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Clarify the Dirac distribution example and CDF boundary #483

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21 changes: 11 additions & 10 deletions src/distribution/dirac.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -2,16 +2,23 @@ use crate::distribution::ContinuousCDF;
use crate::statistics::*;

/// Implements the [Dirac Delta](https://en.wikipedia.org/wiki/Dirac_delta_function#As_a_distribution)
/// distribution
/// distribution.
///
/// All probability mass lies at a single value. Its cumulative distribution
/// function jumps there, so it has no ordinary probability density function.
/// Use [`ContinuousCDF`] to query the probability on either side of the jump.
///
/// # Examples
///
/// ```
/// use statrs::distribution::{Dirac, Continuous};
/// use statrs::distribution::{ContinuousCDF, Dirac};
/// use statrs::statistics::Distribution;
///
/// let n = Dirac::new(3.0).unwrap();
/// assert_eq!(n.mean().unwrap(), 3.0);
/// assert_eq!(n.cdf(2.0), 0.0);
/// assert_eq!(n.cdf(3.0), 1.0);
/// assert_eq!(n.sf(3.0), 0.0);
/// ```
#[derive(Debug, Copy, Clone, PartialEq)]
pub struct Dirac(f64);
Expand Down Expand Up @@ -91,18 +98,12 @@ impl ::rand::distr::Distribution<f64> for Dirac {
}

impl ContinuousCDF<f64, f64> for Dirac {
/// Calculates the cumulative distribution function for the
/// dirac distribution at `x`
///
/// Where the value is 1 if x > `v`, 0 otherwise.
/// Returns 0 below the point mass and 1 at or above it.
fn cdf(&self, x: f64) -> f64 {
if x < self.0 { 0.0 } else { 1.0 }
}

/// Calculates the survival function for the
/// dirac distribution at `x`
///
/// Where the value is 0 if x > `v`, 1 otherwise.
/// Returns 1 below the point mass and 0 at or above it.
fn sf(&self, x: f64) -> f64 {
if x < self.0 { 1.0 } else { 0.0 }
}
Expand Down