String handling basics in C++ - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a string variable named greeting.

C++

#include <iostream>
#include <string>

int main() {
    std::string [1] = "Hello";
    std::cout << greeting << std::endl;
    return 0;
}

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

Ahello

Bstr

Cmessage

Dgreeting

Attempts:

3 left

💡 Hint

Common Mistakes

Using a different variable name than the one printed.

Forgetting to include header.

The variable name should be greeting to match the output line.

2fill in blank

medium

Complete the code to get the length of the string greeting.

C++

#include <iostream>
#include <string>

int main() {
    std::string greeting = "Hello";
    std::cout << greeting.[1]() << std::endl;
    return 0;
}

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

Asize

Blength

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().

3fill in blank

hard

Fix the error in the code to concatenate two strings correctly.

C++

#include <iostream>
#include <string>

int main() {
    std::string first = "Hello, ";
    std::string second = "World!";
    std::string combined = first [1] second;
    std::cout << combined << std::endl;
    return 0;
}

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

A*

B&

C+

D&&

Attempts:

3 left

💡 Hint

Common Mistakes

Using & which is for addresses or references.

Using * which is for multiplication.

In C++, strings are concatenated using the + operator.

4fill in blank

hard

Fill both blanks to create a substring from greeting starting at index 2 with length 3.

C++

#include <iostream>
#include <string>

int main() {
    std::string greeting = "Hello";
    std::string part = greeting.substr([1], [2]);
    std::cout << part << std::endl;
    return 0;
}

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

A2

B3

C1

D4

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping the start index and length.

Using incorrect indices causing wrong substring.

The substring starts at index 2 and has length 3.

5fill in blank

hard

Fill all three blanks to check if the string greeting contains the character 'e' using find method.

C++

#include <iostream>
#include <string>

int main() {
    std::string greeting = "Hello";
    if (greeting.[1]([2]) [3] std::string::npos) {
        std::cout << "Found 'e'" << std::endl;
    } else {
        std::cout << "Not found" << std::endl;
    }
    return 0;
}

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

Afind

B'e'

C!=

D==

Attempts:

3 left

💡 Hint

Common Mistakes

Using == instead of != in the condition.

Searching for a string instead of a character.

The find method returns the position or std::string::npos if not found. We check if it is not equal to npos to confirm presence.

Practice

1. What does the following C++ code do?
std::string s = "Hello";
int len = s.length();

easy

A. Changes the string s to "length"

B. Stores the length of the string "Hello" in variable len

C. Prints the string length to the screen

D. Deletes the string s

Solution

1. Step 1: Understand the string initialization

   The code creates a string variable s with the value "Hello".

2. Step 2: Understand the length() function

   The length() function returns the number of characters in the string, which is 5 for "Hello".

3. Final Answer:

   Stores the length of the string "Hello" in variable len -> Option B

4. Quick Check:

   string length = 5 [OK]

Hint: length() returns number of characters in a string [OK]

Common Mistakes:

• Thinking length() changes the string
• Confusing length() with printing
• Assuming length() deletes the string

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

easy

A. std::string s = "Hello";

B. string s = 'Hello';

C. char s = "Hello";

D. string s = Hello;

Solution

1. Step 1: Check string declaration syntax

   In C++, strings are declared using std::string and initialized with double quotes "".

2. Step 2: Analyze each option

   std::string s = "Hello"; uses std::string with double quotes correctly. string s = 'Hello'; uses single quotes which are for single characters. char s = "Hello"; uses char which holds one character, not a string. string s = Hello; misses quotes around Hello.

3. Final Answer:

   std::string s = "Hello"; -> Option A

4. Quick Check:

   Correct string syntax uses std::string and double quotes [OK]

Hint: Use std::string and double quotes for strings [OK]

Common Mistakes:

• Using single quotes for strings
• Using char for multiple characters
• Forgetting quotes around text

3. What is the output of this C++ code?

std::string s = "Hello";
char c = s[1];
std::cout << c << std::endl;

medium

A. o

B. H

C. l

D. e

Solution

1. Step 1: Understand string indexing

   Strings in C++ are zero-indexed, so s[0] = 'H', s[1] = 'e'.

2. Step 2: Identify the character at index 1

   The code stores s[1] in c, which is 'e', then prints it.

3. Final Answer:

   e -> Option D

4. Quick Check:

   Index 1 character = 'e' [OK]

Hint: Remember string indexing starts at 0 [OK]

Common Mistakes:

• Thinking indexing starts at 1
• Confusing characters at different positions
• Expecting the whole string instead of one char

4. Find the error in this code snippet:

std::string s = "Hello";
char c = s.at(5);
std::cout << c << std::endl;

medium

A. No error, prints 'o'

B. Compilation error: cannot assign char

C. Runtime error: out_of_range exception

D. Syntax error: wrong function name

Solution

1. Step 1: Check string length and indexing

   The string "Hello" has length 5, valid indices are 0 to 4.

2. Step 2: Understand s.at(5) behavior

   Accessing s.at(5) is out of range and throws a runtime exception.

3. Final Answer:

   Runtime error: out_of_range exception -> Option C

4. Quick Check:

   Accessing index 5 causes out_of_range error [OK]

Hint: s.at(index) throws error if index invalid [OK]

Common Mistakes:

• Assuming s.at() works like s[] without error
• Ignoring zero-based indexing
• Confusing syntax and runtime errors

5. Given a string std::string s = "apple,banana,cherry";, which code correctly extracts the substring "banana"?

hard

A. std::string sub = s.substr(6, 6);

B. std::string sub = s.substr(5, 6);

C. std::string sub = s.substr(6, 7);

D. std::string sub = s.substr(7, 6);

Solution

1. Step 1: Identify the start index of "banana"

   "apple," has 6 characters (indices 0 to 5), so 'b' starts at index 6.

2. Step 2: Determine length of "banana"

   "banana" has 6 characters.

3. Step 3: Use substr(start, length)

   Calling s.substr(6, 6) extracts 6 characters starting at index 6, which is "banana".

4. Final Answer:

   std::string sub = s.substr(6, 6); -> Option A

5. Quick Check:

   substr(6,6) = "banana" [OK]

Hint: Count start index and length carefully for substr() [OK]

Common Mistakes:

• Off-by-one errors in start index
• Wrong length parameter
• Confusing substr parameters order