String functions overview in C++ - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the std::string::length() function do?

It returns the number of characters in the string, excluding the null terminator.

beginner

How do you convert a std::string to a C-style string?

Use the c_str() function, which returns a pointer to a null-terminated character array.

intermediate

What is the purpose of std::string::substr()?

It extracts a substring from the string, starting at a given position and optionally for a given length.

intermediate

How does std::string::find() work?

It searches for a substring or character inside the string and returns the position of the first occurrence or std::string::npos if not found.

beginner

What does std::string::append() do?

It adds characters or another string to the end of the current string, increasing its length.

Which function returns the length of a std::string?

A strlen()

B size()

C count()

D length()

length() and size() both return the string length in C++, but length() is the most common for strings. strlen() is for C-style strings.

What does std::string::find() return if the substring is not found?

A <code>std::string::npos</code>

B -1

C 0

D Throws an exception

If the substring is not found, find() returns std::string::npos, a special constant indicating no match.

Which function would you use to get a part of a string starting at position 3 with length 5?

A slice(3, 5)

B substring(3, 5)

C substr(3, 5)

D cut(3, 5)

substr() extracts a substring from a string starting at the given position and length.

How do you add "world" to the end of a string s?

A s.push_back("world")

B s.add("world")

C s.insert("world")

D s.append("world")

append() adds characters or strings to the end of a std::string.

Which function returns a pointer to a C-style string from a std::string?

A c_str()

B data()

C to_cstring()

D ptr()

c_str() returns a pointer to a null-terminated C-style string.

Explain how to find a substring inside a std::string and handle the case when it is not found.

Think about the function that searches and what it returns if no match.

Describe how to extract a part of a string and how to add text to the end of a string in C++.

Two common string operations: cutting and joining.

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