---
title: keep() function
description: >
  `keep()` returns a stream of tables containing only the specified columns.
menu:
  flux_v0_ref:
    name: keep
    parent: universe
    identifier: universe/keep
weight: 101
flux/v0/tags: [transformations]
introduced: 0.7.0
---

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`keep()` returns a stream of tables containing only the specified columns.

Columns in the group key that are not specified in the `columns` parameter or
identified by the `fn` parameter are removed from the group key and dropped
from output tables. `keep()` is the inverse of `drop()`.

##### Function type signature

```js
(<-tables: stream[A], ?columns: [string], ?fn: (column: string) => bool) => stream[B] where A: Record, B: Record
```

{{% caption %}}
For more information, see [Function type signatures](/flux/v0/function-type-signatures/).
{{% /caption %}}

## Parameters

### columns

Columns to keep in output tables. Cannot be used with `fn`.



### fn

Predicate function that takes a column name as a parameter (column) and
returns a boolean indicating whether or not the column should be kept in
output tables. Cannot be used with `columns`.



### tables

Input data. Default is piped-forward data (`<-`).




## Examples

- [Keep a list of columns](#keep-a-list-of-columns)
- [Keep columns matching a predicate](#keep-columns-matching-a-predicate)

### Keep a list of columns

```js
import "sampledata"

sampledata.int()
    |> keep(columns: ["_time", "_value"])

```

{{< expand-wrapper >}}
{{% expand "View example input and output" %}}

#### Input data

| _time                | _value  | *tag |
| -------------------- | ------- | ---- |
| 2021-01-01T00:00:00Z | -2      | t1   |
| 2021-01-01T00:00:10Z | 10      | t1   |
| 2021-01-01T00:00:20Z | 7       | t1   |
| 2021-01-01T00:00:30Z | 17      | t1   |
| 2021-01-01T00:00:40Z | 15      | t1   |
| 2021-01-01T00:00:50Z | 4       | t1   |

| _time                | _value  | *tag |
| -------------------- | ------- | ---- |
| 2021-01-01T00:00:00Z | 19      | t2   |
| 2021-01-01T00:00:10Z | 4       | t2   |
| 2021-01-01T00:00:20Z | -3      | t2   |
| 2021-01-01T00:00:30Z | 19      | t2   |
| 2021-01-01T00:00:40Z | 13      | t2   |
| 2021-01-01T00:00:50Z | 1       | t2   |


#### Output data

| _time                | _value  |
| -------------------- | ------- |
| 2021-01-01T00:00:00Z | -2      |
| 2021-01-01T00:00:10Z | 10      |
| 2021-01-01T00:00:20Z | 7       |
| 2021-01-01T00:00:30Z | 17      |
| 2021-01-01T00:00:40Z | 15      |
| 2021-01-01T00:00:50Z | 4       |
| 2021-01-01T00:00:00Z | 19      |
| 2021-01-01T00:00:10Z | 4       |
| 2021-01-01T00:00:20Z | -3      |
| 2021-01-01T00:00:30Z | 19      |
| 2021-01-01T00:00:40Z | 13      |
| 2021-01-01T00:00:50Z | 1       |

{{% /expand %}}
{{< /expand-wrapper >}}

### Keep columns matching a predicate

```js
import "sampledata"

sampledata.int()
    |> keep(fn: (column) => column =~ /^_?t/)

```

{{< expand-wrapper >}}
{{% expand "View example input and output" %}}

#### Input data

| _time                | _value  | *tag |
| -------------------- | ------- | ---- |
| 2021-01-01T00:00:00Z | -2      | t1   |
| 2021-01-01T00:00:10Z | 10      | t1   |
| 2021-01-01T00:00:20Z | 7       | t1   |
| 2021-01-01T00:00:30Z | 17      | t1   |
| 2021-01-01T00:00:40Z | 15      | t1   |
| 2021-01-01T00:00:50Z | 4       | t1   |

| _time                | _value  | *tag |
| -------------------- | ------- | ---- |
| 2021-01-01T00:00:00Z | 19      | t2   |
| 2021-01-01T00:00:10Z | 4       | t2   |
| 2021-01-01T00:00:20Z | -3      | t2   |
| 2021-01-01T00:00:30Z | 19      | t2   |
| 2021-01-01T00:00:40Z | 13      | t2   |
| 2021-01-01T00:00:50Z | 1       | t2   |


#### Output data

| _time                | *tag |
| -------------------- | ---- |
| 2021-01-01T00:00:00Z | t1   |
| 2021-01-01T00:00:10Z | t1   |
| 2021-01-01T00:00:20Z | t1   |
| 2021-01-01T00:00:30Z | t1   |
| 2021-01-01T00:00:40Z | t1   |
| 2021-01-01T00:00:50Z | t1   |

| _time                | *tag |
| -------------------- | ---- |
| 2021-01-01T00:00:00Z | t2   |
| 2021-01-01T00:00:10Z | t2   |
| 2021-01-01T00:00:20Z | t2   |
| 2021-01-01T00:00:30Z | t2   |
| 2021-01-01T00:00:40Z | t2   |
| 2021-01-01T00:00:50Z | t2   |

{{% /expand %}}
{{< /expand-wrapper >}}