String as character array in Data Structures Theory - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding String Storage

How is a string typically stored in memory when treated as a character array?

A As a linked list where each node contains a character and a pointer to the next character.

B As a sequence of characters stored in contiguous memory locations, ending with a special null character.

C As a hash table mapping each character to its frequency in the string.

D As a binary tree where each leaf node represents a character.

💡 Hint

Think about how arrays store elements in memory.

Strings as character arrays are stored as a sequence of characters in contiguous memory locations, typically ending with a null character to mark the end.

📋 Factual

intermediate

String Length Calculation

When a string is stored as a character array ending with a null character, how is its length usually determined?

A By storing the length in the first element of the array.

B By counting characters until a space character is found.

C By counting characters until the null character is found.

D By using a separate variable that tracks the length.

💡 Hint

Consider how functions like strlen work in C.

The length is found by counting characters until the null character, which marks the end of the string.

🔍 Analysis

advanced

Modifying Characters in a String Array

Given a string stored as a character array, what happens if you modify a character in the middle of the array?

A The null character is moved to the modified position.

B The entire string is deleted and replaced with the new character.

C The string becomes immutable and cannot be changed.

D The character at that position changes, and the rest of the string remains unchanged.

💡 Hint

Think about how arrays allow element updates.

Since strings as character arrays are mutable, changing one character updates that position without affecting others.

❓ Comparison

advanced

String as Character Array vs Immutable String

Which of the following is a key difference between strings stored as character arrays and immutable string objects?

A Character arrays allow direct modification of characters; immutable strings do not.

B Immutable strings are stored in contiguous memory; character arrays are not.

C Character arrays always use more memory than immutable strings.

D Immutable strings do not have a null terminator; character arrays do not either.

💡 Hint

Consider mutability and how strings behave in different languages.

Character arrays can be changed directly, while immutable strings cannot be altered after creation.

❓ Reasoning

expert

Effect of Missing Null Terminator in Character Array String

What is the likely outcome if a character array intended to represent a string does not have a null terminator?

A Functions that rely on the null terminator will read beyond the array, causing undefined behavior.

B The string will be automatically terminated at the last character of the array.

C The string length will be zero because no terminator is found.

D The program will throw a compile-time error.

💡 Hint

Think about how string functions detect the end of a string.

Without a null terminator, functions keep reading memory past the array, leading to unpredictable results or crashes.

Practice

1. What does it mean to treat a string as a character array?

easy

A. Changing the string into a list of words

B. Storing numbers instead of letters

C. Combining multiple strings into one

D. Accessing each letter individually by its position

Solution

1. Step 1: Understand string as sequence of characters

   A string is made up of letters arranged in order, like beads on a string.

2. Step 2: Accessing letters by position

   Treating a string as a character array means you can get each letter by its place number (index).

3. Final Answer:

   Accessing each letter individually by its position -> Option D

4. Quick Check:

   String as character array = access letters by position [OK]

Hint: Think of string as a list of letters you can pick one by one [OK]

Common Mistakes:

• Confusing string with list of words
• Thinking string stores numbers only
• Believing strings cannot be accessed by position

2. Which of the following is the correct way to access the third character of a string s in most programming languages?

easy

A. s[2]

B. s(3)

C. s[3]

D. s{2}

Solution

1. Step 1: Recall zero-based indexing

   Most languages start counting positions from 0, so first character is s[0], second s[1], third s[2].

2. Step 2: Identify correct syntax for indexing

   Square brackets [] are used to access characters by index, so s[2] is correct.

3. Final Answer:

   s[2] -> Option A

4. Quick Check:

   Third character index = 2, use s[2] [OK]

Hint: Remember counting starts at zero, so third char is index 2 [OK]

Common Mistakes:

• Using parentheses instead of brackets
• Using 1-based indexing (s[3])
• Using curly braces for indexing

3. Given the string word = "hello", what is the output of word[1]?

medium

A. "h"

B. "e"

C. "l"

D. "o"

Solution

1. Step 1: Identify the string characters and their indexes

   The string "hello" has characters: h(0), e(1), l(2), l(3), o(4).

2. Step 2: Access character at index 1

   Index 1 corresponds to the second character, which is "e".

3. Final Answer:

   "e" -> Option B

4. Quick Check:

   word[1] = "e" [OK]

Hint: Index 1 means second letter in the string [OK]

Common Mistakes:

• Choosing first character instead of second
• Confusing index 1 with index 2
• Thinking indexing starts at 1

4. What is wrong with this code snippet if text = "world" and we try to print text[5]?

medium

A. It will cause an error because index 5 is out of range

B. It will print a space character

C. It will print the last character

D. It will print the first character

Solution

1. Step 1: Check string length and valid indexes

   The string "world" has length 5, with valid indexes 0 to 4.

2. Step 2: Understand index out of range error

   Accessing index 5 is beyond the last character, causing an error in most languages.

3. Final Answer:

   It will cause an error because index 5 is out of range -> Option A

4. Quick Check:

   Index 5 invalid for length 5 string [OK]

Hint: Indexes go from 0 to length-1; 5 is too big here [OK]

Common Mistakes:

• Assuming index 5 returns last character
• Expecting a space or empty character
• Thinking indexing starts at 1

5. You want to change the first letter of the string name = "alice" to uppercase by treating it as a character array. Which approach is correct?

hard

A. Directly assign name[0] = 'A' (works in all languages)

B. Use name.upper()[0] to change first letter only

C. Convert string to list, change first element, then join back

D. Strings are immutable; you cannot change any character

Solution

1. Step 1: Understand string immutability

   In many languages, strings cannot be changed directly because they are immutable.

2. Step 2: Use list conversion to modify characters

   Convert the string to a list (array) of characters, change the first element, then join back to a string.

3. Final Answer:

   Convert string to list, change first element, then join back -> Option C

4. Quick Check:

   Modify string by list conversion and join [OK]

Hint: Strings can't change directly; convert to list first [OK]

Common Mistakes:

• Trying to assign directly to string index
• Using upper() on whole string but expecting partial change
• Believing strings are mutable