How to write this algorithm in C++?

I'm kind of stumped as to how to write this in C++, seeing as I need some way of making the out of range array references = 0.

Source, filter and output arrays:

Code:

---

S [7] = { 1,2,3,4,5,6,7 };
Filter [3] = { a0,a1,a2 };
out [x];

---

out[0] = a₀.V[0] + a₁.V[X]+ a₂.V[X]
out[1] = a₀.V[1] + a₁.V[0] + a₂.V[X]
out[2] = a₀.V[2] + a₁.V[1] + a₂.V[0]
out[3] = a₀.V[3] + a₁.V[2] + a₂.V[1]
out[4] = a₀.V[4] + a₁.V[3] + a₂.V[2]
out[5] = a₀.V[5] + a₁.V[4] + a₂.V[3]
out[6] = a₀.V[6] + a₁.V[5] + a₂.V[4]
out[7] = a₀.V[X] + a₁.V[6] + a₂.V[5]
out[8] = a₀.V[X] + a₁.V[X] + a₂.V[6]

The only way I can see to do it would be to include an 'if' statement; e.g. "if (i<0) i=0;", but that seems horribly inefficient.

Has anyone got a better idea?

Any help would be greatly appreciated!

An if statement shouldn't be that bad, as branch prediction is pretty good about things that only happen near array edges. However, if you want, you can do the inner portion of the filter first and then the ends as special cases.

Well if that's the case, then I guess it's ok.
I only ask as it's for a coursework that's assessed on the code being 'nice'...

thanks

If you have a choice between readability and efficiency, choose readability unless you have a compelling reason to do otherwise.