---
title: strings.index() function
description: >
  `strings.index()` returns the index of the first instance of a substring in a string.
  If the substring is not present, it returns `-1`.
menu:
  flux_0_x_ref:
    name: strings.index
    parent: strings
    identifier: strings/index
weight: 101
---

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`strings.index()` returns the index of the first instance of a substring in a string.
If the substring is not present, it returns `-1`.



##### Function type signature

```js
(substr: string, v: string) => int
```

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

## Parameters

### v
({{< req >}})
String value to search.



### substr
({{< req >}})
Substring to search for.




## Examples

### Find the index of the first occurrence of a substring

```js
import "sampledata"
import "strings"

sampledata.string()
    |> map(fn: (r) => ({r with _value: strings.index(v: r._value, substr: "g")}))

```

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

#### Input data

| _time                | *tag | _value      |
| -------------------- | ---- | ----------- |
| 2021-01-01T00:00:00Z | t1   | smpl_g9qczs |
| 2021-01-01T00:00:10Z | t1   | smpl_0mgv9n |
| 2021-01-01T00:00:20Z | t1   | smpl_phw664 |
| 2021-01-01T00:00:30Z | t1   | smpl_guvzy4 |
| 2021-01-01T00:00:40Z | t1   | smpl_5v3cce |
| 2021-01-01T00:00:50Z | t1   | smpl_s9fmgy |

| _time                | *tag | _value      |
| -------------------- | ---- | ----------- |
| 2021-01-01T00:00:00Z | t2   | smpl_b5eida |
| 2021-01-01T00:00:10Z | t2   | smpl_eu4oxp |
| 2021-01-01T00:00:20Z | t2   | smpl_5g7tz4 |
| 2021-01-01T00:00:30Z | t2   | smpl_sox1ut |
| 2021-01-01T00:00:40Z | t2   | smpl_wfm757 |
| 2021-01-01T00:00:50Z | t2   | smpl_dtn2bv |


#### Output data

| _time                | _value  | *tag |
| -------------------- | ------- | ---- |
| 2021-01-01T00:00:00Z | 5       | t1   |
| 2021-01-01T00:00:10Z | 7       | t1   |
| 2021-01-01T00:00:20Z | -1      | t1   |
| 2021-01-01T00:00:30Z | 5       | t1   |
| 2021-01-01T00:00:40Z | -1      | t1   |
| 2021-01-01T00:00:50Z | 9       | t1   |

| _time                | _value  | *tag |
| -------------------- | ------- | ---- |
| 2021-01-01T00:00:00Z | -1      | t2   |
| 2021-01-01T00:00:10Z | -1      | t2   |
| 2021-01-01T00:00:20Z | 6       | t2   |
| 2021-01-01T00:00:30Z | -1      | t2   |
| 2021-01-01T00:00:40Z | -1      | t2   |
| 2021-01-01T00:00:50Z | -1      | t2   |

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