npm package: @stdlib/slice-seq2slice
Description: Convert a subsequence string to a Slice object.
Author: The Stdlib Authors
Latest version: 0.2.3

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# seq2slice

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> Convert a subsequence string to a [`Slice`][@stdlib/slice/ctor] object.

<!-- Section to include introductory text. Make sure to keep an empty line after the intro `section` element and another before the `/section` close. -->

<section class="intro">

</section>

<!-- /.intro -->

<!-- Package usage documentation. -->

<section class="installation">

## Installation

```bash
npm install @stdlib/slice-seq2slice
```

</section>

<section class="usage">

## Usage

```javascript
var seq2slice = require( '@stdlib/slice-seq2slice' );
```

<a name="main"></a>

#### seq2slice( str, len, strict )

Converts a subsequence string to a [`Slice`][@stdlib/slice/ctor] object, where `len` specifies the maximum number of elements allowed in the slice.

```javascript
var s = seq2slice( ':5', 10, false );
// returns <Slice>

var v = s.start;
// returns 0

v = s.stop;
// returns 5

v = s.step;
// returns 1
```

A subsequence string has the following format:

```text
<start>:<stop>:<increment>
```

where

-   If an `increment` is not specified, the default increment is `1`. An increment of zero is **not** allowed.
-   The `start` index is **inclusive**.
-   The `stop` index is **exclusive**.
-   Both `start` and `stop` indices are _optional_. If not provided, `start` and `stop` default to index extremes. Which extremes correspond to which index depends on whether the `increment` is positive or negative.
-   Both `start` and `stop` can be negative; in which case, the corresponding index is resolved by subtracting the respective value from the provided length `len`.
-   Both `start` and `stop` can use the `end` keyword (e.g., `end-2::2`, `end-3:`, etc), which supports basic subtraction and division.
-   The `end` keyword resolves to the provided length `len`. Thus, `:-1` is equivalent to `:end-1`, `:-2` is equivalent to `:end-2`, and so on and so forth. The exception is when performing a division operation when the `increment` is less than zero; in which case, `end` is equal to `len-1` in order to preserve user expectations when `end/d` equals a whole number and slicing from right-to-left. The result from a division operation is **rounded down** to the nearest integer value.

```javascript
var s = seq2slice( 'end:2:-1', 10, false );
// returns <Slice>

var v = s.start;
// returns 9

v = s.stop;
// returns 2

v = s.step;
// returns -1

s = seq2slice( 'end-2:2:-1', 10, false );
// returns <Slice>

v = s.start;
// returns 8

v = s.stop;
// returns 2

v = s.step;
// returns -1

s = seq2slice( 'end/2:2:-1', 10, false );
// returns <Slice>

v = s.start;
// returns 4

v = s.stop;
// returns 2

v = s.step;
// returns -1
```

When `strict` is `true`, the function throws an error if a subsequence string resolves to a slice exceeding index bounds.

<!-- run throws: true -->

```javascript
var s = seq2slice( '10:20', 10, true );
// throws <RangeError>
```

</section>

<!-- /.usage -->

<!-- Package usage notes. Make sure to keep an empty line after the `section` element and another before the `/section` close. -->

<section class="notes">

## Notes

-   When `len` is zero, the function always returns a Slice object equivalent to `0:0:<increment>`.
-   When `strict` is `false`, the resolved slice start is clamped to the slice index bounds (i.e., `[0, len)`).
-   When `strict` is `false`, the resolved slice end is upper bound clamped to `len` (i.e., one greater than the last possible index).
-   When the increment is negative, the resolved slice end value may be `null`, thus indicating that a non-empty slice should include the first index.
-   The function ensures that results satisfy the convention that `:n` combined with `n:` is equivalent to `:` (i.e., selecting all elements). This convention matches Python slice semantics, but diverges from the MATLAB convention where `:n` and `n:` overlap by one element.
-   Unlike MATLAB, but like Python, the subsequence string is upper-bound exclusive. For example, in Python, `0:2` corresponds to the sequence `{0,1}`. In MATLAB, `1:3` corresponds to `{1,2,3}`.

</section>

<!-- /.notes -->

<!-- Package usage examples. -->

<section class="examples">

## Examples

<!-- eslint no-undef: "error" -->

```javascript
var seq2slice = require( '@stdlib/slice-seq2slice' );

var s = seq2slice( ':', 5, false );
console.log( 'start: %s. stop: %s. step: %s.', s.start, s.stop, s.step );
// => 'start: 0. stop: 5. step: 1.'

s = seq2slice( '2:', 5, false );
console.log( 'start: %s. stop: %s. step: %s.', s.start, s.stop, s.step );
// => 'start: 2. stop: 5. step: 1.'

s = seq2slice( ':3', 5, false );
console.log( 'start: %s. stop: %s. step: %s.', s.start, s.stop, s.step );
// => 'start: 0. stop: 3. step: 1.'

s = seq2slice( '2:4', 5, false );
console.log( 'start: %s. stop: %s. step: %s.', s.start, s.stop, s.step );
// => 'start: 2. stop: 4. step: 1.'

s = seq2slice( '1:4:2', 5, false );
console.log( 'start: %s. stop: %s. step: %s.', s.start, s.stop, s.step );
// => 'start: 1. stop: 4. step: 2.'

s = seq2slice( '2::2', 5, false );
console.log( 'start: %s. stop: %s. step: %s.', s.start, s.stop, s.step );
// => 'start: 2. stop: 5. step: 2.'

s = seq2slice( ':-2', 5, false );
console.log( 'start: %s. stop: %s. step: %s.', s.start, s.stop, s.step );
// => 'start: 0. stop: 3. step: 1.'

s = seq2slice( ':-1:2', 5, false );
console.log( 'start: %s. stop: %s. step: %s.', s.start, s.stop, s.step );
// => 'start: 0. stop: 4. step: 2.'

s = seq2slice( '-4:-1:2', 5, false );
console.log( 'start: %s. stop: %s. step: %s.', s.start, s.stop, s.step );
// => 'start: 1. stop: 4. step: 2.'

s = seq2slice( '-5:-1', 5, false );
console.log( 'start: %s. stop: %s. step: %s.', s.start, s.stop, s.step );
// => 'start: 0. stop: 4. step: 1.'

s = seq2slice( '::-1', 5, false );
console.log( 'start: %s. stop: %s. step: %s.', s.start, s.stop, s.step );
// => 'start: 4. stop: null. step: -1.'

s = seq2slice( ':0:-1', 5, false );
console.log( 'start: %s. stop: %s. step: %s.', s.start, s.stop, s.step );
// => 'start: 4. stop: 0. step: -1.'

s = seq2slice( '3:0:-1', 5, false );
console.log( 'start: %s. stop: %s. step: %s.', s.start, s.stop, s.step );
// => 'start: 3. stop: 0. step: -1.'

s = seq2slice( '-1:-4:-2', 5, false );
console.log( 'start: %s. stop: %s. step: %s.', s.start, s.stop, s.step );
// => 'start: 4. stop: 1. step: -2.'

s = seq2slice( ':end', 5, false );
console.log( 'start: %s. stop: %s. step: %s.', s.start, s.stop, s.step );
// => 'start: 0. stop: 5. step: 1.'

s = seq2slice( ':end-1', 5, false );
console.log( 'start: %s. stop: %s. step: %s.', s.start, s.stop, s.step );
// => 'start: 0. stop: 4. step: 1.'

s = seq2slice( ':end/2', 5, false );
console.log( 'start: %s. stop: %s. step: %s.', s.start, s.stop, s.step );
// => 'start: 0. stop: 2. step: 1.'

s = seq2slice( 'end/2::-1', 5, false );
console.log( 'start: %s. stop: %s. step: %s.', s.start, s.stop, s.step );
// => 'start: 2. stop: null. step: -1.'

s = seq2slice( 'end-2::-1', 5, false );
console.log( 'start: %s. stop: %s. step: %s.', s.start, s.stop, s.step );
// => 'start: 3. stop: null. step: -1.'

s = seq2slice( 'end/2:', 5, false );
console.log( 'start: %s. stop: %s. step: %s.', s.start, s.stop, s.step );
// => 'start: 2. stop: 5. step: 1.'
```

</section>

<!-- /.examples -->

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</section>

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<section class="related">

* * *

## See Also

-   <span class="package-name">[`@stdlib/slice-ctor`][@stdlib/slice/ctor]</span><span class="delimiter">: </span><span class="description">slice constructor.</span>

</section>

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* * *

## Notice

This package is part of [stdlib][stdlib], a standard library for JavaScript and Node.js, with an emphasis on numerical and scientific computing. The library provides a collection of robust, high performance libraries for mathematics, statistics, streams, utilities, and more.

For more information on the project, filing bug reports and feature requests, and guidance on how to develop [stdlib][stdlib], see the main project [repository][stdlib].

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---

## License

See [LICENSE][stdlib-license].


## Copyright

Copyright &copy; 2016-2026. The Stdlib [Authors][stdlib-authors].

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