for(vector & lt； int & gt* size _ type IX = 0； ix & ltivec . size()- 1； ix = ix + 2)

Ix is a vector.

In this statement, its initial value is 0, and its value will be +2 when the condition does not satisfy IX.

((IX+ 1)% 6 = = 0) is a module, that is, a remainder.

It means that ix+ 1 is divisible by 6.