Trouble with simple functions - c++

Question

I am writing code to call functions having to do with force, mass, and acceleration equations. The functions are called correctly, but the inputs are not multiplied as they should be. My output for the first function is a crazy small number, and the output for the first function is always 0.
Here is my code. Any feedback wold be very helpful. Thanks.
#include <iostream>
#include <cstdlib>
using namespace std;
void displayMenu();
double force(double);
double secondForce(double,double);
int main(int argc, char** argv)
{
int menuOption;
displayMenu();
system("PAUSE");
return 0;
}
void displayMenu(void)
{
int menuOption;
double weight, accel;
cout << " Main Menu" << endl;
cout << "Enter 1 for Force calculation with acceleration = 9.8m/s^2.\n";
cout << "Enter 2 for Force calculation with user defined acceleration.\n";
cout << "Enter 3 to quit the program.\n";
cin >> menuOption;
if(menuOption==1)
{
cout << "Enter a mass.\n";
cin >> weight;
cout << "The force is ";
cout << force(weight);
cout << "N.";
}
else if(menuOption==2){
cout << "Enter a mass.\n";
cin >> weight;
cout << "Enter an acceleration.\n";
cin >> accel;
cout << "The force is ";
cout << secondForce(weight, accel);
cout << "N.";
}
}
double force(double weight)
{
double force, mass;
force=(mass*(9.8));
return force;
}
double secondForce(double secondMass, double secondWeight)
{
double secondForce, mass, acceleration;
secondForce=(mass*acceleration);
return secondForce;
}

Answer 1

You are using uninitialized variables in your functions. Using these uninitialized variables in your program is undefined behavior.
force=(mass*(9.8)); << mass has a garbage value
secondForce=(mass*acceleration); << mass and acceleration have a garbage value
I think you meant to have
double force(double weight)
{
return weight * 9.8;
}
double secondForce(double secondMass, double secondAccel)
{
return secondMAss * secondAccel;
}

Answer 2

The problem is you are using variables in your calculation that are undefined. See my embedded comments below:
double secondForce(double secondMass, double secondWeight)
{
double secondForce, mass, acceleration;
secondForce=(mass*acceleration); //what is the value of mass and acceleration here??
return secondForce;
}
As a side note: Consider stepping through your code inside of a debugger like gdb so you can see how your program executes. Trying to reason through it is very difficult for large programs. In your example, it's easy to spot for an experienced programmer.

Answer 3

You're multiplying the data by garbage values
double force(double weight)
{
double force, mass;
force=(mass*(9.8));
return force;
}
double secondForce(double secondMass, double secondWeight)
{
double secondForce, mass, acceleration;
secondForce=(mass*acceleration);
return secondForce;
}
What's the value of mass and acceleration here? These variables are not initialized. You have to assign some values to them first.

Answer 4

You should enable compiler warnings and pay attention to them. For example, here is what MSVC says even on the lowest warning level:
warning C4700: uninitialized local variable 'mass' used
warning C4700: uninitialized local variable 'mass' used
warning C4700: uninitialized local variable 'acceleration' used
In C++, using an uninitialised variable is never a good idea. Unlike in other languages, most variables in C++ are not automatically initialised with 0 values. This causes your program to inhibit unspecified or undefined behaviour when the value contained in the variable is to be accessed.
Be safe and correct. Initialise! For example:
double force = 0.0;
double mass = 0.0;
Note that the language also allows you to first initialise, then assign, then use the variable. For example:
double force;
force = 0.0;
cout << force;
But that's bad style. Why leave a variable uninitialised and then later assign something when you can do everything in one step? Leaving the variable uninitialised first also prevents you from making it const.
Ensuring correct initialisation of all variables will make the behaviour of your program defined and consistent. You will have other problems, though, because you multiply the 0 mass, which will always result in 0 and is certainly not what you intend. However, deterministic behaviour is a good starting point to fix the wrong math.