Program help

[kethgr]

What's wrong with my program? It will compile and run but doesn't do the calculations. It also says my variables are being used without being initialized. Please help. Thanks!

#include<iostream>

using namespace std;

int main ()
{
double stMileage;
double endMileage;
double stFuel;
double endFuel;

double miTrvld = endMileage - stMileage;
double fuelUsed = stFuel - endFuel;
double MPG = miTrvld / fuelUsed;

const double kmTrvld = miTrvld * 1.609;
const double fuelUsedL = fuelUsed * 3.785;
const double litersHundredKm = fuelUsedL / (kmTrvld * 100);

cout << "Enter starting mileage: ";
cin >> stMileage;
cout << endl;

cout << "Enter ending mileage: ";
cin >> endMileage;
cout << endl;

cout << "Enter fuel at start in gallons: ";
cin >> stFuel;
cout << endl;

cout << "Enter fuel at end in gallons: ";
cin >> endFuel;
cout << endl;

cout << "Number of miles traveled: "
<< miTrvld << " miles"
<< endl;

cout << "Fuel used: "
<< fuelUsed << " gallons"
<< endl;

cout << "Miles per gallon of fuel: "
<< MPG << endl;

cout << endl;

cout << "Number of kms traveled: "
<< kmTrvld << " kms"
<< endl;

cout << "Fuel used: "
<< fuelUsedL << " liters"
<< endl;

cout << "Liters per 100 kms: "
<< litersHundredKm
<< endl << endl;

system("pause");

return 0;

}

[Peter87]

You use these variables before they have been initialized

double stMileage;
double endMileage;
double stFuel;
double endFuel;

[kethgr]

I'm sorry, I'm not really sure how to fix that.
I wrote another program with a similar format and it worked fine.
Is the positioning of the variable declarations wrong?

[cire]

No, it's mostly that you use them before they've been assigned any meaningful value.

double miTrvld = endMileage - stMileage;
double fuelUsed = stFuel - endFuel;

endMileage, stMileaage, stFuel and endFuel have random junk values. So miTrvld and fuelUsed is also going to be some random junk value.

And, you know what they say: Garbage in, garbage out.

The solution is to perform the calculations after getting the user input.

[kethgr]

That makes sense to me, but my teacher keeps telling us to put all of our declarations at the beginning of the function.

So, should I declare them at the top, then put their calculations after the user input?

[cire]

If only to satisfy your instructor.

Best practice is to declare/define variables as close as possible to first use.

[TheBeardedQuack]

It's a simple mistake that i've made many times (and occasionally still do)...

for example: you're trying to declare "miTrvld" as "stMilage - endMileage" but these two variables are still set as 0.
So what's happening is "miTrvld = 0 * 0"

FIX= Have all your questions BEFORE your calculations.
Then you'll have the actual values to be able to do the calculations with!


=======================================================

EDIT: you can still declare your variables first if you like and then assign them later but if you do this DO NOT declare you final values as constants...

example:

double miTrvld;
double stMileage;
double endMileage;

//then questions

miTrvld = endMileage - stMileage;

[kethgr]

Got it. Thank you. :)