Reading from/writing to files using functions [closed] - c++

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Hi I am working on a simple wage application. The following code contains a two option menu using switch statements. The code is also linked to a text file called "shop-account". The file simply contains the value 100.
For option 1 the user is suppose to be able to transfer an amount from the file. The user should be able to make as many transfers as they choose without overdrawing the account. And the code should be able to output the current balance. I believe I am suppose to be using the void function but I have never used it before and am really struggling. I was hoping someone could look at the code and tell me where I am going wrong. Thanks
int read_balance (void);
void write_balance (int balance);
using namespace std;
int _tmain(int argc, _TCHAR* argv[])
{
int selection;
int total;
int balance;
int NewAmount;
do {
cout << "1. Transfer an amount" <<endl;
cout << "2. Quit" << endl;
cout << "Please enter menu number"<<endl;
cin >> selection;
switch(selection)
{
case 1:
cout << "You have choosen to transfer an amount" << endl;
cout << "How much do you wish to transfer from the shop account?"<<endl;
cin >> NewAmount;
balance -= NewAmount;
write_balance (balance);
cout << balance << endl;
break;
case 2:
return 0;
break;
default:
cout << "Ooops, invalid selection!" << endl;
break;
}
}while(selection != 2);
system("pause");
return 0;
}
int read_balance (void)
{
fstream f;
f.open("shop-account.txt");
f >> balance; //error: "balance" is unidentified
f.close();
return balance;
}
void write_balance (int balance)
{
fstream f;
f.open("shop-account.txt");
f << balance;
f.close();
}

Answer 1

Like others mentioned, you didn't declare the int balance in the right scope (function level scope, in this case).
In fact, it looks like you forgot call read_balance alltogether, so your calculations on balance used an indeterminate value, which is Undefined Behaviour.
Next up: declare your variables where they are used, that way you prevent this whole situation when you decide to extract pieces of code into subfunctions, and it gets easier to see where variables are used. Consequently, it gets easier to see whether the code is correct.
Next: Error handling. If you have none, your program is worthless. In fact, even when fixing the above issue,
simply entering invalid input once would run a loop and just keep substracting indeterminate values from the account balance and writing those wrong values to disk. That's probably not what you wanted.
making the shop-account.txt read-only was enough to fool the program into
transfer unlimited amounts without updating the file
having the wrong idea of the balance in the file (since it never checks)
Here is a cleaned-up version that does a minimum amount of checking, and adds an option to just 'check the account balance'. Seemed useful.
See it Live On Coliru
I hope some of this helps. For one thing, I hope your teacher was going to mention most of this :/
int read_balance(void);
void write_balance(int balance);
#include <iostream>
#include <limits>
int main()
{
while(std::cin.good())
{
std::cout << "0. Request balance" << std::endl;
std::cout << "1. Transfer an amount" << std::endl;
std::cout << "2. Quit" << std::endl;
std::cout << "Please enter menu number" << std::endl;
int selection = 2;
if(std::cin >> selection)
{
std::cout << "DEBUG: selection:" << selection << "\n";
switch(selection)
{
case 0:
std::cout << "The current account balance is " << read_balance() << std::endl;
break;
case 1:
{
std::cout << "You have choosen to transfer an amount" << std::endl;
std::cout << "How much do you wish to transfer from the shop account?"<<std::endl;
int amount = 0;
if (std::cin >> amount)
{
std::cout << "DEBUG: amount:" << amount << "\n";
int balance = read_balance();
if(amount<=0)
{
std::cout << "Amount must be positive\n";
}
else if(balance < amount)
{
std::cout << "Insufficient funds\n";
}
else
{
int new_balance = balance - amount;
write_balance(new_balance);
std::cout << "New account balance: " << new_balance << std::endl;
}
} else
{
// bad input cleared outside the switch
}
}
break;
case 2:
return 0;
break;
default:
std::cout << "Ooops, invalid selection!" << std::endl;
break;
}
}
if(std::cin.eof())
{
std::cout << "Bye\n";
return 0;
}
if(std::cin.fail())
{
std::cin.clear();
std::cin.ignore(99999, '\n');
std::cout << "Invalid input\n";
// note eof() can be true here
}
}
}
#include <fstream>
int read_balance(void)
{
std::ifstream f;
f.exceptions(std::ios::failbit | std::ios::badbit);
f.open("shop-account.txt");
int balance;
f >> balance;
f.close();
return balance;
}
void write_balance(int balance)
{
std::ofstream f;
f.exceptions(std::ios::failbit | std::ios::badbit);
f.open("shop-account.txt");
f << balance;
f.close();
}

Answer 2

your function does not have a balance varible declartion !! you have to add a balance declaration in your function
int read_balance(void)
{
fstream f;
f.open("shop-account.txt");
int balance;
f >> balance; //now it's defined
f.close();
return balance;
}