Fix #1: Implement Median function

src/primary/median.rs

@@ -0,0 +1,37 @@
+/// Returns the median (middle) value of a sequence of `f64` values
+///
+/// Empty input returns `None`.
+///
+/// # Example
+/// ```
+/// use rustats::primary::median;
+///
+/// let values = [4.0, 3.0, 1.0, 3.0];
+/// let med = median(&values);
+/// assert_eq!(med, Some(3.0));
+/// ```
+
+pub fn median(values: &[f64]) -> Option<f64> {
+    if values.is_empty() {
+        return None;
+    }
+
+    let mut sorted = values.to_vec();
+    let n = sorted.len();
+
+    sorted.sort_by(|a, b| {
+        a.partial_cmp(b)
+            .expect("Median cannot be computed for NaN values!")
+    });
+
+    let m;
+    if n % 2 == 0 {
+        let i1 = n / 2;
+        m = (sorted[i1 - 1] + sorted[i1]) / 2.0;
+    } else {
+        let i2 = (n + 1) / 2;
+        m = sorted[i2 - 1];
+    }
+
+    Some(m)
+}

src/primary/mod.rs

@@ -1,5 +1,7 @@
 pub mod mean;
 pub mod mode;
+pub mod median;
 
 pub use mean::mean;
 pub use mode::mode;
+pub use median::median;