String functions overview in C++ - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of string find function

What is the output of this C++ code snippet?

C++

#include <iostream>
#include <string>

int main() {
    std::string s = "hello world";
    std::cout << s.find("world") << std::endl;
    return 0;
}

A 11

B -1

C 0

D 6

💡 Hint

Remember that string::find returns the index of the first character of the found substring or std::string::npos if not found.

The substring "world" starts at index 6 in "hello world". So s.find("world") returns 6.

❓ Predict Output

intermediate

Result of string substr function

What does this C++ code print?

C++

#include <iostream>
#include <string>

int main() {
    std::string s = "programming";
    std::cout << s.substr(3, 4) << std::endl;
    return 0;
}

A gram

B prog

C ming

D ramm

💡 Hint

substr(pos, len) returns a substring starting at pos with length len.

Starting at index 3 (0-based), the substring of length 4 is "gram".

❓ Predict Output

advanced

Output of string compare function

What is the output of this C++ program?

C++

#include <iostream>
#include <string>

int main() {
    std::string a = "apple";
    std::string b = "apricot";
    std::cout << a.compare(b) << std::endl;
    return 0;
}

A -1

B positive number

C negative number less than -1

D 0

💡 Hint

string::compare returns 0 if equal, positive if first string is lexicographically greater, negative if smaller.

"apple" is lexicographically smaller than "apricot", so compare returns a negative number.

❓ Predict Output

advanced

Effect of string erase function

What is the output of this C++ code?

C++

#include <iostream>
#include <string>

int main() {
    std::string s = "abcdef";
    s.erase(2, 3);
    std::cout << s << std::endl;
    return 0;
}

A abdef

B abf

C abcdef

D ab

💡 Hint

erase(pos, len) removes len characters starting at pos.

Removing 3 characters starting at index 2 removes 'c', 'd', 'e', leaving "abf".

🧠 Conceptual

expert

Behavior of string::npos in find function

What is the value and meaning of std::string::npos in C++ string functions?

A It is the index 0, meaning the start of the string.

B It is a constant representing the maximum possible size_t value, used to indicate 'not found' in string operations.

C It is a negative integer used to indicate an error in string functions.

D It is a boolean true value returned when a substring is found.

💡 Hint

Check the documentation for string::npos and its role in find functions.

std::string::npos is a static constant representing the largest possible size_t value. It signals that a substring was not found.

Practice

1. Which C++ string function returns the number of characters in a string?

easy

A. substr()

B. append()

C. length()

D. find()

Solution

1. Step 1: Understand string length function

   The length() function returns how many characters are in the string.

2. Step 2: Compare with other functions

   append() adds text, substr() extracts part, find() searches for text.

3. Final Answer:

   length() -> Option C

4. Quick Check:

   String length = length() [OK]

Hint: Length means count characters, use length() [OK]

Common Mistakes:

• Confusing length() with append()
• Using substr() to get length
• Thinking find() returns length

2. Which of the following is the correct syntax to add " world" to a string str in C++?

easy

A. str.append(" world");

B. str.add(" world");

C. str.concat(" world");

D. str.push_back(" world");

Solution

1. Step 1: Identify correct append syntax

   In C++, append() adds text to the end of a string: str.append(" world");.

2. Step 2: Check other options

   add() and concat() are not string methods in C++. push_back() adds a single character, not a string.

3. Final Answer:

   str.append(" world"); -> Option A

4. Quick Check:

   Use append() to add text [OK]

Hint: Use append() to add text, not add() or concat() [OK]

Common Mistakes:

• Using add() or concat() which don't exist
• Using push_back() with string argument
• Missing semicolon at end

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

std::string s = "hello world";
std::cout << s.substr(6, 5);

medium

A. worlds

B. hello

C. hello world

D. world

Solution

1. Step 1: Understand substr parameters

   substr(6, 5) means start at index 6 and take 5 characters.

2. Step 2: Extract substring from "hello world"

   Index 6 is 'w', next 5 characters are "world".

3. Final Answer:

   world -> Option D

4. Quick Check:

   substr(6,5) = "world" [OK]

Hint: Count from zero, substr(start, length) extracts text [OK]

Common Mistakes:

• Starting count at 1 instead of 0
• Taking wrong number of characters
• Confusing substr with find

4. Identify the error in this C++ code snippet:

std::string s = "example";
s = s.substr(3, 10);

medium

A. substr length exceeds string length but no error occurs

B. substr start index is invalid

C. substr requires two arguments but only one given

D. Cannot assign substr result to string variable

Solution

1. Step 1: Check substr parameters

   Start index 3 is valid. Length 10 is longer than remaining string length.

2. Step 2: Understand substr behavior with long length

   substr returns substring from index 3 to end without error even if length is too big.

3. Final Answer:

   substr length exceeds string length but no error occurs -> Option A

4. Quick Check:

   substr handles long length safely [OK]

Hint: substr length can be longer than string safely [OK]

Common Mistakes:

• Expecting runtime error for long length
• Thinking start index is invalid
• Confusing substr arguments count

5. Given a string text = "Hello C++ World", which code snippet converts it to all uppercase letters?

hard

A. text.toUpperCase();

B. for (auto &c : text) c = toupper(c);

C. text.upper();

D. transform(text.begin(), text.end(), text.begin(), tolower);

Solution

1. Step 1: Identify correct uppercase conversion

   Looping over each character and applying toupper() converts all letters to uppercase.

2. Step 2: Check other options

   toUpperCase() and upper() are not C++ string methods. transform(text.begin(), text.end(), text.begin(), tolower); uses tolower which converts to lowercase.

3. Final Answer:

   for (auto &c : text) c = toupper(c); -> Option B

4. Quick Check:

   Use loop + toupper() for uppercase [OK]

Hint: Use loop with toupper() to change case [OK]

Common Mistakes:

• Using non-existent string methods
• Using tolower instead of toupper
• Not using reference in loop to modify string