Adding Thomas Algorithm in Swift (#383)

* Adding Thomas Algorithm in Swift

* Changed function to pass by value

Author: Wesley-Arrington

contents/thomas_algorithm/code/swift/thomas.swift

@@ -0,0 +1,37 @@
+func thomas(a: [Double], b: [Double], c: [Double], d: [Double]) -> [Double] {
+    var a = a
+    var b = b
+    var c = c
+    var d = d
+
+    // set the initial elements
+    c[0] = c[0] / b[0]
+    d[0] = d[0] / b[0]
+
+    let n = d.count // number of equations to solve
+    for i in 1..<n {
+        // scale factor for c and d
+        let scale = 1 / (b[i] - c[i-1] * a[i])
+
+        c[i] = c[i] * scale
+        d[i] = (d[i] - a[i] * d[i-1]) * scale
+    }
+
+    // do the back substitution
+    for i in stride(from: n-2, to: -1, by: -1) {
+        d[i] = d[i] - c[i] * d[i+1]
+    }
+
+    return d
+}
+
+func main() {
+    let a = [0.0, 2.0, 3.0]
+    let b = [1.0, 3.0, 6.0]
+    let c = [4.0, 5.0, 0.0]
+    let d = [7.0, 5.0, 3.0]
+
+    print(thomas(a: a, b: b, c: c, d: d))
+}
+
+main()

contents/thomas_algorithm/thomas_algorithm.md

@@ -111,6 +111,8 @@ You will find this algorithm implemented [in this project](https://scratch.mit.e
 [import, lang:"java"](code/java/thomas.java)
 {% sample lang="hs" %}
 [import, lang:"haskell"](code/haskell/thomas.hs)
+{% sample lang="swift" %}
+[import, lang:"swift"](code/swift/thomas.swift)
 {% endmethod %}
 
 <script>