What is the best way to get an elements opposite index in an array?

Question

Say I have array

{0, 1, 2, 3, 4}

• index 0 and 4 are opposites
• index 1 and 3 are opposites
• index 2's opposite is undefined

Or

 {0, 1, 2, 3, 4, 5}

• index 0 and 5 are opposites
• index 1 and 4 are opposites
• index 2 and 3 are opposites

I remember seeing a really clever way to do this. Something like

i%array.length 

Answer 1

Try this one:

oppIndex = array.length - firstIndex - 1;