Probability Density Function

Chi distribution probability density function (PDF).

The probability density function (PDF) for a chi random variable is

where k is the degrees of freedom and Γ denotes the gamma function.

Installation

npm install @stdlib/stats-base-dists-chi-pdf

Usage

var pdf = require( '@stdlib/stats-base-dists-chi-pdf' );

pdf( x, k )

Evaluates the probability density function (PDF) for a chi distribution with degrees of freedom k.

var y = pdf( 0.1, 1.0 );
// returns ~0.794

y = pdf( 0.5, 2.0 );
// returns ~0.441

y = pdf( -1.0, 4.0 );
// returns 0.0

If provided NaN as any argument, the function returns NaN.

var y = pdf( NaN, 1.0 );
// returns NaN

y = pdf( 0.0, NaN );
// returns NaN

If provided k < 0, the function returns NaN.

var y = pdf( 2.0, -2.0 );
// returns NaN

If provided k = 0, the function evaluates the PDF of a degenerate distribution centered at 0.

var y = pdf( 2.0, 0.0 );
// returns 0.0

y = pdf( 0.0, 0.0 );
// returns Infinity

pdf.factory( k )

Returns a function for evaluating the PDF for a chi distribution with degrees of freedom k.

var myPDF = pdf.factory( 6.0 );

var y = myPDF( 3.0 );
// returns ~0.337

y = myPDF( 1.0 );
// returns ~0.076

Examples

var uniform = require( '@stdlib/random-array-uniform' );
var logEachMap = require( '@stdlib/console-log-each-map' );
var pdf = require( '@stdlib/stats-base-dists-chi-pdf' );

var opts = {
    'dtype': 'float64'
};
var x = uniform( 20, 0.0, 10.0, opts );
var k = uniform( 20, 0.0, 10.0, opts );

logEachMap( 'x: %0.4f, k: %0.4f, f(x;k): %0.4f', x, k, pdf );

C APIs

Usage

#include "stdlib/stats/base/dists/chi/pdf.h"

stdlib_base_dists_chi_pdf( x, k )

Evaluates the probability density function (PDF) for a chi distribution with degrees of freedom k.

double out = stdlib_base_dists_chi_pdf( 2.0, 2.0 );
// returns ~0.2707

The function accepts the following arguments:

• x: [in] double input value.
• k: [in] double degrees of freedom.

double stdlib_base_dists_chi_pdf( const double x, const double k );

Examples

#include "stdlib/stats/base/dists/chi/pdf.h"
#include <stdlib.h>
#include <stdio.h>

static double random_uniform( const double min, const double max ) {
    double v = (double)rand() / ( (double)RAND_MAX + 1.0 );
    return min + ( v*(max-min) );
}

int main( void ) {
    double x;
    double k;
    double y;
    int i;

    for ( i = 0; i < 25; i++ ) {
        x = random_uniform( 0.0, 10.0 );
        k = random_uniform( 0.1, 10.0 );
        y = stdlib_base_dists_chi_pdf( x, k );
        printf( "x: %lf, k: %lf, f(x;k): %lf\n", x, k, y );
    }
}

Notice

This package is part of 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, see the main project repository.

Community

License

See LICENSE.

Copyright

Copyright © 2016-2026. The Stdlib Authors.