x = j:k creates a unit-spaced vector x with elements [j,j+1,j+2,...,j+m] ...
x = j:i:k creates a regularly-spaced vector x using i as the increment ...
Thus the default value of the step, i, is +1 (one).
>> vec = -1:+1:-5
vec =
1×0 empty double row vector
and
>> vec = -1:+5
vec =
-1 0 1 2 3 4 5
