String handling basics in C++ - Step-by-Step Execution

Concept Flow - String handling basics

Start

↓

Declare string variable

↓

Assign value to string

↓

Access or modify string

↓

Use string functions

↓

Output or process string

↓

End

This flow shows how a string variable is declared, assigned, accessed or modified, and then used or output.

Execution Sample

C++

#include <iostream>
#include <string>

int main() {
    std::string s = "Hello";
    s += " World";
    std::cout << s << std::endl;
    return 0;
}

This code creates a string, adds more text to it, and prints the result.

Execution Table

Step	Action	String s	Output
1	Declare string s	""
2	Assign "Hello" to s	"Hello"
3	Append " World" to s	"Hello World"
4	Print s	"Hello World"	Hello World
5	End of program	"Hello World"	Hello World

💡 Program ends after printing the final string.

Variable Tracker

Variable	Start	After 1	After 2	After 3	Final
s	""	""	"Hello"	"Hello World"	"Hello World"

Key Moments - 2 Insights

Why does the string s change after step 3?

Because in step 3, we use the += operator to add " World" to the existing string s, updating its value as shown in the execution_table row 3.

What is printed in step 4 and why?

Step 4 prints the current value of s, which is "Hello World" after the append operation, as shown in the Output column of execution_table row 4.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of string s after step 2?

A ""

B "Hello"

C "Hello World"

D "World"

💡 Hint

Check the 'String s' column in execution_table row 2.

At which step does the string s get its final value?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at when s changes to "Hello World" in the execution_table.

If we remove step 3, what would be printed at step 4?

A "Hello"

B "Hello World"

C ""

D Compilation error

💡 Hint

Without appending, s remains as assigned in step 2; see variable_tracker.

Concept Snapshot

String handling basics in C++:
- Declare with std::string s;
- Assign with s = "text";
- Append with s += "more";
- Access characters with s[index];
- Print with std::cout << s;
Strings are mutable and support many useful functions.

Full Transcript

This example shows how to handle strings in C++. First, we declare a string variable s. Then we assign it the value "Hello". Next, we add more text " World" to s using the += operator. Finally, we print the full string "Hello World". The execution table tracks each step, showing how s changes and what is output. Key moments clarify why s changes after appending and what is printed. The quiz tests understanding of string value changes and output. This teaches basic string creation, modification, and output in C++.

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