Merge pull request kodecocodes#28 from chris-pilcher/Select-Minimum-Maximum

Handling empty lists when selecting minimum maximum

Author: hollance

Select Minimum Maximum/Maximum.swift

@@ -1,9 +1,12 @@
 /*
   Finds the maximum value in an array in O(n) time.
-  The array must not be empty.
 */
 
 func maximum<T: Comparable>(var array: [T]) -> T? {
+  guard !array.isEmpty else {
+    return nil
+  }
+  
   var maximum = array.removeFirst()
   for element in array {
     maximum = element > maximum ? element : maximum

Select Minimum Maximum/Minimum.swift

@@ -1,9 +1,12 @@
 /*
   Finds the minimum value in an array in O(n) time.
-  The array must not be empty.
 */
 
 func minimum<T: Comparable>(var array: [T]) -> T? {
+  guard !array.isEmpty else {
+    return nil
+  }
+
   var minimum = array.removeFirst()
   for element in array {
     minimum = element < minimum ? element : minimum

Select Minimum Maximum/MinimumMaximumPairs.swift

@@ -1,10 +1,13 @@
 /*
   Finds the maximum and minimum value in an array in O(n) time.
-  The array must not be empty.
 */
 
-func minimumMaximum<T: Comparable>(var array: [T]) -> (minimum: T, maximum: T)
+func minimumMaximum<T: Comparable>(var array: [T]) -> (minimum: T, maximum: T)?
 {
+  guard !array.isEmpty else {
+    return nil
+  }
+
   var minimum = array.first!
   var maximum = array.first!
 

Select Minimum Maximum/README.markdown

@@ -24,6 +24,10 @@ Here is a simple implementation in Swift:
 
 ```swift
 func minimum<T: Comparable>(var array: [T]) -> T? {
+  guard !array.isEmpty else {
+    return nil
+  }
+
   var minimum = array.removeFirst()
   for element in array {
     minimum = element < minimum ? element : minimum
@@ -32,6 +36,10 @@ func minimum<T: Comparable>(var array: [T]) -> T? {
 }
 
 func maximum<T: Comparable>(var array: [T]) -> T? {
+  guard !array.isEmpty else {
+    return nil
+  }
+
   var maximum = array.removeFirst()
   for element in array {
     maximum = element > maximum ? element : maximum
@@ -71,8 +79,12 @@ The result is a minimum of `3` and a maximum of `9`.
 Here is a simple implementation in Swift:
 
 ```swift
-func minimumMaximum<T: Comparable>(var array: [T]) -> (minimum: T, maximum: T)
+func minimumMaximum<T: Comparable>(var array: [T]) -> (minimum: T, maximum: T)?
 {
+  guard !array.isEmpty else {
+    return nil
+  }
+
   var minimum = array.first!
   var maximum = array.first!
 
@@ -109,7 +121,7 @@ Put this code in a playground and test it like so:
 
 ```swift
 
-let result = minimumMaximum(array)
+let result = minimumMaximum(array)!
 result.minimum // This will return 3
 result.maximum // This will return 9
 ```

Select Minimum Maximum/SelectMinimumMaximum Tests/SelectMinimumMaximumTests/MaximumTests.swift

@@ -35,6 +35,14 @@ class MaximumTests: XCTestCase {
     XCTAssertEqual(result, 9)
   }
 
+  func testMaximumGivenAnEmptyList() {
+    let array = [Int]()
+
+    let result = maximum(array)
+
+    XCTAssertNil(result)
+  }
+
   func testMaximumMatchesSwiftLibraryGivenARandomList() {
     for _ in 0...10 {
       for n in 1...100 {

Select Minimum Maximum/SelectMinimumMaximum Tests/SelectMinimumMaximumTests/MinimumTests.swift

@@ -35,6 +35,14 @@ class MinimumTests: XCTestCase {
     XCTAssertEqual(result, 3)
   }
 
+  func testMinimumGivenAnEmptyList() {
+    let array = [Int]()
+
+    let result = minimum(array)
+
+    XCTAssertNil(result)
+  }
+
   func testMinimumMatchesSwiftLibraryGivenARandomList() {
     for _ in 0...10 {
       for n in 1...100 {

Select Minimum Maximum/SelectMinimumMaximum Tests/SelectMinimumMaximumTests/SelectMinimumMaximumTests.swift

@@ -6,7 +6,7 @@ class SelectMinimumMaximumTests: XCTestCase {
   func testMinimumAndMaximumGivenAListContainingOneElement() {
     let array = [ 8 ]
 
-    let result = minimumMaximum(array)
+    let result = minimumMaximum(array)!
 
     XCTAssertEqual(result.minimum, 8)
     XCTAssertEqual(result.maximum, 8)
@@ -15,7 +15,7 @@ class SelectMinimumMaximumTests: XCTestCase {
   func testMinimumAndMaximumGivenAListContainingMultipleElementsWithTheSameValue() {
     let array = [ 8, 16, 8, 8 ]
 
-    let result = minimumMaximum(array)
+    let result = minimumMaximum(array)!
 
     XCTAssertEqual(result.minimum, 8)
     XCTAssertEqual(result.maximum, 16)
@@ -24,7 +24,7 @@ class SelectMinimumMaximumTests: XCTestCase {
   func testMimimumAndMaximumGivenAListContainingAnEvenNumberOfElements() {
     let array = [ 3, 4, 6, 8 ]
 
-    let result = minimumMaximum(array)
+    let result = minimumMaximum(array)!
 
     XCTAssertEqual(result.minimum, 3)
     XCTAssertEqual(result.maximum, 8)
@@ -33,7 +33,7 @@ class SelectMinimumMaximumTests: XCTestCase {
   func testMimimumAndMaximumGivenAListContainingAnOddNumberOfElements() {
     let array = [ 8, 3, 9, 4, 6 ]
 
-    let result = minimumMaximum(array)
+    let result = minimumMaximum(array)!
 
     XCTAssertEqual(result.minimum, 3)
     XCTAssertEqual(result.maximum, 9)
@@ -42,7 +42,7 @@ class SelectMinimumMaximumTests: XCTestCase {
   func testMimimumAndMaximumGivenAListOfOrderedElements() {
     let array = [ 3, 4, 6, 8, 9 ]
 
-    let result = minimumMaximum(array)
+    let result = minimumMaximum(array)!
 
     XCTAssertEqual(result.minimum, 3)
     XCTAssertEqual(result.maximum, 9)
@@ -51,18 +51,26 @@ class SelectMinimumMaximumTests: XCTestCase {
   func testMimimumAndMaximumGivenAListOfReverseOrderedElements() {
     let array = [ 9, 8, 6, 4, 3 ]
 
-    let result = minimumMaximum(array)
+    let result = minimumMaximum(array)!
 
     XCTAssertEqual(result.minimum, 3)
     XCTAssertEqual(result.maximum, 9)
   }
 
+  func testMinimumAndMaximumGivenAnEmptyList() {
+    let array = [Int]()
+
+    let result = minimumMaximum(array)
+
+    XCTAssertNil(result)
+  }
+
   func testMinimumAndMaximumMatchSwiftLibraryGivenARandomList() {
     for _ in 0...10 {
       for n in 1...100 {
         let array = createRandomList(n)
 
-        let result = minimumMaximum(array)
+        let result = minimumMaximum(array)!
 
         XCTAssertEqual(result.minimum, array.minElement())
         XCTAssertEqual(result.maximum, array.maxElement())

Select Minimum Maximum/SelectMinimumMaximum.playground/Contents.swift

@@ -1,5 +1,9 @@
 // Compare each item to find minimum
 func minimum<T: Comparable>(var array: [T]) -> T? {
+  guard !array.isEmpty else {
+    return nil
+  }
+
   var minimum = array.removeFirst()
   for element in array {
     minimum = element < minimum ? element : minimum
@@ -9,6 +13,10 @@ func minimum<T: Comparable>(var array: [T]) -> T? {
 
 // Compare each item to find maximum
 func maximum<T: Comparable>(var array: [T]) -> T? {
+  guard !array.isEmpty else {
+    return nil
+  }
+
   var maximum = array.removeFirst()
   for element in array {
     maximum = element > maximum ? element : maximum
@@ -17,8 +25,12 @@ func maximum<T: Comparable>(var array: [T]) -> T? {
 }
 
 // Compare in pairs to find minimum and maximum
-func minimumMaximum<T: Comparable>(var array: [T]) -> (minimum: T, maximum: T)
+func minimumMaximum<T: Comparable>(var array: [T]) -> (minimum: T, maximum: T)?
 {
+  guard !array.isEmpty else {
+    return nil
+  }
+
   var minimum = array.first!
   var maximum = array.first!
 
@@ -56,7 +68,7 @@ minimum(array)
 maximum(array)
 
 // Test of minimumMaximum function
-let result = minimumMaximum(array)
+let result = minimumMaximum(array)!
 result.minimum
 result.maximum