我需要课堂帮助。 （C ++）

Question

Well I am trying to do an exercise in my programming book and it's difficult for me to grasp exactly what it wants.

My "enterAccountData()" function is suppose to ask the user for an account number and a balance neither of which can be negative and the account number cannot be less than 1000

The second one is the one I am stuck on "computeInterest()" THis function is suppose to accept an integer argument that represents the number of years the account will earn interest. The function then displays the account number from the previous function and displays its ending balance at the end of each year based on the interest rate attached to the BankAccount class. (The interest rate on "BankAccount" has to be a constant static field which is set at 3 percent(0.03)).

So My question is this: How do I set up "computeInterest()" too allow it to calculate the interest using the constant static field when my debugger will not allow me to actually keep the field as a constant? I am not trying to stop any random errors from happening for now, just trying to get the jist of what the book is exactly asking for. Here is my code.

#include <iostream>
#include <iomanip>

using namespace std;

class BankAccount
{
  private:
    int accountNum;
    double accountBal;
    static double annualIntRate;
  public:
    void enterAccountData(int, double);
    void computeInterest();
    void displayAccount();
};

//implementation section:
double BankAccount::annualIntRate = 0.03;
void BankAccount::enterAccountData(int number, double balance)
{
    cout << setprecision(2) << fixed;
    accountNum = number;
    accountBal = balance;

    cout << "Enter the account number " << endl;
    cin >> number;

    while(number < 0 || number < 999)
    {
        cout << "Account numbers cannot be negative or less than 1000 " <<
                "Enter a new account number: " << endl;
        cin >> number;
    }

    cout << "Enter the account balance " << endl;
    cin >> balance;

    while(balance < 0)
    {
        cout << "Account balances cannot be negative. " <<
                "Enter a new account balance: " << endl;
        cin >> balance;
    }
    return;
}
void BankAccount::computeInterest()
{
    const int MONTHS_IN_YEAR = 12;
    int months;
    double rate = 0; 
    int counter = 0;
    BankAccount::annualIntRate = rate;

    cout << "How many months will the account be held for? ";
    cin >> months;
    counter = 0;
    do
    {
        balance = accountBal * rate + accountBal;
        counter++;
    }while(months < counter);
    cout << "Balance is:$" << accountBal << endl;
}

int main()
{
    const int QUIT = 0;
    const int MAX_ACCOUNTS = 10;
    int counter;
    int input;
    int number = 0;
    double balance = 0;

    BankAccount accounts[MAX_ACCOUNTS];
    //BankAccount display;

    counter = 0;

    do
    {

        accounts[counter].enterAccountData(number, balance);
        cout << " Enter " << QUIT << " to stop, or press 1 to proceed.";
        cin >> input;
        counter++;
    }while(input != QUIT && counter != 10);

    accounts[counter].computeInterest();

    system("pause");
    return 0;
}

Answer 1

The constant field is easy enough:

class BankAccount
{
  ...
  const static double annualIntRate = 0.03;
  ...
}

(Does your debugger complain about that? I'm using gcc 4.2.1) But there are other troubling things in your code, like the way computeInterest tries to set rate to zero, and the while loop... needs work.

One good principle is worth a hundred specific corrections. When you develop a complicated function, don't try to do it all at once; start with a simple piece, get that working perfectly, then build up. F'rinstance, computeInterest . You have several independent parts to get working: going through the while loop the correct number of times, calculating interest increments, keeping track of the balance-- right now computeInterest does none of these correctly. Tackle them one at a time, or in parallel if you want, but never combine parts that don't work.

Answer 2

boy it's been a LONG time since I've worked in C++, but I think all you have to do is this:

static double annualIntRate =.03;

in the "private" section of your code.

and then you can use annualIntRate as though it was a global (to each instance of the class) variable.