Previous-frame values

A cell that reads its own last value is how you build a counter, a running total, a smoothed signal, or anything that animates. In most spreadsheets this is a circular reference — an error you work around by turning on "iterative calculation" and hoping.

In OctoMY you say it directly: prev(B1) means the value B1 had at the last frame.

The problem it solves

Write this and you get an error, correctly:

B1 = =B1 + A1        ✗ circular reference

B1 cannot be its own input. There is no order to evaluate that in.

Write this instead:

A1 = 1
B1 = =prev(B1) + A1        ✓

prev(B1) is not a reference to B1. It is a reference to a stored value from the last frame, which is an ordinary input — so the sheet stays a plain dependency graph with no loop in it, and B1 computes in one pass like every other cell.

Frames

prev() only changes value when the sheet advances a frame. Until then it holds still:

A1 = 1
B1 = =prev(B1) + A1

           B1
start       1
advance     2
advance     3
advance     4

That last part matters more than it looks: editing a cell does not advance the frame. If you fix a typo in an unrelated cell, or paste a block, or change a colour, B1 does not tick. Only advancing does.

That is deliberate. If edits advanced the frame, =prev(B1)+1 would count your keystrokes, which is a different feature and a confusing one.

What it is for

A running total or counter — the example above.

Smoothing a noisy input. Blend the new value with the last one:

A1 = (some live input)
B1 = =prev(B1) * 0.9 + A1 * 0.1

Animation. A value that advances every frame is a clock, and a clock drives motion:

A1 = =prev(A1) + 0.016        a time value, advancing ~1/60 s per frame
B1 = =sin(A1)                  an oscillation
C1 = =vec3(0, B1 * 10, 0)      a position that moves up and down

That last cell is a three-component value ready to drive something — see cell types for what makes a cell hold three numbers.

Rules worth knowing

A cell that is read through prev() still evaluates normally. It is not frozen; it computes this frame's value like any other cell. prev() just reads what it was.

prev() on a cell that has never been evaluated reads zero. Start values come from wherever you set them — often a plain literal in another cell, added in on the first frame.

A real circular reference is still an error. prev() is how you declare that a dependency crosses a frame boundary; without it, a loop is still a loop, and every cell in the loop is marked.

See also

In this section