PS2: How to rename things? - Qualité de Développement

Objectives¶

The goal of this session is to understand the following points:

Exercise 1 : Magic numbers¶

magic-numbers.cpp

#include <iostream>
#include <string>

using namespace std;

bool qualifiesForDiscount(double originalPrice) {
    return originalPrice >= 10;
}

double calculateDiscountPercentage(double originalPrice, bool isStudent, bool isLoyalCustomer) {
    double discountPercentage = 0.0;

    if (qualifiesForDiscount(originalPrice)) {
        if (isStudent) {
            discountPercentage += 10; 
        }
        if (isLoyalCustomer) {
            discountPercentage += 10;
        }
    }

    return discountPercentage;
}

double calculateDiscountAmount(double originalPrice, double discountPercentage) {
    return originalPrice * discountPercentage/100;
}

double calculateFinalPrice(double originalPrice, bool isStudent, bool isLoyalCustomer){
    double discountPercentage = calculateDiscountPercentage(originalPrice, isStudent, isLoyalCustomer);
    double discountAmount = calculateDiscountAmount(originalPrice, discountPercentage);
    double finalPrice = originalPrice - discountAmount;
    return finalPrice;
}

double redeemStudentPoints(double discountAmount){
    return discountAmount*(100-10)/100;
}

double redeemLoyaltyPoints(double discountAmount){
    return discountAmount*(100-2*10)/100;
}

double calculateRewardPoints(double discountAmount, bool isStudent, bool isLoyalCustomer){
    double rewardPoints = 0.0;
    if (isStudent){
        rewardPoints = redeemStudentPoints(discountAmount);
    } else if (isLoyalCustomer){
        rewardPoints = redeemLoyaltyPoints(discountAmount);
    }
    return rewardPoints;
}

int main() {
    const double originalPrice = 100;
    const bool isStudent = true;
    const bool isLoyalCustomer = true;

    cout << "Original price: " << originalPrice << " euros." << endl;

    double finalPrice = calculateFinalPrice(originalPrice,isStudent,isLoyalCustomer);
    cout << "Final price with student and loyalty discounts: " << finalPrice << " euros." << endl;

    double discountAmount = originalPrice - finalPrice;
    cout << "Your reward points: " << calculateRewardPoints(discountAmount,isStudent,isLoyalCustomer) << endl;

    return 0;
}

Save and compile the code (with g++ -o magic-numbers magic-numbers.cpp).

Run ./magic-numbers.
Do you understand what this code does?

Although the names of the elements are appropriate, this code is not clean. Can you spot the reasons?

Name the magic numbers.

User input¶

To avoid recompiling the code every time you modify originalPrice, isStudent, and isLoyalCustomer, we can retrieve the values from the keyboard during execution.

Replace the following lines with the code below.

const double originalPrice = 100;
const bool isStudent = true;
const bool isLoyalCustomer = true;

double userOriginalPrice;
cout << "Enter the original price: ";
cin >> userOriginalPrice;
const double& originalPrice = userOriginalPrice;

bool userIsStudent;
cout << "Are you a student? (1:Yes, 0:No) ";
cin >> userIsStudent;
const bool& isStudent = userIsStudent;

bool userIsLoyalCustomer;
cout << "Do you have a loyalty card? (1:Yes, 0:No) ";
cin >> userIsLoyalCustomer;
const bool& isLoyalCustomer = userIsLoyalCustomer;

Test your code and answer the questions in the Quiz.

Exercise 2 : Classes and methods¶

classes-and-methods.cpp

#include <iostream>
using namespace std;

double b1 = 0.0;
double b2 = 100.0;

void d1(double x) {
    b1 += x;
}

void d2(double x) {
    b1 += x;
}

void w1(double x) {
    if (x + 4 <= b1) {
        b1 -= (x + 4);
    } else {
        cout << "Insufficient funds!" << endl;
    }
}

void w2(double x) {
    if (x + 2 <= b2) {
        b2 -= (x + 2);
    } else {
        cout << "Insufficient funds!" << endl;
    }
}

int main() {
    d1(100.0);
    w1(50.0);
    w2(50.0);
    cout << "First account balance: " << b1 << endl;
    cout << "Second account balance: " << b2 << endl;
    return 0;
}

Do you understand what this code does?

Quiz: Which of the two accounts has the higher balance after these transactions?
Replace the previous transactions with the following:

d1(100.0);
w1(50.0);
d2(100.0);
w2(50.0);

Quiz: Now, which account has the higher balance?

To improve this code, we will rewrite it as follows:

classes-and-methods.cpp

#include <iostream>
using namespace std;

enum t {B, S};

class b {
private:
    double mB1;
    t mT;
    double mF;

public:
    b(double b1, t t1) : mB1(b1), mT(t1) {
        switch(t1){
            case B:
                mF = 4;
                break;
            case S: 
                mF = 2;
                break;
        }
    }

    void d(double x) {
        mB1 += x;
    }

    void w(double x) {
        if (x + mF <= mB1) {
            mB1 -= (x + mF);
        } else {
            cout << "Insufficient funds!" << endl;
        }
    }

    double getB1() const {
        return mB1;
    }

    string tToString() const {
        switch(mT){
            case B: return "Basic";
            case S: return "Standard";
            default: return "Unknown";
        }
    }
};

int main() {
    b a1(0.0,B);
    b a2(100.0,S);
    a1.d(100.0);
    a1.w(50.0);
    a2.d(100.0);
    a2.w(50.0);
    cout << "First account type: " << a1.tToString() << " and balance: " << a1.getB1() << endl;
    cout << "Second account type: " << a2.tToString() << " and balance: " << a2.getB1() << endl;
    return 0;
}

C++ Class Members

Private members of a class in C++ are often prefixed with m (e.g., mB1, mT, and mF). This prefix helps distinguish them from local variables used in the constructor (e.g., double b1), which are used to initialize members like mB1 (with mB1(b1)). Other conventions exist, such as using the _ prefix (e.g., _b1).

However, this practice is not always systematic. For example, it is possible to use shadowing, where a local variable and a class member share the same name. In this case, the distinction is made by using this->variable to refer to the class attribute and simply variable for the local variable. In Java, shadowing is the most popular convention (more details on this in your Object-Oriented Development course).

In this course, we will adopt the prefix convention (either m, or _) to clarify the distinction between class members and other variables.
You must stay consistent in your repo, do not mix m and _.

const method in a class

A const method is a member function of a class that, by placing the const keyword at the end of its declaration, guarantees that it does not modify the attributes of the object. This particularly applies to getters, which are often defined as follows:

<attributeType> getAttribute() const {
    return mAttribute;
}

Do you understand what this code does?

Hints

The t object of type enum can take the values B (for “Basic”) or S (for “Standard”).
The b class represents a bank account.
It has the following attributes:
- mB1, which represents its balance.
- mT, which indicates its type (Basic or Standard).
- mF, which is its withdrawal fee.

The following code is the constructor of the class:

b(double b1, t t1) : mB1(b1), mT(t1) {
    switch(t1){
        case B:
            mF = 4;
            break;
        case S: 
            mF = 2;
            break;
    }
}

An object of class b is constructed with two arguments: double b1, which is its initial balance, and t t1, which defines the account type.
The switch(t1) manages the different account types.
- If t1 == B, the withdrawal fee is 4 euros.
- If t1 == S, the withdrawal fee is 2 euros.
The tToString() method displays the types as corresponding strings.

Quiz: This time, which account has the highest balance?
This code compiles and works as it should, but it is a real eyesore. It hurts me physically to look at it. It is essential to rename these horrors.

Recompile and test your code.
Add a third “Premium” type with a withdrawal fee of 1 euro.
In the int main() function, replace the existing code with the following instructions:

Create three accounts with the three different types (Basic, Standard, and Premium), each having an initial balance of 100.0 euros.
Make a deposit of 50 euros into each account.
Make a withdrawal of 148 euros from each account.
Display the information (types and balances) of the three accounts.

Quiz: What is the sum of the balances of all three accounts?

Return to the objectives and check the points you have mastered. Review the points you have not fully understood yet. Ask your instructor for help if needed.

Practical Sessions

PS1: Discover Git and GitLab

Practical Sessions

PS3: Introduction to refactoring