When you say
for k = start : step : stop
the first expression, (i-1) in your case, will be the first/starting value of the loop iterator k. Then the next iteration k will be start+step, or i-1-1 = i-2 in your case. It keeps going until it gets to the "stop" value or goes past it (i.e., goes above it if the step is positive, or below it if the step value is negative). This is basic stuff. Please read the getting started section of the help for more essential information.
