. Step 1: Understand string immutability and list comprehension

   Since strings cannot be changed, create new strings with capitalized first letters using list comprehension.

2. Step 2: Analyze options

   [word[0].upper() + word[1:] for word in words] correctly creates new strings by combining uppercase first letter and rest of string. for word in words: word[0] = word[0].upper() tries to assign to string index, causing error. words.map(lambda w: w.capitalize()) uses invalid syntax in Python. words = words.upper() tries to call upper() on list, which is invalid.

3. Final Answer:

   [word[0].upper() + word[1:] for word in words] -> Option C

4. Quick Check:

   Use list comprehension with new strings to handle immutability [OK]

Hint: Use list comprehension to create new strings, not modify in place [OK]

Common Mistakes:

• Trying to assign to string characters
• Using invalid methods on lists
• Confusing Python with other languages' syntax