# betainc
> [Incomplete beta function][incomplete-beta-function].
## Usage
```javascript
var betainc = require( '@stdlib/math/base/special/betainc' );
```
#### betainc( x, a, b\[, regularized\[, upper]] )
By default, evaluates the regularized lower [incomplete beta function][incomplete-beta-function] for inputs `x`, `a > 0` and `b > 0`. The fourth and fifth parameters of the function can be used to specify whether instead to evaluate the non-regularized and/or upper incomplete beta functions, respectively.
```javascript
var y = betainc( 0.5, 2.0, 2.0 );
// returns 0.5
y = betainc( 0.5, 2.0, 2.0, false );
// returns ~0.083
y = betainc( 0.2, 1.0, 2.0 );
// returns 0.36
y = betainc( 0.2, 1.0, 2.0, true, true );
// returns 0.64
```
If provided `NaN` as any argument, the function returns `NaN`.
```javascript
var y = betainc( NaN, 1.0, 1.0 );
// returns NaN
y = betainc( 0.8, NaN, 1.0 );
// returns NaN
y = betainc( 0.8, 1.0, NaN );
// returns NaN
```
If provided a `x` outside the interval `[0,1]`, the function returns `NaN`.
```javascript
var y = betainc( 1.5, 1.0, 1.0 );
// returns NaN
y = betainc( -0.5, 1.0, 1.0 );
// returns NaN
```
If provided a negative `a`, the function returns `NaN`.
```javascript
var y = betainc( 0.5, -2.0, 2.0 );
// returns NaN
```
If provided a negative `b`, the function returns `NaN`.
```javascript
var y = betainc( 0.5, 2.0, -2.0 );
// returns NaN
```
## Examples
```javascript
var randu = require( '@stdlib/random/base/randu' );
var betainc = require( '@stdlib/math/base/special/betainc' );
var i;
var x;
var a;
var b;
for ( i = 0; i < 100; i++ ) {
x = randu();
a = randu() * 10.0;
b = randu() * 10.0;
console.log( 'x: %d, \t a: %d, \t b: %d, \t f(x,a,b): %d', x.toFixed( 4 ), a.toFixed( 4 ), b.toFixed( 4 ), betainc( x, a, b ).toFixed( 4 ) );
}
```
[incomplete-beta-function]: https://en.wikipedia.org/wiki/Incomplete_beta_function