String handling basics in C++ - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of string concatenation and length

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

C++

#include <iostream>
#include <string>

int main() {
    std::string a = "Hello";
    std::string b = "World";
    std::string c = a + " " + b;
    std::cout << c << " " << c.length() << std::endl;
    return 0;
}

A Hello World 10

B HelloWorld 10

C Hello World 11

D HelloWorld 11

💡 Hint

Remember that adding a space between strings adds one character to the length.

The strings "Hello" and "World" are concatenated with a space in between, so the result is "Hello World" which has 11 characters including the space.

❓ Predict Output

intermediate

Output of string find method

What will this program print?

C++

#include <iostream>
#include <string>

int main() {
    std::string s = "programming";
    size_t pos = s.find("gram");
    std::cout << pos << std::endl;
    return 0;
}

A 3

B 0

C -1

D 7

💡 Hint

The find method returns the index where the substring starts.

The substring "gram" starts at index 3 in "programming" (p=0, r=1, o=2, g=3).

❓ Predict Output

advanced

Output of string substr and concatenation

What is the output of this code?

C++

#include <iostream>
#include <string>

int main() {
    std::string s = "abcdef";
    std::string t = s.substr(1, 3) + s.substr(4);
    std::cout << t << std::endl;
    return 0;
}

A bcdef

B fedcb

C cdef

D bcde

💡 Hint

Check carefully the substr parameters and concatenation.

s.substr(1,3) is "bcd" and s.substr(4) is "ef", concatenated gives "bcdef".

❓ Predict Output

advanced

Output of string comparison

What does this program print?

C++

#include <iostream>
#include <string>

int main() {
    std::string a = "apple";
    std::string b = "banana";
    if (a < b) {
        std::cout << "a < b" << std::endl;
    } else {
        std::cout << "a >= b" << std::endl;
    }
    return 0;
}

A Runtime error

B a >= b

C Compilation error

D a < b

💡 Hint

Strings are compared lexicographically (dictionary order).

"apple" is lexicographically less than "banana", so the output is "a < b".

❓ Predict Output

expert

Output of string erase and insert

What is the output of this program?

C++

#include <iostream>
#include <string>

int main() {
    std::string s = "hello world";
    s.erase(5, 1);
    s.insert(5, ",");
    std::cout << s << std::endl;
    return 0;
}

A hello world

B hello,world

C hello, world

D helloworld

💡 Hint

Erase removes the space, insert adds a comma at the same position.

The space at index 5 is erased, then a comma is inserted at index 5, resulting in "hello,world".

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