npm package: @stdlib/math-base-special-cosm1
Description: Compute the cosine of a number minus one.
Author: The Stdlib Authors
Latest version: 0.3.1

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# cosm1

[![NPM version][npm-image]][npm-url] [![Build Status][test-image]][test-url] [![Coverage Status][coverage-image]][coverage-url] <!-- [![dependencies][dependencies-image]][dependencies-url] -->

> Compute `cos(x) - 1`.

<section class="installation">

## Installation

```bash
npm install @stdlib/math-base-special-cosm1
```

</section>

<section class="usage">

## Usage

```javascript
var cosm1 = require( '@stdlib/math-base-special-cosm1' );
```

#### cosm1( x )

Computes `cos(x) - 1`, where `cos` is the [cosine][@stdlib/math/base/special/cos] of a `number` (in radians). This function should be used instead of manually calculating `cos(x) - 1` when the argument is near unity.

```javascript
var PI = require( '@stdlib/constants-float64-pi' );

var v = cosm1( 0.0 );
// returns 0.0

v = cosm1( PI/4.0 );
// returns ~-0.293

v = cosm1( -PI/6.0 );
// returns ~-0.134

v = cosm1( NaN );
// returns NaN
```

</section>

<!-- /.usage -->

<section class="examples">

## Examples

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

```javascript
var uniform = require( '@stdlib/random-array-uniform' );
var logEachMap = require( '@stdlib/console-log-each-map' );
var PI = require( '@stdlib/constants-float64-pi' );
var cosm1 = require( '@stdlib/math-base-special-cosm1' );

var opts = {
    'dtype': 'float64'
};
var x = uniform( 100, 0.0, 2.0*PI, opts );

logEachMap( 'cos(%0.4f) - 1 = %0.4f', x, cosm1 );
```

</section>

<!-- /.examples -->

<!-- C interface documentation. -->

* * *

<section class="c">

## C APIs

<!-- 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 -->

<!-- C usage documentation. -->

<section class="usage">

### Usage

```c
#include "stdlib/math/base/special/cosm1.h"
```

#### stdlib_base_cosm1( x )

Computes `cos(x) - 1`, where `cos` is the [cosine][@stdlib/math/base/special/cos] of a `number` (in radians). This function should be used instead of manually calculating `cos(x) - 1` when the argument is near unity.

```c
double out = stdlib_base_cosm1( 0.0 );
// returns 0.0

out = stdlib_base_cosm1( 3.141592653589793238 / 4.0 );
// returns ~-0.293
```

The function accepts the following arguments:

-   **x**: `[in] double` input value.

```c
double stdlib_base_cosm1( const double x );
```

</section>

<!-- /.usage -->

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

<section class="notes">

</section>

<!-- /.notes -->

<!-- C API usage examples. -->

<section class="examples">

### Examples

```c
#include "stdlib/math/base/special/cosm1.h"
#include <stdio.h>

int main( void ) {
    const double x[] = { 0.0, 0.523, 0.785, 1.047, 3.14 };

    double y;
    int i;
    for ( i = 0; i < 5; i++ ) {
        y = stdlib_base_cosm1( x[ i ] );
        printf( "cosm1(%lf) = %lf\n", x[ i ], y );
    }
}
```

</section>

<!-- /.examples -->

</section>

<!-- /.c -->

<!-- Section for related `stdlib` packages. Do not manually edit this section, as it is automatically populated. -->

<section class="related">

* * *

## See Also

-   <span class="package-name">[`@stdlib/math-base/special/cos`][@stdlib/math/base/special/cos]</span><span class="delimiter">: </span><span class="description">compute the cosine of a number.</span>

</section>

<!-- /.related -->

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<section class="main-repo" >

* * *

## 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].

#### Community

[![Chat][chat-image]][chat-url]

---

## Copyright

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

</section>

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[@stdlib/math/base/special/cos]: https://www.npmjs.com/package/@stdlib/math-base-special-cos

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</section>

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