How to initialize local variable to unkown value C++ - c++

Question

trying to make my first post right so here goes.
I ran into this question and have not been able to figure it out. I keep receiving the error:
error C4700: uninitialized local variable 'miles' used
I have scavenged over all of StackOverflow and keep running into the same answer: I have to initialize my local variable, but when I do that I am creating a set value. I want to set my local variable 'miles' to an unknown value because I want the user to be able to set the value when the program runs.
Everything ran great until I tried to cast the end value 'miles'so that it would truncate.
Please correct me if I'm using incorrect terminology. Fresh-out-of-the-womb-to-programming. And thank you to everyone in advance.
Question:
Write a program that prompts the capacity, in gallons, of an automobile fuel tank and the miles per gallon the automobile can be driven. The program outputs the number of miles the automobile can be driven without refueling. Numbers entered for capacity must allow entry of capacity being an integer and the miles per gallon in decimals. The number of miles must be output to the next lowest integer (without decimals).
#include "stdafx.h"
//include statement
#include<iostream>
//include namespace statement
using namespace std;
//main function
int main()
{
//variable declaration
double capacity_Gallons;
double miles_Gallon;
double miles = static_cast<int>(miles < 0 ? miles - 0.5 : miles + 0.5);
//inputting capacity of automobile
cout << "Enter the capacity of the automobile fuel in gallons: ";
cin >> capacity_Gallons;
cout << endl;
//inputting the miles per Gallons
cout << "Enter the miles per gallons the automobile can be driven: ";
cin >> miles_Gallon;
cout << endl;
//calculating miles
miles = capacity_Gallons * miles_Gallon;
//display output data
cout << "Number of miles driven wihtout refueling: " << miles << endl;
//pause system for some time for user continuation
system("pause");
} //end main

Answer 1

You should take out that line entirely, and change the later line to double miles = capacity_Gallons * miles_Gallon;.
Instead of your handcrafted rounding code it would be better to use the standard rounding function in the display statement, ... << std::lround(miles) << ... although your assignment stipulation says you should round down , not round to nearest as you are currently doing. (So you can just cast to int there).

Answer 2

You don't need to declare miles there, you can declare it at the point it has a value.
#include<iostream>
int main()
{
//inputting capacity of automobile
double capacity_Gallons;
std::cout << "Enter the capacity of the automobile fuel in gallons: ";
std::cin >> capacity_Gallons;
std::cout << endl;
//inputting the miles per Gallons
double miles_Gallon;
std::cout << "Enter the miles per gallons the automobile can be driven: ";
std::cin >> miles_Gallon;
std::cout << endl;
//calculating miles
double miles = capacity_Gallons * miles_Gallon;
//display output data
std::cout << "Number of miles driven wihtout refueling: " << miles << std::endl;
//pause system for some time for user continuation
system("pause");
}
As an aside, using namespace std is a bad habit.