You can think of a function in two ways.
It is important to distinguish the two. For example, let's return to the distance function.
// distance(x1,y1,x2,y2) returns the distance between
// points (x1,y1) and (x2,y2) in the plane.
double distance(double x1, double y1, double x2, double y2)
{
double deltaX = x1 - x2;
double deltaY = y1 - y2;
return sqrt(deltaX*deltax + deltaY*deltaY);
}
Distance accomplishes what the comment says: distance(x1, y1, x2, y2) returns the distance between points (x1,y1) and (x1,y2) in the plane. Distance works by employing the distance formula that you learn about when you study algebra.
When you think about writing a function, the first question that you should ask is: What is this function supposed to accomplish? After you have answered that question precisely, think about how it should work.
Once you have finished writing a function definition, compare your answer to What should it accomplish to your answer to how should it work, as written in the function body. Does the function actually do what it is supposed to do?
Student programs frequently contain functions that do not do what they are supposed to do. Often, that is due to an error. But an astonishing fraction of the time, it is by design! In fact, it is clear that the student knows the function does not do its job, because another function compensates for the error.
That is a very poor way to write software. You will never get a complex thing to work if its individual components do not do what they are supposed to do. That should be obvious.
If a function does not accomplish exactly what it is supposed to accomplish, fix that function. Do not compensate for the error elsewhere.
It is not enough for a function to do the right thing on some parameter values. It must do the exact right thing on all values of parameters that make sense. The standards for this course require that.
If you have defined a function correctly, then you should show some confidence in that. Do not try to work around some perceived mistake that is not even present. Look at the following code that uses distance.
if(a == 0 && b == 0)
{
r = sqrt(c*c + d*d);
}
else
{
r = distance(a,b,c,d);
}
Clearly, the author of this code believes that distance(a, b, c, d)
does not work correctly when a and b are both 0, so he or she
adds a special case to work around the perceived error in distance
In fact, distance(a, b, c, d) works just fine
when a and b are both 0. So the code above should be replaced by
r = distance(a,b,c,d);
Any statement of what a function accomplishes must explain exactly how its parameters influence that. For example, the distance function does not just compute the distance between two points. It returns the distance between points (x1,y1) and (x2,y2).
If your function has a parameter that is so mysterious that you do not understand how that parameter influences what the function is supposed to accomplish, ask yourself why that parameter is there at all.