327 lines
9.4 KiB
Markdown
327 lines
9.4 KiB
Markdown
---
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title: difference() function
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description: >
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`difference()` returns the difference between subsequent values.
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menu:
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flux_v0_ref:
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name: difference
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parent: universe
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identifier: universe/difference
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weight: 101
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flux/v0/tags: [transformations]
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introduced: 0.7.1
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---
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<!------------------------------------------------------------------------------
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IMPORTANT: This page was generated from comments in the Flux source code. Any
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edits made directly to this page will be overwritten the next time the
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documentation is generated.
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To make updates to this documentation, update the function comments above the
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function definition in the Flux source code:
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https://github.com/influxdata/flux/blob/master/stdlib/universe/universe.flux#L375-L384
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Contributing to Flux: https://github.com/influxdata/flux#contributing
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Fluxdoc syntax: https://github.com/influxdata/flux/blob/master/docs/fluxdoc.md
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------------------------------------------------------------------------------->
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`difference()` returns the difference between subsequent values.
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### Subtraction rules for numeric types
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- The difference between two non-null values is their algebraic difference;
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or `null`, if the result is negative and `nonNegative: true`;
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- `null` minus some value is always `null`;
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- Some value `v` minus `null` is `v` minus the last non-null value seen
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before `v`; or `null` if `v` is the first non-null value seen.
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- If `nonNegative` and `initialZero` are set to `true`, `difference()`
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returns the difference between `0` and the subsequent value.
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If the subsequent value is less than zero, `difference()` returns `null`.
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### Output tables
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For each input table with `n` rows, `difference()` outputs a table with
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`n - 1` rows.
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##### Function type signature
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```js
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(
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<-tables: stream[A],
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?columns: [string],
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?initialZero: bool,
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?keepFirst: bool,
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?nonNegative: bool,
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) => stream[B] where A: Record, B: Record
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```
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{{% caption %}}
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For more information, see [Function type signatures](/flux/v0/function-type-signatures/).
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{{% /caption %}}
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## Parameters
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### nonNegative
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Disallow negative differences. Default is `false`.
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When `true`, if a value is less than the previous value, the function
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assumes the previous value should have been a zero.
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### columns
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List of columns to operate on. Default is `["_value"]`.
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### keepFirst
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Keep the first row in each input table. Default is `false`.
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If `true`, the difference of the first row of each output table is null.
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### initialZero
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Use zero (0) as the initial value in the difference calculation
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when the subsequent value is less than the previous value and `nonNegative` is
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`true`. Default is `false`.
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### tables
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Input data. Default is piped-forward data (`<-`).
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## Examples
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- [Calculate the difference between subsequent values](#calculate-the-difference-between-subsequent-values)
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- [Calculate the non-negative difference between subsequent values](#calculate-the-non-negative-difference-between-subsequent-values)
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- [Calculate the difference between subsequent values with null values](#calculate-the-difference-between-subsequent-values-with-null-values)
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- [Keep the first value when calculating the difference between values](#keep-the-first-value-when-calculating-the-difference-between-values)
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### Calculate the difference between subsequent values
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```js
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import "sampledata"
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sampledata.int()
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|> difference()
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```
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{{< expand-wrapper >}}
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{{% expand "View example input and output" %}}
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#### Input data
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| _time | _value | *tag |
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| -------------------- | ------- | ---- |
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| 2021-01-01T00:00:00Z | -2 | t1 |
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| 2021-01-01T00:00:10Z | 10 | t1 |
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| 2021-01-01T00:00:20Z | 7 | t1 |
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| 2021-01-01T00:00:30Z | 17 | t1 |
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| 2021-01-01T00:00:40Z | 15 | t1 |
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| 2021-01-01T00:00:50Z | 4 | t1 |
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| _time | _value | *tag |
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| -------------------- | ------- | ---- |
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| 2021-01-01T00:00:00Z | 19 | t2 |
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| 2021-01-01T00:00:10Z | 4 | t2 |
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| 2021-01-01T00:00:20Z | -3 | t2 |
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| 2021-01-01T00:00:30Z | 19 | t2 |
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| 2021-01-01T00:00:40Z | 13 | t2 |
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| 2021-01-01T00:00:50Z | 1 | t2 |
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#### Output data
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| _time | _value | *tag |
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| -------------------- | ------- | ---- |
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| 2021-01-01T00:00:10Z | 12 | t1 |
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| 2021-01-01T00:00:20Z | -3 | t1 |
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| 2021-01-01T00:00:30Z | 10 | t1 |
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| 2021-01-01T00:00:40Z | -2 | t1 |
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| 2021-01-01T00:00:50Z | -11 | t1 |
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| _time | _value | *tag |
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| -------------------- | ------- | ---- |
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| 2021-01-01T00:00:10Z | -15 | t2 |
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| 2021-01-01T00:00:20Z | -7 | t2 |
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| 2021-01-01T00:00:30Z | 22 | t2 |
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| 2021-01-01T00:00:40Z | -6 | t2 |
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| 2021-01-01T00:00:50Z | -12 | t2 |
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{{% /expand %}}
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{{< /expand-wrapper >}}
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### Calculate the non-negative difference between subsequent values
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```js
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import "sampledata"
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sampledata.int()
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|> difference(nonNegative: true)
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```
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{{< expand-wrapper >}}
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{{% expand "View example input and output" %}}
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#### Input data
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| _time | _value | *tag |
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| -------------------- | ------- | ---- |
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| 2021-01-01T00:00:00Z | -2 | t1 |
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| 2021-01-01T00:00:10Z | 10 | t1 |
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| 2021-01-01T00:00:20Z | 7 | t1 |
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| 2021-01-01T00:00:30Z | 17 | t1 |
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| 2021-01-01T00:00:40Z | 15 | t1 |
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| 2021-01-01T00:00:50Z | 4 | t1 |
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| _time | _value | *tag |
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| -------------------- | ------- | ---- |
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| 2021-01-01T00:00:00Z | 19 | t2 |
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| 2021-01-01T00:00:10Z | 4 | t2 |
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| 2021-01-01T00:00:20Z | -3 | t2 |
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| 2021-01-01T00:00:30Z | 19 | t2 |
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| 2021-01-01T00:00:40Z | 13 | t2 |
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| 2021-01-01T00:00:50Z | 1 | t2 |
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#### Output data
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| _time | _value | *tag |
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| -------------------- | ------- | ---- |
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| 2021-01-01T00:00:10Z | 12 | t1 |
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| 2021-01-01T00:00:20Z | | t1 |
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| 2021-01-01T00:00:30Z | 10 | t1 |
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| 2021-01-01T00:00:40Z | | t1 |
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| 2021-01-01T00:00:50Z | | t1 |
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| _time | _value | *tag |
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| -------------------- | ------- | ---- |
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| 2021-01-01T00:00:10Z | | t2 |
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| 2021-01-01T00:00:20Z | | t2 |
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| 2021-01-01T00:00:30Z | 22 | t2 |
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| 2021-01-01T00:00:40Z | | t2 |
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| 2021-01-01T00:00:50Z | | t2 |
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{{% /expand %}}
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{{< /expand-wrapper >}}
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### Calculate the difference between subsequent values with null values
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```js
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import "sampledata"
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sampledata.int(includeNull: true)
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|> difference()
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```
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{{< expand-wrapper >}}
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{{% expand "View example input and output" %}}
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#### Input data
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| _time | _value | *tag |
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| -------------------- | ------- | ---- |
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| 2021-01-01T00:00:00Z | -2 | t1 |
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| 2021-01-01T00:00:10Z | | t1 |
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| 2021-01-01T00:00:20Z | 7 | t1 |
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| 2021-01-01T00:00:30Z | | t1 |
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| 2021-01-01T00:00:40Z | | t1 |
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| 2021-01-01T00:00:50Z | 4 | t1 |
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| _time | _value | *tag |
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| -------------------- | ------- | ---- |
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| 2021-01-01T00:00:00Z | | t2 |
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| 2021-01-01T00:00:10Z | 4 | t2 |
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| 2021-01-01T00:00:20Z | -3 | t2 |
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| 2021-01-01T00:00:30Z | 19 | t2 |
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| 2021-01-01T00:00:40Z | | t2 |
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| 2021-01-01T00:00:50Z | 1 | t2 |
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#### Output data
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| _time | _value | *tag |
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| -------------------- | ------- | ---- |
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| 2021-01-01T00:00:10Z | | t1 |
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| 2021-01-01T00:00:20Z | 9 | t1 |
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| 2021-01-01T00:00:30Z | | t1 |
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| 2021-01-01T00:00:40Z | | t1 |
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| 2021-01-01T00:00:50Z | -3 | t1 |
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| _time | _value | *tag |
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| -------------------- | ------- | ---- |
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| 2021-01-01T00:00:10Z | | t2 |
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| 2021-01-01T00:00:20Z | -7 | t2 |
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| 2021-01-01T00:00:30Z | 22 | t2 |
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| 2021-01-01T00:00:40Z | | t2 |
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| 2021-01-01T00:00:50Z | -18 | t2 |
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{{% /expand %}}
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{{< /expand-wrapper >}}
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### Keep the first value when calculating the difference between values
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```js
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import "sampledata"
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sampledata.int()
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|> difference(keepFirst: true)
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```
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{{< expand-wrapper >}}
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{{% expand "View example input and output" %}}
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#### Input data
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| _time | _value | *tag |
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| -------------------- | ------- | ---- |
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| 2021-01-01T00:00:00Z | -2 | t1 |
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| 2021-01-01T00:00:10Z | 10 | t1 |
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| 2021-01-01T00:00:20Z | 7 | t1 |
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| 2021-01-01T00:00:30Z | 17 | t1 |
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| 2021-01-01T00:00:40Z | 15 | t1 |
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| 2021-01-01T00:00:50Z | 4 | t1 |
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| _time | _value | *tag |
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| -------------------- | ------- | ---- |
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| 2021-01-01T00:00:00Z | 19 | t2 |
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| 2021-01-01T00:00:10Z | 4 | t2 |
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| 2021-01-01T00:00:20Z | -3 | t2 |
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| 2021-01-01T00:00:30Z | 19 | t2 |
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| 2021-01-01T00:00:40Z | 13 | t2 |
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| 2021-01-01T00:00:50Z | 1 | t2 |
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#### Output data
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| _time | _value | *tag |
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| -------------------- | ------- | ---- |
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| 2021-01-01T00:00:00Z | | t1 |
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| 2021-01-01T00:00:10Z | 12 | t1 |
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| 2021-01-01T00:00:20Z | -3 | t1 |
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| 2021-01-01T00:00:30Z | 10 | t1 |
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| 2021-01-01T00:00:40Z | -2 | t1 |
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| 2021-01-01T00:00:50Z | -11 | t1 |
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| _time | _value | *tag |
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| -------------------- | ------- | ---- |
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| 2021-01-01T00:00:00Z | | t2 |
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| 2021-01-01T00:00:10Z | -15 | t2 |
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| 2021-01-01T00:00:20Z | -7 | t2 |
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| 2021-01-01T00:00:30Z | 22 | t2 |
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| 2021-01-01T00:00:40Z | -6 | t2 |
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| 2021-01-01T00:00:50Z | -12 | t2 |
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{{% /expand %}}
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{{< /expand-wrapper >}}
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