String handling basics in C++ - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a string in C++?

A string in C++ is a sequence of characters used to store text. It is often handled using the <code>std::string</code> class from the Standard Library.

beginner

How do you declare and initialize a string variable in C++?

You declare a string variable using std::string and initialize it with text inside double quotes, for example: std::string name = "Alice";

beginner

How can you find the length of a string in C++?

Use the .length() or .size() method on a string object, for example: name.length() returns the number of characters in the string.

beginner

How do you concatenate two strings in C++?

You can use the + operator to join two strings, for example: std::string fullName = firstName + " " + lastName;

beginner

How do you access individual characters in a string?

You can use the square brackets [] with an index to access characters, for example: char firstChar = name[0]; gets the first character.

Which header file must you include to use std::string in C++?

A <code>&lt;string&gt;</code>

B <code>&lt;iostream&gt;</code>

C <code>&lt;vector&gt;</code>

D <code>&lt;cmath&gt;</code>

The <string> header defines the std::string class.

What does str.length() return if str is a string?

A First character of <code>str</code>

B Number of characters in <code>str</code>

C Memory size of <code>str</code>

D Last character of <code>str</code>

.length() returns how many characters the string contains.

How do you combine two strings a and b in C++?

A <code>a + b</code>

B <code>a - b</code>

C <code>a * b</code>

D <code>a / b</code>

The + operator joins two strings together.

What is the index of the first character in a C++ string?

A Depends on the string length

B 1

C -1

D 0

String indexing in C++ starts at 0.

Which of these is a valid way to declare a string variable named city with value "Paris"?

A <code>std::string city = Paris;</code>

B <code>string city = 'Paris';</code>

C <code>std::string city = "Paris";</code>

D <code>char city = "Paris";</code>

Strings use double quotes and must be declared with std::string.

Explain how to create, access, and find the length of a string in C++.

Think about how you handle words or sentences in code.

Describe how to join two strings together in C++ and why this might be useful.

Imagine putting two pieces of text side by side.

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