Compare payload value to last received payload

General
1

I receive a payload value that rises every second, after a while it'll stop for a moment and reset to 0 again.

What I want to do is capture that max value.

I'm struggling to hold onto the last value to compare it against the new one.

2

You will need to save the last value. You could use a function node to save the value as a node variable and check it each time a msg comes thru or you could use a switch node and a change node to test and save (if the new value is greater) the value as a flow variable.

If you haven't worked with context, take a read of the documentation here: Working with context : Node-RED

1 Like
3

You can also look at the smooth node - as that also can return the max value seen (as well as mean, min, etc) - and can be reset by a msg.reset message.

1 Like
4

Solution:

pl = msg.payload.CycleTimeMach;
pl2 = msg.payload.CycleTimeMan;

avg = flow.get("AvgOldMach.S8")
avg2 = flow.get("AvgOldMan.S8")

if (pl < avg) {
    flow.set("AvgNewMach", {S8: avg})
}
if (pl2 < avg2) {
    flow.set("AvgNewMan", {S8: avg})
}

flow.set("AvgOldMach", {S8: pl})
flow.set("AvgOldMan", {S8: pl2})

I was trying to do it with the flow context last night but my head just wouldn't compute. I guess all I really needed was a good coffee and fresh eyes.

Edit:
Probably worth adding that I setup the context in another node using

flow.set("AvgOldMach", {S8: 0})
flow.set("AvgNewMach", {S8: 0})

flow.set("AvgOldMan", {S8: 0})
flow.set("AvgNewMan", {S8: 0})

from a one-hit timestamp injection that fires every time the flow starts

5

In the function node you could use this

avg = flow.get("AvgOldMach.S8") || 0  // get flow var, it it doesn't exist use 0
avg2 = flow.get("AvgOldMan.S8") || 0  // get flow var, it it doesn't exist use 0

and eliminate the need to have the other node to set up the context.

1 Like
6

Ah brilliant, I couldn't figure out how to achieve this.

thanks

7

The users guide is very informative: Writing Functions : Node-RED

8

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