String Basics and Memory Representation in DSA C++ - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a string variable named name.

DSA C++

std::string [1];

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

Aname

Bstr

Cword

Dtext

Attempts:

3 left

💡 Hint

Common Mistakes

Using a different variable name than 'name'.

Forgetting to use std::string type.

The variable name is declared as name using std::string.

2fill in blank

medium

Complete the code to assign the value "hello" to the string variable greeting.

DSA C++

std::string greeting = [1];

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

Ahello

B'hello'

C"hello"

Dhello"

Attempts:

3 left

💡 Hint

Common Mistakes

Using single quotes instead of double quotes.

Not using any quotes around the string.

String literals in C++ must be enclosed in double quotes, so "hello" is correct.

3fill in blank

hard

Fix the error in the code to get the length of the string word.

DSA C++

int length = word.[1]();

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

Alength

Bsize

Ccount

Dlen

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'len()' which is not a C++ string method.

Using 'count()' which does not exist for strings.

The correct method to get string length in C++ is length().

4fill in blank

hard

Fill in the blank to create a substring from text starting at index 2 with length 4.

DSA C++

std::string sub = text.[1](2, 4);

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

Asubstring

Bsubstr

Csubstr()

Dsub

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'substring' which is not a C++ string method.

Including parentheses in the method name in the blank.

The method is 'substr' and the starting index is 2.

5fill in blank

hard

Fill all three blanks to create a map of words to their lengths for words longer than 3 characters.

DSA C++

std::map<std::string, int> lengths;
for (const auto& [1] : words) if ([2] > 3) lengths[[3]] = [2];

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

Aword

Bword.length()

Dwords

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'words' as the loop variable instead of 'word'.

Using incorrect expressions for length.

The map uses 'word' as key, 'word.length()' as value, and iterates over 'word' in 'words'.

Practice

1. What does the following C++ code represent?
char str[] = "hello";

easy

A. An array of characters ending with a null character

B. A pointer to a constant string literal

C. A string object from the C++ standard library

D. A dynamically allocated string

Solution

1. Step 1: Understand the declaration type

   The code declares str as a character array initialized with the string "hello".

2. Step 2: Recognize the null terminator in C-style strings

   In C++, character arrays initialized with string literals automatically include a null character '\0' at the end to mark the string's end.

3. Final Answer:

   An array of characters ending with a null character -> Option A

4. Quick Check:

   C-style string = char array + '\0' [OK]

Hint: Remember C strings always end with '\0' null character [OK]

Common Mistakes:

• Confusing array with pointer
• Forgetting the null terminator
• Thinking it's a C++ string object

2. Which of the following is the correct way to declare a string literal pointer in C++?

easy

A. char *str = "hello";

B. char str = "hello";

C. char str[] = 'hello';

D. string str = 'hello';

Solution

1. Step 1: Check pointer declaration syntax

   char *str = "hello"; declares a pointer to a string literal correctly.

2. Step 2: Identify errors in other options

   char str = "hello"; uses a single char variable for a string literal, which is invalid.
   char str[] = 'hello'; uses single quotes for multiple characters, which is invalid.
   string str = 'hello'; uses single quotes with string type, which requires double quotes.

3. Final Answer:

   char *str = "hello"; -> Option A

4. Quick Check:

   Pointer to string literal uses char * and double quotes [OK]

Hint: String literals need char* with double quotes [OK]

Common Mistakes:

• Using single quotes for strings
• Assigning string literal to char variable
• Confusing char array and pointer syntax

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

char str[] = "abc";
std::cout << sizeof(str) << std::endl;

medium

A. 3

B. Undefined behavior

C. 5

D. 4

Solution

1. Step 1: Understand sizeof on char array initialized with string

   The array str contains characters 'a', 'b', 'c' plus a null terminator '\0'. So total 4 characters.

2. Step 2: sizeof returns total bytes of array

   Since each char is 1 byte, sizeof(str) returns 4.

3. Final Answer:

   4 -> Option D

4. Quick Check:

   sizeof(char array with "abc") = 4 [OK]

Hint: Count characters plus null terminator for sizeof [OK]

Common Mistakes:

• Ignoring null terminator in size
• Confusing sizeof with string length
• Expecting runtime output instead of compile-time size

4. Identify the error in the following code snippet:

char *str = "hello";
str[0] = 'H';
std::cout << str << std::endl;

medium

A. Output will be empty string

B. Pointer str is not initialized

C. Modifying a string literal causes undefined behavior

D. Syntax error in assignment

Solution

1. Step 1: Understand string literal immutability

   String literals are stored in read-only memory. Modifying them causes undefined behavior.

2. Step 2: Analyze the code's modification attempt

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

3. Final Answer:

   Modifying a string literal causes undefined behavior -> Option C

4. Quick Check:

   String literals are immutable in C++ [OK]

Hint: Never modify string literals through pointers [OK]

Common Mistakes:

• Assuming string literals are modifiable arrays
• Ignoring const correctness
• Confusing pointer initialization with mutability

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

char str1[] = "abc";
char *str2 = str1;
str2[1] = 'z';
std::cout << str1 << std::endl;

hard

A. abc

B. azc

C. a\0c

D. Runtime error

Solution

1. Step 1: Understand array and pointer relationship

   str1 is a modifiable character array. str2 points to the same array.

2. Step 2: Modify character via pointer

   Changing str2[1] changes the second character of str1 from 'b' to 'z'.

3. Step 3: Print the modified array

   Printing str1 outputs "azc" reflecting the change.

4. Final Answer:

   azc -> Option B

5. Quick Check:

   Pointer modifies array content directly [OK]

Hint: Pointer to array modifies original array content [OK]

Common Mistakes:

• Thinking string literals are modifiable
• Confusing pointer and array copy
• Expecting original string unchanged