updated map note
Scott Anderson
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@ -16,118 +16,4 @@ related:
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- /influxdb/cloud/reference/flux/stdlib/contrib/rows/map/
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---
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The `map()` function applies a function to each record in the input tables.
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The modified records are assigned to new tables based on the group key of the input table.
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The output tables are the result of applying the map function to each record of the input tables.
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When the output record contains a different value for the group key, the record is regrouped into the appropriate table.
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When the output record drops a column that was part of the group key, that column is removed from the group key.
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_**Function type:** Transformation_
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```js
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map(fn: (r) => ({ _value: r._value * r._value }))
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```
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## Parameters
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{{% note %}}
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Make sure `fn` parameter names match each specified parameter. To learn why, see [Match parameter names](/influxdb/cloud/reference/flux/language/data-model/#match-parameter-names).
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{{% /note %}}
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### fn
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A single argument function to apply to each record.
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The return value must be a record.
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_**Data type:** Function_
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{{% note %}}
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Records evaluated in `fn` functions are represented by `r`, short for "record" or "row".
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{{% /note %}}
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## Important notes
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#### Preserve columns
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By default, `map()` drops any columns that:
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1. Are not part of the input table's group key.
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2. Are not explicitly mapped in the `map()` function.
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This often results in the `_time` column being dropped.
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To preserve the `_time` column and other columns that do not meet the criteria above,
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use the `with` operator to map values in the `r` record.
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The `with` operator updates a column if it already exists,
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creates a new column if it doesn't exist, and includes all existing columns in
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the output table.
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```js
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map(fn: (r) => ({ r with newColumn: r._value * 2 }))
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```
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## Examples
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###### Square the value of each record
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```js
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from(bucket:"example-bucket")
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|> filter(fn: (r) =>
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r._measurement == "cpu" and
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r._field == "usage_system" and
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r.cpu == "cpu-total"
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)
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|> range(start:-12h)
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|> map(fn: (r) => ({ r with _value: r._value * r._value}))
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```
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###### Create a new table with new format
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```js
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from(bucket:"example-bucket")
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|> filter(fn: (r) =>
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r._measurement == "cpu" and
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r._field == "usage_system"
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)
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|> range(start:-12h)
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// create a new table by copying each row into a new format
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|> map(fn: (r) => ({
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time: r._time,
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app_server: r.host
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}))
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```
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###### Add new columns and preserve existing columns
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```js
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from(bucket:"example-bucket")
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|> filter(fn: (r) =>
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r._measurement == "cpu" and
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r._field == "usage_system"
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)
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|> range(start:-12h)
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// create a new table by copying each row into a new format
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|> map(fn: (r) => ({
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r with
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app_server: r.host,
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valueInt: int(v: r._value)
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}))
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```
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## Troubleshooting
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### Map object property is not supported in a flux table
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Flux tables can only support certain value types. The presently supported types are:
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* float
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* integer
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* unsigned
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* string
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* boolean
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* time
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In the future, [duration](https://github.com/influxdata/flux/issues/1803) and [bytes](https://github.com/influxdata/flux/issues/1804) will be supported, but these two types are not presently supported.
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If any unsupported type is included in the record returns by `map`, an error with the name of the column type and the column that attempted to use the unsupported type will be returned.
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If you are using a duration, you can convert it to a time or integer value using `time(v:)` or `int(v:)`.
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For the bytes type, the `string(v:)` converter is the most likely candidate.
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{{< duplicate-oss >}}
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@ -112,3 +112,32 @@ from(bucket:"example-bucket")
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valueInt: int(v: r._value)
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}))
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```
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## Troubleshooting
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### Map object property is not supported in a Flux table
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Flux tables only support the following value types:
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- float
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- integer
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- unsigned integer
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- string
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- boolean
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- time
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If `map()` returns a record with an unsupported type, Flux returns an error with
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the name of the column that attempted to use the unsupported type.
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If mapping a **duration** value, use [`time()`](/influxdb/v2.0/reference/flux/stdlib/built-in/transformations/type-conversions/time/)
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to convert it to a time value or [`int()`](/influxdb/v2.0/reference/flux/stdlib/built-in/transformations/type-conversions/int/)
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to convert it to an integer.
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For the **bytes** type, use [`string()`](/influxdb/v2.0/reference/flux/stdlib/built-in/transformations/type-conversions/string/) to convert the value to a string.
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{{% note %}}
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For information about supporting other data types in Flux tables, see the
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following Github issues:
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- [Add support for duration column value](https://github.com/influxdata/flux/issues/1803)
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- [Add support for bytes column value](https://github.com/influxdata/flux/issues/1804)
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{{% /note %}}
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