C++ Program Help (MPG calculator)

Hello, I have this project due today for class and I cant seem to get it compiled without errors, it gives me one error and its not specific enough for me to fix it on my own, so I am asking for help here. Here is the code, and what its "supposed" to do.

"A liter is 0.264179 gallons. Write a program that will read in the number of liters of gasoline consumed by the user's car and the number of miles traveled by the car, and will then output the number of miles per gallon the car delivered. Your program should allow the user to repeat this calculation as often as the user wishes. Define a function to compute the number of miles per gallon. Your program should use a globally defines constant for the number of liters per gallon."

And here is my code;

Code:

---

//Assignment 1
//Gas
//september-27-05
//


#include <iostream>
using namespace std;


int main()
{
int liters;
double distance;
double gallon = (0.264179*liters);

do
{
cout << "Please input how many liters of gasoline is in your vehicle";
cin >> liters;
cout << "Please input the distance in miles you traveled in your vehicle.";
cin >> distance;

cout << "Your vehicle's MPG is:" << (distance/liters) << endl;


}while(liters >0);

return 0;

}

---

Thanks in advance to anyone who can help.

-Brent

Code:

---

double gallon = (0.264179*liters);

---

You can't do that since liters is undefined at the point of usage. Basically what you want is have it as a constant ratio and then multiply it (factor-label) in the MPG equation:

Code:

---

const double GALLONS_PER_LITER = 0.264179;
...
double mpg = distance / (liters * GALLONS_PER_LITER);

---

int liters = 0;

and what speedy said.

Thanks speed and fooddude, i changed the int liters to equal 0, then i changed out the 2 lines that speedy gave me, and i seem to have more errors, i may have took what speedy said wrong and did the wrong things, this is my code as of now ;

Code:

---

//Gas
//september-27-05
//


#include <iostream>
using namespace std;


int main()
{
int liters = 0;
double mpg = distance / (liters * GALLONS_PER_LITER);
const double GALLONS_PER_LITER = 0.264179;

do
{
cout << "Please input how many liters of gasoline is in your vehicle";
cin >> liters;
cout << "Please input the distance in miles you traveled in your vehicle.";
cin >> distance;

cout << "Your vehicle's MPG is:" << (distance/liters) << endl;


return 0;

---

and the errors now are;
C:\Documents and Settings\classics\Desktop\mpgproject.cpp(15) : error C2065: 'GALLONS_PER_LITER' : undeclared identifier
C:\Documents and Settings\classics\Desktop\mpgproject.cpp(15) : error C2563: mismatch in formal parameter list
C:\Documents and Settings\classics\Desktop\mpgproject.cpp(23) : error C2679: binary '>>' : no operator defined which takes a right-hand operand of type '' (or there is no acceptable conversion)
C:\Documents and Settings\classics\Desktop\mpgproject.cpp(25) : error C2563: mismatch in formal parameter list
C:\Documents and Settings\classics\Desktop\mpgproject.cpp(31) : fatal error C1004: unexpected end of file found

Pay attention to the errors.

You use GALLONS_PER_LITER in a calculation before you define it.

You never delare distance. (and, you probably don't want to use it in a calcualtion until after it's been input by the user)

Your do while loop has no closing brace... or a while()

Im still confused about the gallons_per_liter error... I fixed everything else and now im only recieving 2 errors about the gallon_per_liter part.. heres my code and errors, if anyone knows a solution (explained in dummy terms for a C++ noob) it would be awesome and greatly appreciated.

Code:

---

/Assignment 1
//Gas
//september-27-05
//frfrf


#include <iostream>
using namespace std;


int main()
{
int liters = 0;
double mpg = distance / (liters*GALLONS_PER_LITER);
const double GALLONS_PER_LITER = 0.264179;
double distance = 0;

do
{
cout << "Please input how many liters of gasoline is in your vehicle";
cin >> liters;
cout << "Please input the distance in miles you traveled in your vehicle.";
cin >> distance;

cout << "Your vehicle's MPG is" << (distance/liters) << endl;

 }while(liters >-1);

return 0;

}

---

C:\Documents and Settings\classics\Desktop\mpgproject.cpp(14) : error C2065: 'GALLONSPERLITER' : undeclared identifier
C:\Documents and Settings\classics\Desktop\mpgproject.cpp(14) : error C2563: mismatch in formal parameter list

Look here, these two lines are in the wrong order. You use GALLONS_PER_LITER and then you declare it:

Code:

---

double mpg = distance / (liters*GALLONS_PER_LITER);
const double GALLONS_PER_LITER = 0.264179;

---

Put them the other way around.

distance is also in the wrong place. And this line is a syntax error:

Code:

---

/Assignment 1

---

The calculation of mpg should be moved much farther down. You must wait until the user enters the liters and the distance before you calculate the mpg. Follow the program flow from top to bottom on paper or in your head. You will see that it doesn't make sense as written.

Yeah, right now it always calulates 0 litres.

how would i move it down? i thought ints and const and doubles ect need to be determined at the top of the code?

i switched the 2 lines around and now only 1 error...

C:\Documents and Settings\classics\Desktop\mpgproject.cpp(15) : error C2563: mismatch in formal parameter list

Quote:

---

i thought ints and const and doubles ect need to be determined at the top of the code?

---

In C, they do. In C++, they don't. But you want to declare the variables at the top of the function, and preform the calculation[s] on them later on.

>> i thought ints and const and doubles ect need to be determined at the top of the code?

Nope, that is not true. There is a style of programming that declares these things at the top of code, but it is not appropriate for C++ anyway. If your instructor is telling you to do that, you can still assign a value later. Initialize mpg to 0.0, and later assign it when you know the values of liters and distance.

Post your new code to get help with the new error.

>> you want to declare the variables at the top of the function, and preform the calculation[s] on them later on

This is not true for C++. Declare your variables only when and where you need them, and initialize them fully when they are declared.

You can do this, you know:

Code:

---

int main() {
    int x, y = 3;

    x = y * y;
}

---

sorry i forgot to post it i guess

Code:

---


//Gas
//september-27-05
//frfrf


#include <iostream>
using namespace std;


int main()
{
int liters = 0;
const double GALLONS_PER_LITER = 0.264179;
double mpg = distance / (liters*GALLONS_PER_LITER);
double distance = 0;

do
{
cout << "Please input how many liters of gasoline is in your vehicle";
cin >> liters;
cout << "Please input the distance in miles you traveled in your vehicle.";
cin >> distance;

cout << "Your vehicle's MPG is" << (distance/liters) << endl;

 }while(liters >-1);

return 0;

}

---

ok so say i turn double mpg = distance / (liters*GALLONS_PER_LITER);

into


double mpg = 0.0

then where would i put the distance / (liters*GALLONS_PER_LITER); ?

After you have the variables involved, of course.

You also need to save the result of the calculation. You can't just go

Code:

---

distance / (liters*GALLONS_PER_LITER);

---

but rather have to go

Code:

---

mpg = distance / (liters*GALLONS_PER_LITER);

---

Quote:

---

then where would i put thYou are dividing distance by (liters*GALLONS_PER_LITER);?

---

Would it not make sense to do that after you have input the distance? Other wise you are just dividing 0 by something (which will always be zero).

think about the flow of your program.

-Ask two questions.
-store the answers.
-Use the answers in a calculation.
-display the results.

The value of your variable can be changed as many times as you want throughout your program, so don't get confused with thinking that you have to have the final result at the time of declaration.

Also, as stated, in C++ you do not have to declare your variables at the top of your code.

Code:

---

#include <iostream>
using namespace std;


int main()
{
  const double GALLONS_PER_LITER = 0.264179; //give a meaningful name to conversion constant
 
  int liters = 0;        //will be set by cin before we use in calculation, but
  double distance = 0.0;  //it's nice to get in the habit of initializing variables
 
  double mpg = 0.0; //could (should) wait to declare this until closer to where we need it

  do
  {
      cout << "Please input how many liters of gasoline is in your vehicle: ";
      cin >> liters;
     
      cout << "Please input the distance in miles you traveled in your vehicle: ";
      cin >> distance;
     
      //We've got our user input... let's do our calculation.
      mpg = distance / (liters * GALLONS_PER_LITER);
      // OR
      //we could just as well declare mpg here and omit mpg lines above
      //double mpg = distance / (liters * GALLONS_PER_LITER);
     
      //display results
      cout << "Your vehicle's MPG is: " << mpg << endl;

  }while(liters >-1);

  return 0;
}

---

thank you everyone who helped!

-brent

Quote:

---

Originally Posted by mrwooter

"A liter is 0.264179 gallons. Write a program that will read in the number of liters of gasoline consumed by the user's car and the number of miles traveled by the car, and will then output the number of miles per gallon the car delivered. Your program should allow the user to repeat this calculation as often as the user wishes. Define a function to compute the number of miles per gallon. Your program should use a globally defines constant for the number of liters per gallon."

---

You need to make sure you read your assignment carefully, he wants a global constant that stores the 0.264179 conversion rate. Global means its defined outside of any function.

ie:

Code:

---


#include <stdio.h>

const double foo = 0.264179; /* globally defined, accessible from
                                                    any function */

int main(void)
{
  /* do things here */

  return 0;
}

---

Note: this is a C example obviously, but it shows the concept your teacher is looking for.