v:v,|v:1+!5
  v
1 2 3 4 5 5 4 3 2 1

  avg:{(+/x)%#x}
  sma:{avg'x@(,\!y),(1+!y)+\:!y}

  sma[v;5]
1 1.5 2 2.5 3 3.8 4.2 4.2 3.8 3