String Basics and Memory Representation in DSA Python - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to get the length of the string.

DSA Python

word = "hello"
length = len([1])
print(length)

Drag options to blanks, or click blank then click option'

A"word"

Bword

Clen

Dhello

Attempts:

3 left

💡 Hint

Common Mistakes

Using quotes around the variable name like "word" which makes it a string literal.

Using the function name len inside len().

The len() function returns the number of characters in the string variable word.

2fill in blank

medium

Complete the code to get the first character of the string.

DSA Python

word = "world"
first_char = word[[1]]
print(first_char)

Drag options to blanks, or click blank then click option'

A-1

B1

Cword

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using 1 as the index which gives the second character.

Using -1 which gives the last character.

String indexing starts at 0, so word[0] gives the first character.

3fill in blank

hard

Fix the error in the code to get the last character of the string.

DSA Python

word = "python"
last_char = word[[1]]
print(last_char)

Drag options to blanks, or click blank then click option'

A-len(word)

B0

Clen(word) - 1

Dlen(word)

Attempts:

3 left

💡 Hint

Common Mistakes

Using len(word) which is out of range.

Using 0 which gives the first character.

The last character is at index len(word) - 1 because indexing starts at 0.

4fill in blank

hard

Fill both blanks to create a substring from the second to the fourth character (inclusive).

DSA Python

word = "example"
substring = word[[1]:[2]]
print(substring)

Drag options to blanks, or click blank then click option'

A1

B4

C5

D3

Attempts:

3 left

💡 Hint

Common Mistakes

Using end index 3 which excludes the fourth character.

Using start index 2 which skips the second character.

Substring slicing uses start index inclusive and end index exclusive. To get characters 2 to 4, use word[1:4].

5fill in blank

hard

Fill all three blanks to create a dictionary with words as keys and their lengths as values, only for words longer than 3 characters.

DSA Python

words = ["cat", "house", "dog", "elephant"]
lengths = [1]: [2] for word in words if len(word) [3] 3
print(lengths)

Drag options to blanks, or click blank then click option'

A{word}

Blen(word)

C>

Dword

Attempts:

3 left

💡 Hint

Common Mistakes

Missing the opening curly brace.

Using the wrong comparison operator.

Swapping key and value positions.

The dictionary comprehension syntax is {key: value for item in list if condition}. Here, keys are words and values are their lengths. The condition filters words longer than 3 characters.

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