npm package: @stdlib/math-base-special-fibonacci-indexf
Description: Compute the Fibonacci number index of a single-precision floating-point number.
Author: The Stdlib Authors
Latest version: 0.1.1

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# Fibonacci Number Index

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

> Compute the [Fibonacci number][fibonacci-number] index of a single-precision floating-point number.

<section class="intro">

The [Fibonacci number][fibonacci-number] index is given by

<!-- <equation class="equation" label="eq:fibonacci_number_indexf" align="center" raw="n = \left \lfloor{\log_\varphi \biggl(F \cdot \sqrt{5} + \tfrac{1}{2}\biggr)}\right \rfloor" alt="Formula to compute the Fibonacci number index."> -->

<!-- </equation> -->

where `φ` is the [golden ratio][golden-ratio] and `F > 1`.

</section>

<!-- /.intro -->

<section class="installation">

## Installation

```bash
npm install @stdlib/math-base-special-fibonacci-indexf
```

</section>

<section class="usage">

## Usage

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

#### fibonacciIndexf( F )

Computes the [Fibonacci number][fibonacci-number] index of a single-precision floating-point number.

```javascript
var n = fibonacciIndexf( 2 );
// returns 3

n = fibonacciIndexf( 3 );
// returns 4

n = fibonacciIndexf( 5 );
// returns 5
```

If provided either a non-integer or `F_n <= 1`, the function returns `NaN`.

```javascript
var n = fibonacciIndexf( -1 );
// returns NaN

n = fibonacciIndexf( 3.14 );
// returns NaN
```

If provided `NaN`, the function returns `NaN`.

```javascript
var n = fibonacciIndexf( NaN );
// returns NaN
```

</section>

<!-- /.usage -->

<section class="notes">

</section>

<!-- /.notes -->

<section class="examples">

## Examples

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

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

var F1;
var F2;
var FN;
var n;
var i;

F1 = 1;
F2 = 1;
for ( i = 3; i < 37; i++ ) {
    FN = F1 + F2;
    F1 = F2;
    F2 = FN;
    n = fibonacciIndexf( FN );
    console.log( 'n(%d) = %d', FN, n );
}
```

</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/fibonacci_indexf.h"
```

#### stdlib_base_fibonacci_indexf( F )

Computes the [Fibonacci number][fibonacci-number] index of a single-precision floating-point number.

```c
float out = stdlib_base_fibonacci_indexf( 2.0f );
// returns 3.0f

out = stdlib_base_fibonacci_indexf( 3.0f );
// returns 4.0f
```

The function accepts the following arguments:

-   **F**: `[in] float` input value.

```c
float stdlib_base_fibonacci_indexf( const float F );
```

</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/fibonacci_indexf.h"
#include <stdio.h>

int main( void ) {
    const float x[] = { 2.0f, 3.0f, 5.0f, 8.0f };

    float y;
    int i;
    for ( i = 0; i < 4; i++ ) {
        y = stdlib_base_fibonacci_indexf( x[ i ] );
        printf( "fibonacci_indexf(%f) = %f\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">

</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]

---

## License

See [LICENSE][stdlib-license].


## Copyright

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

</section>

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[stdlib-license]: https://raw.githubusercontent.com/stdlib-js/math-base-special-fibonacci-indexf/main/LICENSE

[fibonacci-number]: https://en.wikipedia.org/wiki/Fibonacci_number

[golden-ratio]: https://en.wikipedia.org/wiki/Golden_ratio

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</section>

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