1.
Hello
Goodbye
One more time:
Hello
End of program.

2. no

3. 
No impact on program as return statement is optional in void function initialize_screen.
Program would compile.
Program would run.
Program would behave normally.
Return statement is optional  in void function show_results.
No effect on program if return statement is missing.
Function celsius returns a value of double so the return statement is mandatory.
Program would not compile if return statement would be missing from celsius function.

4.
void function (int, int, int);
//Input three integers
//Output product of the three inputs
void main()
{
using namespace std;

int x, y, z;

cout << "Enter three integers & product will be output to screen.\n";
cin >> x
    >> y
    >> z;
}
void function (int a, int b, int c)
{
using namespace std;
cout << a*b*c;
}

5. 
Microsoft Visual Studio 2010 compiles without errors void main() and int main().
No warnings or errors were experienced with int main() and having no return 0; statement in program.

6. Statement, since no values are returned expressions cannot be used.


7.
10 20 30
1 2 3
1 20 3

8. 5 5

9.
par1_value in function call = 111
par2_ref in function call = 222
n1 after function call = 1
n2 after function call = 2

10.
void zero_both( int& a, int& b )
{
a = 0;
b = 0;
}

11.
void add_tax( double tax_rate, double& cost )
{
     cost = cost + (tax_rate * cost)
}

12.Yes, Yes.

13. No.

14. Yes.

15.
// Precondition two int will be given values
// Postcondition two int will be ordered from smaller to larger int

16.
// Precondition one number will be given a value
// Postcondition square root will be returned

17. Every function should be tested in a program in which every other function
in that program has already been fully tested and debugged.

18. A driver program is a minimal & temporary tool to test functions.

19. To test two cout statements I would put statements in main program then run it. If it works
put brackets around code & add function header & place appropriately in program. Then put function
call in main program, then run, and compare outputs.

20. I would write a program that requests from user tax_rate & cost then output calculation to screen.
Then run various inputs and compare results to known answers.

21. A stub is a simplified or partially incomplete function used for testing & filler for program outline. 

22.
double rain_prob(double a, double b, double c)
{
return 1;
}

23. Put assert(z==0); before expression that is being tested.

24. A separate utility program that allows programmer to stop program at any time or step through execution of code
one step at a time.

25. keep an open mind, check common errors, localize error, use second set of eyes, use debugger