Thread: Need help with my first program

enrolled in my first c++ class and have a homework assignment due. my problem is that i declared my variables and wrote what i think is a good start for the program. my problem is that when i enter the information that i want, my actual output does nothing. i cant seem to get my program to do the math it is supposed to. can someone look at this and see what i am missing. this is just part of the project. i want to get on the right track before i write the rest. i just want this to figure the old monthly cost. i use Dev C++. thank you.

Code:

#include <iostream>
#include <cstdlib>
#include <cfloat>

using namespace std;

int main(int argc, char *argv[]) {


float CostOfNewCar; //New vehicle price
float MonthlyMilesDriven; //Mile driven per month
float NewVehicleMPG; //New vehicles mile per gallon of gas
float OldVehicleMPG; //Old vehicles miles per gallon of gas
float CurrentMonthlyCost; //Current monthly cost that a persons spends
float ProposedMonthlyCost; //Proposed monthly cost that person will spend
float PriceOfGas; //Enter the current price of gas per gallon
float NewCarPayment; //New car payment cost
float OldMonthlyGasExpense; //Old vehicle gas expense per month
float NewMonthlyGasExpense; //New vehicle gas expense per month
float OldCarPayment; //Old car payment

// Prompt user for information

cout << "How many mile do you drive a month?:" ;
cin >> MonthlyMilesDriven;

cout << "Current vehichles MPG :" ;
cin >> OldVehicleMPG;

cout << "How much is gas per gallon:" ;
cin >> PriceOfGas;








// Calculations

NewCarPayment = CostOfNewCar / 60 ;
OldCarPayment = 130.00;
OldMonthlyGasExpense = MonthlyMilesDriven / OldVehicleMPG * PriceOfGas;
CurrentMonthlyCost = OldMonthlyGasExpense + OldCarPayment;





// Output results

cout << "Currently monthly cost: ";
cin >> CurrentMonthlyCost;







system("pause");


return(0);




}

first, you should always initialize your variables. this means assigning a value to your variable--even if you dont think you need to. for example, in your code you have:

Code:

NewCarPayment = CostOfNewCar / 60 ;

however CostOfNewCar was never initialized, so the result of this is undefined. one time you run the program, CostOfNewCar may have the value of 10, another time it may have the value of -999.12345. maybe you need to add some code to ask for this information from the user, and of course read it in?

now for the problem your referring to, its pretty simple, your pretty much there!

Code:

// Output results

    cout << "Currently monthly cost: ";
    cin >> CurrentMonthlyCost;

notice in the second line your telling the computer "receive input and store it in the CurrentMonthlyCost variable". you probably want to instruct the computer to "output the value of the CurrentMonthlyCost variable". cout is for output and cin is for input.

edit: also, for the code above you only need the first include statement (iostream). if you know your going to need the other two headers for further development of your project, then you of course must leave them in, but otherwise get rid of them.
more importantly, you should not use something like "system("pause")", as this immediately restricts your program to Windows operating systems. if you submit your source code for marking to your teacher who could be running Linux and he/she tried to compile and then run it, the output would very likely contain an error message saying that "pause" cannot be found. instead of using a windows-specific function, you could do a simple "cout" saying to press enter to exit, and a simple "cin" to accept the enter (into a dummy variable), which would work on any operating system.