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Conditions and Operators: Conditions and Switch - Case: Fall Through¶

Adapted from: “Learn Modern C++” by cpptutor: Learn Modern C++: Conditions and Operators

Program that Demonstrates Conditions and Fall Through in the Switch Case Statement¶

// 03-fallthrough.cpp : demonstrate case clauses without break
 
#include <iostream>
using namespace std;
 
int main() {
    cout << "Please enter an integer between zero and three:\n";
    int n{};
    cin >> n;
    switch (n) {
    case 0:
        cout << "Number is less than 1\n";
        [[fallthrough]];
    case 1:
        cout << "Number is less than 2\n";
        [[fallthrough]];
    case 2:
        cout << "Number is less than 3\n";
        break;
    case 3:
        cout << "Number is exactly 3\n";
        break;
    default:
        cout << "Number out of range!\n";
        break;
    }
}

Explanation of the Above Code¶

This C++ program demonstrates the use of the switch statement with the [[fallthrough]] attribute to explicitly allow fall-through behavior between cases. Here’s a breakdown of the code:

Code Explanation:¶

Header Inclusion:
```
#include <iostream>
```
- The <iostream> header is included to enable input (cin) and output (cout) operations.
Namespace:
```
using namespace std;
```
- This allows the program to use standard library features (like cin and cout) without prefixing them with std::.
Main Function:
```
int main() {
```
- The main function is the entry point of the program.
User Prompt and Input:
```
cout << "Please enter an integer between zero and three:\n";
int n{};
cin >> n;
```
- The program prompts the user to enter an integer between 0 and 3.
- The input is stored in the variable n.
Switch Statement:
```
switch (n) {
```
- The switch statement evaluates the value of n and executes the corresponding case block.
Case Blocks:
- Case 0:
```
case 0:
    cout << "Number is less than 1\n";
    [[fallthrough]];
```
  - Prints “Number is less than 1”.
  - The [[fallthrough]] attribute explicitly allows execution to continue to the next case (case 1).
- Case 1:
```
case 1:
    cout << "Number is less than 2\n";
    [[fallthrough]];
```
  - Prints “Number is less than 2”.
  - The [[fallthrough]] attribute allows execution to continue to the next case (case 2).
- Case 2:
```
case 2:
    cout << "Number is less than 3\n";
    break;
```
  - Prints “Number is less than 3”.
  - The break statement prevents further fall-through and exits the switch block.
- Case 3:
```
case 3:
    cout << "Number is exactly 3\n";
    break;
```
  - Prints “Number is exactly 3”.
  - The break statement exits the switch block.
- Default Case:
```
default:
    cout << "Number out of range!\n";
    break;
```
  - Handles any input that is not 0, 1, 2, or 3.
  - Prints “Number out of range!”.

Key Features:¶

Fall-Through Behavior:
- The [[fallthrough]] attribute is used to explicitly indicate intentional fall-through between cases. This improves code readability and avoids accidental fall-through, which can lead to bugs.
Default Case:
- The default case ensures that invalid inputs are handled gracefully.

Example Input/Output:¶

Input: 0

Please enter an integer between zero and three:
0
Number is less than 1
Number is less than 2
Number is less than 3

Input: 3

Please enter an integer between zero and three:
3
Number is exactly 3

Input: 5

Please enter an integer between zero and three:
5
Number out of range!

Compile and Run Code¶

Use Python to Change to Working Directory¶

import os
root_dir = os.getcwd()

code_dir = root_dir + "/" + "Cpp_Code/03_Conditions_and_Operators"

os.chdir(code_dir)

Use Docker to Compile the Code in a C++23 Environment¶

!docker run --rm -v $(pwd):/app cpp23-clang18:latest clang++-18 -std=c++23 -stdlib=libc++ /app/03-fallthrough.cpp -o /app/03-fallthrough

Use Docker to Run Executable in a C++23 Environment¶

!echo "0" | docker run -i --rm -v $(pwd):/app cpp23-clang18:latest ./03-fallthrough

Please enter an integer between zero and three:
Number is less than 1
Number is less than 2
Number is less than 3

!echo "1" | docker run -i --rm -v $(pwd):/app cpp23-clang18:latest ./03-fallthrough

Please enter an integer between zero and three:
Number is less than 2
Number is less than 3

!echo "2" | docker run -i --rm -v $(pwd):/app cpp23-clang18:latest ./03-fallthrough

Please enter an integer between zero and three:
Number is less than 3

!echo "3" | docker run -i --rm -v $(pwd):/app cpp23-clang18:latest ./03-fallthrough

Please enter an integer between zero and three:
Number is exactly 3

!echo "4" | docker run -i --rm -v $(pwd):/app cpp23-clang18:latest ./03-fallthrough

Please enter an integer between zero and three:
Number out of range!