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Yes in that case an accumulator is needed. One method you could choose would be to pass a table through to accumulate while also allowing you to look back to previous rows:
q)update c2:1_@[;`c2]{y[`c2]:enlist $[(y[`c1][0]>last x[`c2]) or ((last x[`c])<last x[`c2]);y[`c1][0];last x`c2];x,y}/[enlist each {(1#0#x),x}update c2:0 from `c`c1#t] from t c c1 c2 ---------- 30 10 10 40 20 20 25 5 20 20 25 25 4 5 5 4 4 4 4.5 3 4 4.5 3.5 4