Fix#4: Implemented variance function

src/primary/mod.rs

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

src/primary/variance.rs

@@ -0,0 +1,36 @@
+/// Returns the arithmetic mean of a sequence of `f64` values.
+///
+/// Empty input returns `None`.
+///
+/// # Example
+/// ```
+/// use rustats::primary::variance;
+///
+/// let values = [1.0, 2.0, 3.0];
+/// let var = variance(&values);
+/// assert_eq!(var, Some(0.6666666666666666));
+/// ```
+/// 
+
+use super::mean;
+
+
+pub fn variance(values: &[f64]) -> Option<f64> {
+    let mut count: u64 = 0;
+    let mut square_sum: f64 = 0.0;
+    let avg = mean(&values)?;
+
+    for value   in values {
+        square_sum += (*value-avg)*(*value-avg);
+        count += 1;
+    }
+
+    if count < 2{
+        None
+    }
+
+    else {
+       Some((square_sum)/ (count - 1)  as f64) 
+    }
+
+}