Example 2 from Application: Mapped Filters*

[Kai Yu Yeo]

Question: Could someone point out how the "counter is reset to one" when there is a change in run? Thank you!

• Based on following the code's progression it would appear that counter increments or decrements by the current state (i.e. "count !"), rather than resetting based on a change from +1 to -1 or vice versa.  
• "Book1.xlsx" is attached, and illustrates the progression
• Example 2's code is pasted at the bottom as a reference

: mapf ( seq xt -- seq ) ~ { f }
    [: f >R ;] map
    ['] R> filter
;

: positive-negative ( n -- n )
    0 > if 1 else -1 then
;

: changes ( seq -- seq )
    dup tail swap ['] - zipwith
    ['] positive-negative map
;

: freq ( n -- xt )
    0 ref { count } 0 ref { state }
    [:
    dup state @ =
    swap state !
    if state @ count +! 0
    else count @ state @ count ! then
    dup 0 = if false else true then
    ;]
;

nil 95 cons 105 cons 120 cons 110 cons 100 cons 130 cons 140 cons 150 cons 130 cons 120 cons dup .list
changes dup .list
freq mapf .list