C++ basic menu driven program (calling functions) - c++

Question

I'm new to programming and am trying to figure out why this is happening.
Basically I'm asked to design a menu driven program that does some basic deposit/withdrawal calculations. I'm supposed to write different functions for each process and call on them when necessary.
I'm having 2 problems:
1) My functions aren't updating my variables within the program. For example it will run, I can enter my starting balance, I can enter my transaction type and amount but every time I switch from deposit to writing a check, it resets the balance to the original user input.
2) When I want the program to exit, it is still asking me for the double input. Not sure how to make it accept just an "E" instead of "E number"
Thanks in advance.
#include <iostream>
#include <iomanip>
#include <string>
using namespace std;
//Prototypes for my functions that will be called in
void displayMenu();
double checkProcess(double, double);
double depositProcess(double, double);
double totalServCharge(double);
int main()
{
//Variables needed for the problem
char choice; //menu choice
double transAmt, balance, numbServCharge; //transaction amount, current balance, total service charges, number of service charges
numbServCharge = 0; //Start the counter for number of service charges at zero
cout << "Checkbook Program\n\n";
cout << "Enter the beginning balance: ";
cin >> balance; //get the initial balance
cout << endl;
do
{
//Call the display menu function
displayMenu();
//Get user's choice and transaction amount from menu
cout << "Enter a transaction: ";
cin >> choice >> transAmt;
choice = toupper(choice);
cout << endl;
//Create an error message for invalid choice and get a second choice
while((choice != 'C') && (choice != 'D') && (choice != 'E'))
{
cout << "Invalid selection. Please choose C, D or E: ";
cin >> choice;
}
//Set up for option #1 -- using a check
if(choice=='C')
{
//Call check function and service charges function
checkProcess(transAmt, balance);
totalServCharge(numbServCharge);
}
//Set up for option #2 -- deposits
if(choice=='D')
{
//Call deposit function and service charges function
depositProcess(transAmt, balance);
totalServCharge(numbServCharge);
}
}while(choice != 'E'); //Close program if option 3 is selected
//Display final balance
cout << "Processing end of the month";
cout << "\nFinal balance : $ " << fixed << setprecision(2) << balance << endl << endl;
system("pause"); //Pause screen so it doesn't close right away
return 0;
}
void displayMenu()
{
//Highlight menu options
cout << "\nCommands\n";
cout << "C amount - process a check in a specific amount\n";
cout << "D amount - process a deposit in a specific amount\n";
cout << "E - end the program\n\n";
}
double checkProcess(double transAmt, double balance)
{
cout << "\nProcessing check for $" << fixed << setprecision(2) << transAmt;
transAmt = transAmt + .25; //Add the service charge onto the transaction
balance = balance - transAmt; //Update account balence
cout << "\nBalance: $" << fixed << setprecision(2) << balance;
cout << "\nService charge: $0.25 for a check\n";
return balance;
}
double depositProcess(double transAmt, double balance)
{
cout << "\nProcessing Deposit for $" << fixed << setprecision(2) << transAmt << endl;
transAmt = transAmt - 0.25; //Add the service charge onto the deposit
balance = balance + transAmt; //Update account balence
cout << "Balance: $" << fixed << setprecision(2) << balance << endl;
return balance;
}
double totalServCharge(double numbServCharge)
{
double totalServCharge = 0;
numbServCharge++; //Add one to the number of service charges there have been
totalServCharge = .25 * numbServCharge; //Update total cost of service charges
cout << "Total service charges: $" << fixed << setprecision(2) << totalServCharge << endl; //Tell user total service charges so far
return numbServCharge;
}

Answer 1

(1) Your variables aren't updating because you aren't changing them; you're changing the copies passed to your subroutines. You either need to pass the current balance by reference (so that when you change it in the subroutine scope, you're also changing the original) or assign the return value from your subroutines to your current balance.
When you pass a variable by value (as you are now), the program makes a copy of that variable for use in the function scope. When you leave that scope, the copy is deleted. Do this when you want to be sure a subroutine won't alter anything in the scope from which it is called. If you want the subroutine to alter the variables you pass to it, pass the variables by reference. In this function signature, "transAmt" is passed by value and "balance" is passed by reference:
double foo(double transAmt, double& balance);
Incidentally, if you pass the balance by reference there's no reason to return it. You aren't doing anything with the returned value right now anyway.
(2) If you want to not ask for a number when "E" is given, make separate "cin" statements for the number in the conditional blocks for "C" and "D". Right now all your input code is shared between all cases. It would be even better if you arrange things so that you exit the loop before a number is asked for. This way, you only have to write that "cin" statement once.

Answer 2

It's becouse in C++ when you call a function with parameter, you only operate on COPIES of those parameters. If you want to change the original value, pass variable as reference.
See those simple examples:
#include <iostream>
using namespace std;
void normal(int a)
{
a += 10;
}
void by_reference(int &a) //Notice ampersand
{
a += 10;
}
int by_return(int a)
{
return a + 10;
}
int main()
{
int a = 5;
cout << "Start: " << a <<endl;
normal(a);
cout << "Normal: " << a <<endl;
by_reference(a);
cout << "Reference: " << a <<endl;
a = by_return(a);
cout << "Return: " << a <<endl;
}
Now you can see that if you pass the variable as reference it gets changed.
Even if just putting the & before parameter names would fix your problem, I'd recommend you using a return, it generally is simplier and makes your code look better (it's much easier to understand).
Also, correct your indentation. It's very bery important when you work with bigger projects.
Hope this helps. :)

Answer 3

1 | The first problem is occurring because you are not actually modifying the user's input. You are passing the variable in but once the new value is returned you are not storing it, thus the value is forgotten in the main function. Not the best solution, but it might suit you to do this:
if (choice == 'D')
{
//Call deposit function and service charges function
balance = depositProcess(transAmt, balance); // Overwrite balance
numbServCharge = totalServCharge(numbServCharge); // Overwrite numbServCharge
}
Or in a more efficient way, you can pass in the variable by reference. All you would need to do is change the function prototype.
void totalServCharge(double&);
void depositProcess(double&, double&);
Now when the variable is passed in, it will be modified directly by the function. Also note the function data type changed to void because you no longer need to return a value from the function.
2 | Your second problem is the way you handle user input. If you want to ask for multiple input, it might sometimes be best to do it with separate cout and cin statements.
The way it is set in your program is for the user to enter two inputs at once, therefore if you only enter E, it waits patiently as it should for your next input (transAmt). It might be better to process the action desired and then have an amount entered afterwards.
Very general example:
do {
cout << "1 : Transaction or 2 : Exit" << endl;
cin >> choice;
if (choice == 2)
break;
cout << "1. Deposit\n" << "2. Check" << endl;
cout << "Enter transaction type followed by amount" << endl;
cin >> transType >> amount;
// Then do what was specified in choice
} while (true); // Or whatever condition pleases you