# How do I calculate diff between two consecutive payloads
**URL:** https://discourse.nodered.org/t/how-do-i-calculate-diff-between-two-consecutive-payloads/6756
**Category:** General
**Created:** [13 January 2019 19:40 UTC](https://discourse.nodered.org/t/how-do-i-calculate-diff-between-two-consecutive-payloads/6756 "2019-01-13T19:40:25Z")
**Posts on this page:** 15
**Page:** 1
### Author:  [@Connicat](https://discourse.nodered.org/u/Connicat)
#### Post date: [13 January 2019 19:40 UTC](https://discourse.nodered.org/t/how-do-i-calculate-diff-between-two-consecutive-payloads/6756/1 "2019-01-13T19:40:25Z")
I'm trying to calculate the difference between to two consecutive payloads. I'm using SNMP node to query a router interface and return data bytes received. I want to subtract value payload n from payload n+1 to obtain throughput figure per second.
I've looked at context and function nodes but not not sure where to start.
Any suggestions would be appreciated.
Thanks
Chris
---
### Author:  [@ukmoose](https://discourse.nodered.org/u/ukmoose)
#### Post date: [13 January 2019 19:44 UTC](https://discourse.nodered.org/t/how-do-i-calculate-diff-between-two-consecutive-payloads/6756/2 "2019-01-13T19:44:25Z")
Look at the storing data part of [https://nodered.org/docs/writing-functions](https://nodered.org/docs/writing-functions)
---
### Author:  [@leifnel](https://discourse.nodered.org/u/leifnel)
#### Post date: [13 January 2019 22:05 UTC](https://discourse.nodered.org/t/how-do-i-calculate-diff-between-two-consecutive-payloads/6756/3 "2019-01-13T22:05:18Z")
I use a delay node, then store the output in a flow variable
Then the undelayed message go into a function which subtracts the flow variable from the current and divide with the delay time to get the rate.
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### Author:  [@Andrei](https://discourse.nodered.org/u/Andrei)
#### Post date: [14 January 2019 02:06 UTC](https://discourse.nodered.org/t/how-do-i-calculate-diff-between-two-consecutive-payloads/6756/4 "2019-01-14T02:06:45Z")
> [@Connicat](#):
>
> I'm trying to calculate the difference between to two consecutive payloads.
Perhaps this pre-built node could help you ?
> **[GitHub - bartbutenaers/node-red-contrib-interval-length: A Node Red node to...](https://github.com/bartbutenaers/node-red-contrib-interval-length)**
>
> A Node Red node to measure the interval length between successive messages - GitHub - bartbutenaers/node-red-contrib-interval-length: A Node Red node to measure the interval length between successi...
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### Author:  [@Connicat](https://discourse.nodered.org/u/Connicat)
#### Post date: [14 January 2019 09:04 UTC](https://discourse.nodered.org/t/how-do-i-calculate-diff-between-two-consecutive-payloads/6756/5 "2019-01-14T09:04:21Z")
Thanks for your responses. To be clear, I want to subtract content of payload n from payload n+1, not find the time gap between the two.
As suggested, I'm going to run two snmp nodes and fire the second one with a short delay. Store both to context and run function node to subtract one from the other.
---
### Author:  [@ghayne](https://discourse.nodered.org/u/ghayne)
#### Post date: [14 January 2019 10:26 UTC](https://discourse.nodered.org/t/how-do-i-calculate-diff-between-two-consecutive-payloads/6756/6 "2019-01-14T10:26:09Z")
```auto
[{"id":"a014e37e.af038","type":"debug","z":"e8e5ccfb.8b54e","name":"","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"true","targetType":"full","x":630,"y":80,"wires":[]},{"id":"e62bf4.e47cc41","type":"inject","z":"e8e5ccfb.8b54e","name":"","topic":"","payload":"2","payloadType":"num","repeat":"","crontab":"","once":false,"onceDelay":0.1,"x":170,"y":60,"wires":[["6b749ee9.5af15"]]},{"id":"4424359f.3033ec","type":"inject","z":"e8e5ccfb.8b54e","name":"","topic":"","payload":"1","payloadType":"num","repeat":"","crontab":"","once":false,"onceDelay":0.1,"x":170,"y":120,"wires":[["6b749ee9.5af15"]]},{"id":"6b749ee9.5af15","type":"join","z":"e8e5ccfb.8b54e","name":"","mode":"custom","build":"array","property":"payload","propertyType":"msg","key":"topic","joiner":"\\n","joinerType":"str","accumulate":false,"timeout":"","count":"2","reduceRight":false,"reduceExp":"","reduceInit":"","reduceInitType":"","reduceFixup":"","x":330,"y":80,"wires":[["53168641.5a4cb8"]]},{"id":"53168641.5a4cb8","type":"change","z":"e8e5ccfb.8b54e","name":"","rules":[{"t":"set","p":"result","pt":"msg","to":"payload[0]-payload[1]","tot":"jsonata"}],"action":"","property":"","from":"","to":"","reg":false,"x":480,"y":80,"wires":[["a014e37e.af038"]]}]
```
---
### Author:  [@shrickus](https://discourse.nodered.org/u/shrickus)
#### Post date: [14 January 2019 13:45 UTC](https://discourse.nodered.org/t/how-do-i-calculate-diff-between-two-consecutive-payloads/6756/7 "2019-01-14T13:45:04Z")
Here are a couple other contrib nodes that may do some of this for you...
> **[node-red-contrib-combine](https://flows.nodered.org/node/node-red-contrib-combine)**
>
> Node-RED Nodes that output combinations of consecutive incoming messages
> **[node-red-contrib-delta-timed](https://flows.nodered.org/node/node-red-contrib-delta-timed)**
>
> Node for Node-RED that computes the difference between messages, including optionaly a period of time.
---
### Author:  [@Connicat](https://discourse.nodered.org/u/Connicat)
#### Post date: [15 January 2019 14:28 UTC](https://discourse.nodered.org/t/how-do-i-calculate-diff-between-two-consecutive-payloads/6756/8 "2019-01-15T14:28:01Z")
Thanks Leifnel.
Used this technique and have it working fine now.
Chris
---
### Author:  [@cinhcet](https://discourse.nodered.org/u/cinhcet)
#### Post date: [16 January 2019 11:56 UTC](https://discourse.nodered.org/t/how-do-i-calculate-diff-between-two-consecutive-payloads/6756/9 "2019-01-16T11:56:07Z")
What about the following:
First SNMP node, then a change node that stores the payload in an other property of the message, then the second SNMP node with the second request, then a function node that does the subtraction. This way you have no delays, no storage, no race conditions, no parallel execution.
That would be a more flow based programming style
---
### Author:  [@moebius](https://discourse.nodered.org/u/moebius)
#### Post date: [16 January 2019 12:07 UTC](https://discourse.nodered.org/t/how-do-i-calculate-diff-between-two-consecutive-payloads/6756/10 "2019-01-16T12:07:01Z")
Here is another way with joining:
```auto
[{"id":"9e03a15e.eb805","type":"function","z":"c421dfeb.3e752","name":"AJV10","func":"var countmsg = 2; //how many items to join?\n\nvar nodublicates = false; //only join if payload unequal existing payload\nvar limit; //only join if payloads are on both sides of the limit\nvar hysteresis; //hysteresis \nvar clearaftersend = false;//clear queue after sending\nvar sendinstantly = false;\nvar reverse = false; //reverse output array\nvar priomode = false;\nvar storagemode = false; \nvar topicmatch = false;\n//------------ different topic mode ------------------\nvar diftopics = false; //true: join only different topics; false: join only same topics\nvar alphasort = false;\n\nvar inmsg = msg;\nvar msgs = context.get(\"aggmsgs\")||{};\nvar topicmsgs = msgs[inmsg.topic]||[];\n\n\nif (inmsg.topic === \"\")\n return null;\nif (typeof inmsg.reset != \"undefined\") {\n msgs = {};\n topicmsgs = [];\n lastval = undefined;\n context.set(\"aggmsgs\",msgs);\n context.set(\"lastval\",lastval);\n return null;\n}\nif (typeof inmsg.resettopic != \"undefined\") {\n topicmsgs = [];\n lastval = undefined;\n}\nif (typeof hysteresis !== 'undefined' && typeof lastval !== 'undefined') {\n if (Math.abs(inmsg.payload - lastval) <= hysteresis) {\n return null;\n }\n}\nif (typeof lastval !== 'undefined') {\n if (nodublicates && lastval == inmsg.payload)\n return null;\n if (typeof limit !== 'undefined' && countmsg == 2) {\n if (!((lastval <= limit && inmsg.payload >= limit) || (lastval >= limit && inmsg.payload <= limit))) \n return null;\n }\n} \n\nif (topicmatch) {\n if (typeof inmsg.payload.jointopic != \"undefined\") {\n if (topicmsgs.length === 0) {\n topicmsgs.push(inmsg);\n msgs[inmsg.topic] = topicmsgs;\n context.set(\"aggmsgs\",msgs);\n return null; \n }\n else {\n topicmsgs[0] = inmsg;\n }\n }\n else { //only accept topics specified by jointopics\n for (var tmsgs in msgs) {\n if (msgs.hasOwnProperty(tmsgs)) { \n var bfound = false;\n var btopicfound = false;\n var ar = msgs[tmsgs];\n for (var i = 0; i < ar.length; i++) { //search and replace\n if (i === 0) {\n if (ar[i].payload.jointopic == inmsg.topic)\n btopicfound = true;\n } else {\n if (ar[i].topic == inmsg.topic) {\n ar[i] = inmsg;\n bfound = true;\n }\n }\n }\n if (!bfound && btopicfound) //add new\n msgs[tmsgs].push(inmsg);\n }\n }\n }\n}\n\nvar outmsg = { \"topic\": inmsg.topic , \"payload\": \"\"};\nvar lastval = context.get(\"lastval\");\nvar inprio = typeof inmsg.priority != \"undefined\" ? inmsg.priority: countmsg-1 ; //default prio is 1\n\nif (diftopics) {\n if (topicmsgs.length > 0) { //msg with same topic -> replace old msg\n topicmsgs.shift();\n }\n topicmsgs.push(inmsg);\n msgs[inmsg.topic] = topicmsgs; \n if (Object.keys(msgs).length > countmsg) {\n for (var tmsgs in msgs) {\n if (msgs.hasOwnProperty(tmsgs)) { \n delete msgs[tmsgs];\n break;\n }\n }\n }\n context.set(\"aggmsgs\",msgs);\n context.set(\"lastval\",inmsg.payload);\n if (Object.keys(msgs).length == countmsg || sendinstantly) {\n var aout = [];\n outmsg.topic = \"\";\n \n for (var tmsgs in msgs) {\n if (msgs.hasOwnProperty(tmsgs)) \n aout.push(msgs[tmsgs][0]);\n }\n if (clearaftersend) {\n context.set(\"aggmsgs\",undefined);\n context.set(\"lastval\",undefined);\n }\n if (alphasort) \n aout.sort(function(a,b) {return (a.topic > b.topic) ? 1 : ((a.topic > b.topic) ? -1 : 0);});\n\n if (reverse) \n aout.reverse();\n\n for (var i =0; i no priority\n return null;\n for (var i = 0; i < topicmsgs.length; i++) { //only send if storage is full\n if (topicmsgs[i] === undefined)\n return null;\n } \n }\n\n if (topicmatch) {\n for (var tmsgs in msgs) {\n if (msgs.hasOwnProperty(tmsgs)) { \n var ar = msgs[tmsgs];\n if (ar.length == countmsg) {\n for (var i = 0; i < ar.length; i++) {\n if (ar[i].topic == inmsg.topic) {\n outmsg.payload = ar;\n outmsg.topic = ar[0].topic;\n node.send(RED.util.cloneMessage(outmsg));\n }\n }\n }\n }\n }\n } else {\n if (topicmsgs.length == countmsg || sendinstantly) {\n outmsg.payload = topicmsgs;\n if (topicmatch) {\n outmsg.topic = topicmsgs[0].topicori;\n outmsg.payload[0].topic = outmsg.topic;\n }\n \n if (clearaftersend) {\n msgs[inmsg.topic] = undefined;\n context.set(\"aggmsgs\",msgs);\n context.set(\"lastval\",undefined);\n }\n if (reverse) {\n var revArr = outmsg.payload.map(a => Object.assign({}, a));\n revArr.reverse();\n outmsg.payload = revArr;\n }\n return outmsg;\n }\n else\n return null;\n }\n}\n","outputs":1,"noerr":0,"x":580,"y":2223,"wires":[["7dbd79.3f6b3288"]]},{"id":"7e87f6e1.e8b438","type":"inject","z":"c421dfeb.3e752","name":"","topic":"lala","payload":"","payloadType":"date","repeat":"","crontab":"","once":false,"onceDelay":0.1,"x":419,"y":2170,"wires":[["9e03a15e.eb805"]]},{"id":"afd70e5b.5f155","type":"debug","z":"c421dfeb.3e752","name":"","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"true","x":965,"y":2271,"wires":[]},{"id":"7dbd79.3f6b3288","type":"change","z":"c421dfeb.3e752","name":"","rules":[{"t":"set","p":"dif","pt":"msg","to":"payload[1].payload - payload[0].payload","tot":"jsonata"}],"action":"","property":"","from":"","to":"","reg":false,"x":744,"y":2247,"wires":[["afd70e5b.5f155"]]}]
```
---
### Author:  [@aabbas](https://discourse.nodered.org/u/aabbas)
#### Post date: [18 April 2019 07:52 UTC](https://discourse.nodered.org/t/how-do-i-calculate-diff-between-two-consecutive-payloads/6756/11 "2019-04-18T07:52:20Z")
Hi,
I want "2" be all time the first payload and 1 the second to get a positif value (If 1 is befor 2 the result is -1 and If 1 wans sent two times we obtain 0). So I want to fix the range of payload. have you an idea?
Thank you for your help.
---
### Author:  [@Colin](https://discourse.nodered.org/u/Colin)
#### Post date: [18 April 2019 20:11 UTC](https://discourse.nodered.org/t/how-do-i-calculate-diff-between-two-consecutive-payloads/6756/12 "2019-04-18T20:11:56Z")
Are the two values coming from different places?
---
### Author:  [@Energycharges](https://discourse.nodered.org/u/Energycharges)
#### Post date: [26 September 2020 09:40 UTC](https://discourse.nodered.org/t/how-do-i-calculate-diff-between-two-consecutive-payloads/6756/13 "2020-09-26T09:40:14Z")
Hi all
Can you please guide me with the flow , Im having a same kind of issue .
I have raised issue in [[How to calculate the difference in KWH values received every 5 min from modbus data and save it as a new values in csv](https://discourse.nodered.org/t/how-to-calculate-the-difference-in-kwh-values-received-every-5-min-from-modbus-data-and-save-it-as-a-new-values-in-csv/33274/7)]
Please guide me
Appreciate , your effort in notifying the issue
thank you !
Energycharges 😃 😃
---
### Author:  [@Paul-Reed](https://discourse.nodered.org/u/Paul-Reed)
#### Post date: [26 September 2020 09:47 UTC](https://discourse.nodered.org/t/how-do-i-calculate-diff-between-two-consecutive-payloads/6756/14 "2020-09-26T09:47:09Z")
Please keep your discussion in one thread.
You could of tagged the above, from your other thread.
---
### Author:  [@Paul-Reed](https://discourse.nodered.org/u/Paul-Reed)
#### Post date: [26 September 2020 09:47 UTC](https://discourse.nodered.org/t/how-do-i-calculate-diff-between-two-consecutive-payloads/6756/15 "2020-09-26T09:47:23Z")