Keep track of expenses c# - c#-4.0

Question

I am creating an application where the user shall be able to enter the following values:
gross monthly income (before deductions).
estimated monthly tax deducted.
groceries
water and lights
travel costs (including petrol)
cellphone and telephone
other expenses
The user shall be to choose between renting accommodation or buying a property if the the user selects to rents, the user will enter the monthly rental amount
if the user selects to buy a property, the user shall be required to enter the following values for a home loan:
purchase price of property
total deposit
interest rate
number of months to repay
The software shall calculate the monthly home loan repayment for buying a property based of the values that the user entered.
If the monthly home loan repayment is more than a third of the user's gross monthly income, the software shall alert that approval of the home loan is unlikely/
The software shall calculate the available monthly money after all the specified deductions have been made.
The software shall not persist the user data between runs. The data shall only be stored in memory while the software is running.
Please help me out with the code c#.

Answer 1

This should be fairly straight forward, it sounds like the program is supposed to be written entirely in the console which makes things a lot simpler.
Most of the code should use Console.WriteLine(String) and Console.ReadLine().
For example:
double netMonthlyIncome = 0;
Console.WriteLine("Enter gross monthly income:");
double gmi = Double.Parse(Console.ReadLine());
Console.WriteLine("Enter estimated monthly taxes:");
double emt = Double.Parse(Console.ReadLine());
netMonthlyIncome = gmi - emt;
What this code is doing is calling a built-in function to write a string to the console window, and setting up a double variable to hold the result from the console. The ReadLine function will wait for a user to hit enter and will take anything they wrote before enter and return it in a string variable. The Double.Parse() function takes this string and attempts to convert it to a number. The final step is that the gross monthly income variable will have the estimated monthly taxes subtracted from it to a seperate variable, this would be repeated for the other expenses. To go an extra step, you should include some checks to verify the inputs. For example:
double netMonthlyIncome = 0;
Console.WriteLine("Enter gross monthly income:");
String inputText = Console.ReadLine();
while(!Double.TryParse(inputText)) {
Console.WriteLine("That couldn't be interpreted, please re-enter a number:")
inputText = Console.ReadLine();
}
double gmi = Double.Parse(inputText);
In this example, the user would be prompted once to input the gross monthly income, but the TryParse function checks to see if the input is able to be converted to a double, and returns true if it can be. The ! will reverse whatever the result is, so if the TryParse returns true, the ! makes it false and the while() statement is skipped. If the TryParse returns false, the ! makes it true, and the while loop is entered where the user is prompted to re-enter a number, and will continue to be prompted until the TryParse is able to get a number out of it. This is, in some form, good practice to prevent a user from entering something like "Apples" and breaking the program.
After that, you would use similar prompts with new variables to get the cost of groceries, utilities, travel, etc. and use:
netMonthlyIncome = netMonthlyIncome - groceries - utilities - travel - ...;
You'd also use similar prompts to get the loan values, in this example I'll use 'a' as the loan amount, 'r' as the interest, and 'n' as the number of months. The formula should be double monthlyLoanPayment = a * (r(1+r)^n)/((1+r)^n - 1). It's maybe not the most efficient but personally I like to separate out everything to avoid any accidental syntax issues with order of operations.
So, you'd have:
r = r/12; // convert yearly interest to monthly interest
double numerator = r*Math.pow((1+r),n); //this is equivalent to r*(1+r)^n
double denominator = Math.pow(1+r,n) - 1; //this is equivalent to (1+r)^n - 1
double monthlyLoanPayment = a * (numerator/denominator);
if(monthlyLoanPayment >gmi/3) {
Console.WriteLine("This loan will probably get denied");
} else {
Console.WriteLine("This loan will probably get approved");
}
netMonthlyIncome = netMonthlyIncome - netMonthlyLoanPayment;
Console.WriteLine("Money Available each month:");
Console.WriteLine(netMonthlyIncome);