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Variables, Scopes and Namespaces: Constants¶

Adapted from: “Learn Modern C++” by cpptutor: Learn Modern C++: Variables, Scopes and Namespaces

Program that Demonstrates Constants¶

// 02-constants.cpp : introducing the const keyword
 
#include <print>
using namespace std;
 
const double PI = 3.14159265358979;
 
int main() {
    auto const APPROX_E = 3;
    println("pi is almost exactly {}, while e is approximately {}",
        PI, APPROX_E);
}

Explanation of the Above Code¶

This C++ code demonstrates the use of the const keyword to define constants and the use of the println function for formatted output.

Code Explanation:¶

Header Inclusion:
```
#include <print>
```
- The <print> header is part of the C++23 standard library. It provides the println function, which is used for formatted output similar to Python’s print function.
Namespace:
```
using namespace std;
```
- This allows the program to use standard library features (like println) without needing to prefix them with std::.
Constant Definition:
```
const double PI = 3.14159265358979;
```
- PI is defined as a constant of type double. The const keyword ensures that its value cannot be modified after initialization.
Main Function:
```
int main() {
    auto const APPROX_E = 3;
    println("pi is almost exactly {}, while e is approximately {}", PI, APPROX_E);
}
```
- Inside the main function:
  - APPROX_E is declared as a constant using auto const. The auto keyword deduces its type as int based on the assigned value 3.
  - The println function is used to print a formatted string. The placeholders {} are replaced with the values of PI and APPROX_E.

Output:¶

When executed, the program will output:

pi is almost exactly 3.14159265358979, while e is approximately 3

Key Concepts:¶

const Keyword: Used to define immutable variables.
auto Keyword: Automatically deduces the type of a variable.
Formatted Output: The println function simplifies printing formatted strings in C++23.

Compile and Run Code¶

Use Python to Change to Working Directory¶

import os
root_dir = os.getcwd()

code_dir = root_dir + "/" + "Cpp_Code/02_Variables_Scopes_and_Namespaces"

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/02-constants.cpp -o /app/02-constants

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

!docker run --rm -v $(pwd):/app cpp23-clang18:latest ./02-constants

pi is almost exactly 3.14159265358979, while e is approximately 3