String Basics and Memory Representation in DSA Python - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of String Slicing and Concatenation

What is the output of the following Python code?

DSA Python

s = "hello"
result = s[1:4] + s[0]
print(result)

A "helh"

B "ello"

C "elloh"

D "ellh"

💡 Hint

Remember string slicing s[start:end] includes start but excludes end.

The slice s[1:4] takes characters from index 1 to 3, which are 'e', 'l', 'l'. Adding s[0] which is 'h' results in 'ellh'.

🧠 Conceptual

intermediate

Understanding String Immutability

Which statement best describes why strings in Python are immutable?

A Because changing a string would require creating a new string object in memory.

B Because strings are stored in a fixed-size memory block that cannot be changed.

C Because strings are stored as linked lists of characters that cannot be altered.

D Because strings are mutable but Python hides this from the user.

💡 Hint

Think about what happens when you try to change a character in a string.

Strings are immutable because any change creates a new string object; the original string's memory cannot be altered.

❓ Predict Output

advanced

Output of String Interning Behavior

What is the output of this code snippet?

DSA Python

a = "python"
b = "python"
c = ''.join(['p', 'y', 't', 'h', 'o', 'n'])
print(a is b)
print(a is c)

A True\nTrue

B True\nFalse

C False\nFalse

D False\nTrue

💡 Hint

The 'is' operator checks if two variables point to the same object in memory.

String literals like 'python' are interned and share the same memory, so a is b is True. But c is created dynamically, so a is c is False.

🔧 Debug

advanced

Identify the Error in String Modification Attempt

What error does the following code raise?

DSA Python

s = "immutable"
s[0] = 'I'

A TypeError: 'str' object does not support item assignment

B IndexError: string index out of range

C SyntaxError: invalid syntax

D AttributeError: 'str' object has no attribute '0'

💡 Hint

Strings cannot be changed by assigning to an index.

Strings are immutable in Python, so trying to assign a character to an index raises a TypeError.

🚀 Application

expert

Memory Size Comparison of Strings

Given two strings s1 = 'abc' and s2 = 'a' + 'b' + 'c', which statement about their memory usage is correct?

A s1 uses more memory because it is a literal, s2 uses less because it is concatenated.

B s1 and s2 are the same object only if explicitly interned using sys.intern().

C s1 and s2 are different objects with separate memory allocations.

D s1 and s2 always share the same memory location due to interning.

💡 Hint

Consider how Python handles string literals vs runtime concatenation.

String literals like s1 may be interned, but s2 is created at runtime by concatenation, so they are different objects in memory.

Practice

1. Which of the following statements about Python strings is true?

easy

A. Strings are stored as linked lists in memory.

B. Strings are immutable, meaning they cannot be changed after creation.

C. Strings are mutable and can be changed character by character.

D. Strings always use 4 bytes per character regardless of content.

Solution

1. Step 1: Understand string mutability in Python

   Python strings cannot be changed after they are created. Any modification creates a new string.

2. Step 2: Analyze other options

   Strings are not mutable, not stored as linked lists, and use variable bytes depending on encoding.

3. Final Answer:

   Strings are immutable, meaning they cannot be changed after creation. -> Option B

4. Quick Check:

   Strings are immutable [OK]

Hint: Remember: Python strings cannot be changed once made [OK]

Common Mistakes:

• Thinking strings can be changed in place
• Assuming fixed byte size per character
• Confusing string storage with lists

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

easy

A. str = 'Hello World'

B. str = Hello World

C. str = "Hello World

D. str = 'Hello World"

Solution

1. Step 1: Check string syntax rules

   Strings must be enclosed in matching single or double quotes.

2. Step 2: Validate each option

   str = 'Hello World' uses matching single quotes correctly. Options B, C, and D have missing or mismatched quotes causing syntax errors.

3. Final Answer:

   str = 'Hello World' -> Option A

4. Quick Check:

   Matching quotes create strings correctly [OK]

Hint: Strings need matching quotes around text [OK]

Common Mistakes:

• Forgetting quotes around string text
• Using mismatched quotes
• Confusing string with variable names

3. What will be the output of the following code?

text = 'abc'
print(id(text))
text += 'd'
print(id(text))

medium

A. Two different numbers printed

B. Same number printed twice

C. SyntaxError

D. RuntimeError

Solution

1. Step 1: Understand string immutability and id()

   Strings are immutable, so adding 'd' creates a new string object with a new id.

2. Step 2: Trace the code output

   First print shows id of 'abc', second print shows id of new string 'abcd', which is different.

3. Final Answer:

   Two different numbers printed -> Option A

4. Quick Check:

   String immutability changes id after modification [OK]

Hint: String id changes after modification due to immutability [OK]

Common Mistakes:

• Assuming string id stays same after += operation
• Confusing id() with value
• Expecting syntax or runtime errors here

4. Find the error in the following code snippet:

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

medium

A. Output: hello

B. SyntaxError: invalid syntax

C. TypeError: 'str' object does not support item assignment

D. Output: Hello

Solution

1. Step 1: Identify string mutability rules

   Strings cannot be changed by assigning to an index because they are immutable.

2. Step 2: Analyze the error raised

   Assigning to my_str[0] causes a TypeError about item assignment not supported.

3. Final Answer:

   TypeError: 'str' object does not support item assignment -> Option C

4. Quick Check:

   Immutable strings cause TypeError on index assignment [OK]

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

Common Mistakes:

• Expecting string to change after index assignment
• Thinking code runs without error
• Confusing TypeError with SyntaxError

5. Given the code below, what will be the output?

str1 = 'abc'
str2 = 'abc'
print(str1 is str2)
str3 = ''.join(['a', 'b', 'c'])
print(str1 is str3)

hard

A. False\nTrue

B. False\nFalse

C. True\nTrue

D. True\nFalse

Solution

1. Step 1: Understand string interning for literals

   Python reuses the same object for identical string literals, so str1 and str2 point to the same object.

2. Step 2: Understand string creation with join()

   str3 is created at runtime and is a new object, so str1 is not str3.

3. Final Answer:

   True\nFalse -> Option D

4. Quick Check:

   Literal strings share object, runtime strings do not [OK]

Hint: Literal strings share memory; constructed strings do not [OK]

Common Mistakes:

• Assuming all identical strings share same object
• Confusing 'is' with '=='
• Ignoring how strings are created