String Basics and Memory Representation in DSA Python - Build from Scratch

String Basics and Memory Representation

📖 Scenario: Imagine you are working with a simple text message system. You want to understand how strings are stored and how to access their parts.

🎯 Goal: You will create a string variable, find its length, access specific characters, and print the results to see how strings work in memory.

📋 What You'll Learn

Create a string variable with exact content

Create a variable to store the length of the string

Access specific characters by index

Print the string, its length, and accessed characters

💡 Why This Matters

🌍 Real World

Understanding strings and their memory representation helps when working with text data in messaging apps, file processing, and user input handling.

💼 Career

Basic string manipulation is essential for software developers, data analysts, and anyone working with text processing or user interfaces.

1

Create the string variable

Create a string variable called message and assign it the exact value "Hello, World!"

DSA Python

# Create the string variable 'message' with value "Hello, World!"
# Your code here

Hint

Use double quotes around the text exactly as shown.

2

Find the length of the string

Create a variable called length and set it to the length of the string message using the len() function

DSA Python

message = "Hello, World!"
# Create variable 'length' to store length of 'message'
# Your code here

Hint

Use len(message) to get the number of characters.

3

Access specific characters by index

Create two variables: first_char to store the first character of message and last_char to store the last character of message using indexing

DSA Python

message = "Hello, World!"
length = len(message)
# Create 'first_char' as message[0]
# Create 'last_char' as message[length - 1]
# Your code here

Hint

Remember, string indexes start at 0.

4

Print the string, its length, and accessed characters

Print the variables message, length, first_char, and last_char each on a separate line

DSA Python

message = "Hello, World!"
length = len(message)
first_char = message[0]
last_char = message[length - 1]
# Print 'message', 'length', 'first_char', and 'last_char'
# Your code here

Hint

Use separate print() statements for each variable.

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