Uninitialized Local Variable - c++

Question

I'm new and just learning C++ and came across this problem that I've spent maybe an hour trying to fix and researching answers on but I cant seem to figure out what I'm doing wrong. I'm using Visual Studios as my IDE, the most recent version.
#include "stdafx.h"
#include <iostream>
#include "constant.h"
//height of the tower
double towerHeight(double x)
{
using namespace std;
cout << "Enter a height for the tower" << '\n';
cin >> x;
return x;
}
//the number of seconds since the ball has been dropped to determine the distance
double secondsSinceDrop(double x)
{
using namespace std;
cout << "How long has it been since you dropped the ball (Seconds): ";
cin >> x;
return x;
}
//finds how far off the ground the ball is
double currentBallHeight(double x, double y)
{
return y * constant::gravity - x;
}
//prints how far off the ground the ball is
void printResult(double x, double y)
{
using namespace std;
if (currentBallHeight(x, y) < 0)
cout << "At " << y << " the ball is on the ground." << '\n';
else
cout << "At " << y << " the ball is at: " << currentBallHeight(x, y) << '\n';
}
int main()
{
double x = towerHeight(x);
double y = secondsSinceDrop(x);
printResult(x,y);
return 0;
}
This is the Error Code
- chapter 2 comprehensive quiz (part 2).cpp(46): error C4700: uninitialized local variable 'x' used
-Line (46) is - double x = towerHeight(x);
I've been getting this and I've changed my code around to get it down to just this one error but i cant figure out how to fix it. Its probably something simple and I'm dumb for overlooking it but any help would be greatly appreciated.

Answer 1

These lines will be throwing errors
because the variable 'x' you are sending as an argument does not exist in the scope of main
int main()
{
-> double x = towerHeight(x);
-> double y = secondsSinceDrop(x);
printResult(x,y);
return 0;
}
Instead you could try something like this.
#include "stdafx.h"
#include <iostream>
#include "constant.h"
using namespace std;
//height of the tower
double towerHeight()
{
double height;
cout << "Enter a height for the tower" << '\n';
cin >> height
return height;
}
//the number of seconds since the ball has been dropped to determine the distance
double secondsSinceDrop()
{
double seconds;
cout << "How long has it been since you dropped the ball (Seconds): ";
cin >> seconds;
return seconds;
}
//finds how far off the ground the ball is
double currentBallHeight(double x, double y)
{
return y * constant::gravity - x;
}
//prints how far off the ground the ball is
void printResult(double x, double y)
{
if (currentBallHeight(x, y) < 0)
cout << "At " << y << " the ball is on the ground." << '\n';
else
cout << "At " << y << " the ball is at: " << currentBallHeight(x, y) << '\n';
}
int main()
{
double height = towerHeight();
double seconds = secondsSinceDrop();
printResult(height, seconds);
return 0;
}
Some tips that I would recommend
Declare your variables as much as relevant to you instead of using 'x/y/z'
There is no need to add the using namespace std; inside each function

Answer 2

Your first line of code in main() is double x = towerHeight(x);, what value of x are you sending to the function, when you have not initialized it.
When you are using a variable without initializing the value of it is undefined.
You can pass the variable as a reference to your function and accept the values inside it.
//height of the tower
void towerHeight(double &x)
{
using namespace std;
cout << "Enter a height for the tower" << '\n';
cin >> x;
}
//the number of seconds since the ball has been dropped to determine the distance
void secondsSinceDrop(double &y)
{
using namespace std;
cout << "How long has it been since you dropped the ball (Seconds): ";
cin >> y;
}
int main()
{
double x = 0.0, y = 0.0;
towerHeight(x);
secondsSinceDrop(y);
printResult(x, y);
return 0;
}

Answer 3

You seem to be struggling to connect the mental dots on what the computer is doing when you
declare variables with an initial value
define function parameters
return a value from a function
Not sure how this question will fair with the SO community as the preference is for Q/A that is succinct and reusable (maybe some editing can help) but for your benefit let me explain some of these concepts.
Let's start with a variable declaration
int x = 5;
int y = x;
When you define int x; it creates a space in RAM for an integer (4 bytes). Adding the = 5 initializes it immediately. It's important that the value on the right side of = (5 in this case) is known before the computer tries to make space for x.
It's fine to use values that aren't constant for variables like this (notice the second line in the example) but x has to be known before you declare y. In other words, this would obviously be a problem:
int y = x;
int x = 5;
For this same reason, the line: double x = towerHeight(x); is problematic because you're using x when you call towerHeight before ever defining x
When you define a function's parameters:
double towerHeight(double x) {
This tells the computer that you are going to copy the value from whatever called towerHeight to a new place in RAM and call it "x". This means that the value outside of the function doesn't get modified. Consider the following example:
double towerHeight(double x) {
x = 5;
std::cout << x << std::endl; // outputs 5
}
int main() {
double x = 10;
towerHeight(x);
std::cout << x << std::endl; // outputs 10
return 0;
}
Even though x was changed in towerHeight that was a "different copy of x" which also happened to be called the same name.
When you return a value from a function, in the same manner as passing a function argument, the return value is copied and used in places of the function call. Let's modify the previous example slightly:
double towerHeight(double x) {
x = 5;
return x;
}
int main() {
double x = 10;
x = towerHeight(x); // returns the value "5"
std::cout << x << std::endl; // Outputs "5"
return 0;
}
You can think of towerHeight(x) being replaced by "5" so the code would read x = 5;
Conclusion
You should try and use different variable names for
function arguments (the variables/values you pass to the function)
function parameters (what they are called inside the function)
to avoid this kind of confusion. Though there may be times where using the same name makes sense (i.e. passing by reference, which is another question). It's important for you to be aware of what's really going on.
Here is what you probably intend to do:
double towerHeight()
{
double height;
std::cout << "Enter a height for the tower" << std::endl;
std::cin >> height;
return height;
}
double secondsSinceDrop()
{
double seconds;
std::cout << "How long has it been since you dropped the ball (Seconds): ";
std::cin >> seconds;
return seconds;
}
double currentBallHeight(double y0, double t)
{
return y0 - (constant::gravity * t * t / 2);
}
void printResult(double y0, double t)
{
double currentHeight = currentBallHeight(y0, t);
if (currentHeight < 0)
std::cout << "At " << t << "s the ball is on the ground." << std::endl;
else
std::cout << "At " << t << "s the ball is at: " << currentHeight << std::endl;
}
int main()
{
double y0 = towerHeight();
double t = secondsSinceDrop();
printResult(y0, t);
return 0;
}
Summarizing what I've changed:
Renamed x to y0 since y(0)/h(0) is typically used for "initial height" in physics classes, and similarly y with t (though time would be an even better name).
Don't pass anything to towerHeight or secondsSinceDrop; you're not trying to give those functions something, you're trying to get something out of them.
Move the definition of x from a function parameter to a local variable defined in the function for towerHeight and secondsSinceDrop
Removed the duplicated call to currentBallHeight (no need to do the same math twice, it takes time to crunch numbers after all, however small in this case)
Rewrote for proper usage of std::cout and std::endl
Rewrote the currentBallHeight equation to match constant free-fall kinematics (y(t) = y(0) - 0.5g * t^2) as an added bonus (assuming constant::gravity > 0)
At some point it will be valuable for you to become aware of the more technical terminology and definitions for the concepts I've outlined here. Here are some recommended readings (just to get you started; keep learning, always):
Sequence Points
Parameters and Arguments
Passing by Reference vs by Value
Passing pointers vs by Reference
Making sure you understand what using namespace std; does and why you should never use it

Answer 4

Rewrite your function as following:
//height of the tower
double towerHeight()
{
double x;
using namespace std;
cout << "Enter a height for the tower" << '\n';
cin >> x;
return x;
}
and in int main(){} rewrite following line:
double x = towerHeight();
I guess this will do but you can actually modify your double secondsSinceDrop(double x); function this way as it doesn't really need a double value as parameter.