Delta

The Delta operation reports how much a numeric input has moved since the last time it ran. In Delta mode that is the plain difference; in Rate mode it is the difference divided by the elapsed time, giving a per-second rate of change.

Inputs

Port	Type	Required	Description
>	Any	No	Optional gate. When closed, the path stops and no sample is recorded.
In	Integer or Float	Yes	The value to track. A non-numeric or Nil value fails the Node; its On Error verb resolves the failure.

Outputs

Port	Type	Default	Description
Out	Float	Nil	The change since the previous sample — a difference in Delta mode, a per-second rate in Rate mode.

Properties

Name	Type	Default	Description
Mode	Combo	Delta	Delta emits current − previous. Rate emits (current − previous) / seconds elapsed.
On Error	Combo	Stop this branch	Per-op On Error verb (Stop this branch / Continue with the last value / Continue with the fallback). See the Error Handling guide.

State on Reload

Out is preserved across restarts. The remembered sample is not: after a restart the next run is a first sample again and stops the path.

The first run always stops

A change needs two samples. On its first run — and on the first run after every restart — Delta records the value, stops the path, and reports Delta: first sample captured; no delta yet. In Rate mode a second sample arriving at the same instant as the one before it stops the path too, reporting Delta: two samples at the same instant; nothing to divide by — there is no interval to divide by. Both are stops by design, not errors: no verb is consulted. Build downstream Nodes expecting nothing on the first run.

---

How It Works

1. Gate check — if the Gate is connected and closed, the path stops and nothing is recorded.

2. Read the input — In must carry a number. Anything else fails the Node, and its On Error verb resolves the failure.

3. First sample? — with no previous sample held, the value and the moment are recorded and the path stops.

4. Compute — Delta mode subtracts the held value from the current one. Rate mode divides that difference by the seconds since the held sample was taken.

5. Replace and emit — the current value and moment become the held sample, Out is written, and the path continues.

---

Examples

Delta mode

Mode: Delta

Run	In	Out
1	100	(stops — first sample)
2	130	30.0
3	125	-5.0
4	125	0.0

Rate mode

Mode: Rate — runs 10 seconds apart

Run	In	Out
1	1000	(stops — first sample)
2	1050	5.0  (50 over 10 s)
3	1050	0.0

---

Use Cases

• Consumption from a cumulative meter — an energy or water meter reports a total that only rises; Delta turns it into “units used since the last reading”.
• Detecting a runaway — feed a temperature into Rate mode and Compare the result against a ceiling to catch a fast rise long before the absolute value becomes alarming.
• Suppressing a stuck sensor — a Delta of exactly 0 over many runs is a reading that is not moving.

---

Error Handling

Delta fails when In is unconnected, Nil, or not a number; the Node’s On Error verb resolves the failure.

Verb	What it does
Stop this branch (default)	This Node and its dependents stop; Out keeps its value and reads as a failed update.
Continue with the last value	Dependents run on the value Out already holds, not stamped as new; it reads as a failed update.
Continue with the fallback	Dependents run on Out’s per-output Fallback, stamped as this run’s value; it reads as a failed update. The Editor refuses a Task where the Fallback is unset.

---

Limitations

• Delta compares against the previous run, not against a fixed interval. How often the Node runs is decided by whatever triggers its path, so in Delta mode the size of the number depends on that cadence. Use Rate mode when the answer must be independent of it.
• Only one value is tracked per Node. To follow several signals, use one Delta per signal.
• The held sample does not survive a restart.