feat: Added medianlow and medianHigh functions

README.md

@@ -14,6 +14,8 @@ This library is a collection of functions that perform statistical calculations
 * [covarianceSample](#sample-covariance)
 * [max](#max)
 * [median](#median)
+* [medianLow](#medianLow)
+* [medianHigh](#medianHigh)
 * [min](#min)
 * [pearson](#pearson-correlation-coefficient)
 * [standardDeviationPopulation](#population-standard-deviation)
@@ -126,6 +128,34 @@ Sigma.median([1, 12, 19.5, 3, -5])
 // Result: 3
 ```
 
+### Median Low
+
+Returns the [median value](http://en.wikipedia.org/wiki/Median) from the array.
+
+**Note**:
+
+* Returns nil when the array is empty.
+* Returns the lower of the two middle values if there is an even number of items in the array.
+
+```Swift
+Sigma.median([1, 12, 19.5, 10, 3, -5])
+// Result: 3
+```
+
+### Median High
+
+Returns the [median value](http://en.wikipedia.org/wiki/Median) from the array.
+
+**Note**:
+
+* Returns nil when the array is empty.
+* Returns the higher of the two middle values if there is an even number of items in the array.
+
+```Swift
+Sigma.median([1, 12, 19.5, 10, 3, -5])
+// Result: 10
+```
+
 ### Sample variance
 
 Computes [variance](http://en.wikipedia.org/wiki/Variance) based on a sample.
@@ -320,6 +350,7 @@ If you need help or want to extend the library feel free to create an issue or s
 ## Contributors
 
 * [Thomas Fankhauser](https://github.com/southdesign)
+* [John Clema](https://github.com/JohnClema)
 
 ## License
 

SigmaSwiftStatistics/Sigma.swift

@@ -110,7 +110,7 @@ public struct Sigma {
     let sorted = values.sort { $0 < $1 }
 
     if count % 2 == 0 {
-      // Event number of items - return the mean of two middle values
+      // Even number of items - return the mean of two middle values
       let leftIndex = Int(count / 2 - 1)
       let leftValue = sorted[leftIndex]
       let rightValue = sorted[leftIndex + 1]
@@ -121,6 +121,55 @@ public struct Sigma {
     }
   }
 
+  /**
+
+   Returns the central value from the array after it is sorted.
+
+   http://en.wikipedia.org/wiki/Median
+
+   - parameter values: Array of decimal numbers.
+   - returns: The median value from the array. Returns nil for an empty array. Returns the smaller of the two middle values if there is an even number of items in the array.
+
+   Example
+
+   Sigma.median([1, 12, 19.5, 3, -5]) // 3
+
+   */
+  public static func medianLow(values: [Double]) -> Double? {
+    let count = Double(values.count)
+    if count == 0 { return nil }
+    let sorted = values.sort { $0 < $1 }
+
+    if count % 2 == 0 {
+      // Even number of items - return the lower of the two middle values
+      return sorted[Int(count / 2) - 1]
+    } else {
+      // Odd number of items - take the middle item.
+      return sorted[Int(count / 2)]
+    }
+  }
+
+  /**
+
+   Returns the central value from the array after it is sorted.
+
+   http://en.wikipedia.org/wiki/Median
+
+   - parameter values: Array of decimal numbers.
+   - returns: The median value from the array. Returns nil for an empty array. Returns the greater of the two middle values if there is an even number of items in the array.
+
+   Example
+
+   Sigma.median([1, 12, 19.5, 3, -5, ]) // 3
+
+   */
+  public static func medianHigh(values: [Double]) -> Double? {
+    let count = Double(values.count)
+    if count == 0 { return nil }
+    let sorted = values.sort { $0 < $1 }
+    return sorted[Int(count / 2)]
+  }
+
   /**
 
   Computes variance based on a sample.

SigmaSwiftStatisticsTests/SigmaTests.swift

@@ -74,6 +74,48 @@ class SigmaTests: XCTestCase {
     XCTAssert(result == nil)
   }
 
+  // MARK: - Median Low
+  func testMedianLow_oddNumberOfItems() {
+    let result = Sigma.medianLow([1, 12, 19.5, 3, -5])!
+    XCTAssertEqual(3, result)
+  }
+
+  func testMedianLow_eventNumberOfItems() {
+    let result = Sigma.medianLow([1, 12, 19.5, 3, -5, 8])!
+    XCTAssertEqual(3, result)
+  }
+
+  func testMedianLow_oneItem() {
+    let result = Sigma.medianLow([2])!
+    XCTAssertEqual(2, result)
+  }
+
+  func testMedianLow_whenEmpty() {
+    let result = Sigma.medianLow([])
+    XCTAssert(result == nil)
+  }
+
+  //MARK: - Median High
+  func testMedianHigh_oddNumberOfItems() {
+    let result = Sigma.medianHigh([1, 12, 19.5, 3, -5])!
+    XCTAssertEqual(3, result)
+  }
+
+  func testMedianHigh_eventNumberOfItems() {
+    let result = Sigma.medianHigh([1, 12, 19.5, 3, -5, 8])!
+    XCTAssertEqual(8, result)
+  }
+
+  func testMedianHigh_oneItem() {
+    let result = Sigma.medianHigh([2])!
+    XCTAssertEqual(2, result)
+  }
+
+  func testMedianHigh_whenEmpty() {
+    let result = Sigma.medianHigh([])
+    XCTAssert(result == nil)
+  }
+
   // MARK: - Sample variance
 
   func testVarianceSample() {