Thomas Algorithm in Ruby (algorithm-archivists#648)

Author: tarunvelli

contents/thomas_algorithm/code/ruby/thomas.rb

@@ -0,0 +1,38 @@
+# note this example is inplace and destructive
+def thomas(a, b, c, d)
+  # set the initial elements
+  c[0] = c[0] / b[0]
+  d[0] = d[0] / b[0]
+
+  n = d.length # number of equations to solve
+  (1...n).each do |i|
+    scale = 1 / (b[i] - c[i - 1] * a[i]) # scale factor for c and d
+    c[i] *= scale
+    d[i] = (d[i] - a[i] * d[i - 1]) * scale
+  end
+
+  # do the back substitution
+  (n - 2).downto(0).each do |j|
+    d[j] -= c[j] * d[j + 1]
+  end
+
+  d
+end
+
+# example for matrix
+# [1  4  0][x]   [7]
+# [2  3  5][y] = [5]
+# [0  3  6][z]   [3]
+
+#                 [.8666]
+# soln will equal [1.533]
+#                 [-.266]
+# note we index a from 1 and c from 0
+
+a = [0.0, 2.0, 3.0]
+b = [1.0, 3.0, 6.0]
+c = [4.0, 5.0, 0.0]
+d = [7.0, 5.0, 3.0]
+
+soln = thomas(a, b, c, d)
+puts soln

contents/thomas_algorithm/thomas_algorithm.md

@@ -129,6 +129,8 @@ You will find this algorithm implemented [in this project](https://scratch.mit.e
 [import, lang:"crystal"](code/crystal/thomas.cr)
 {% sample lang="kotlin" %}
 [import, lang:"kotlin"](code/kotlin/thomas.kt)
+{% sample lang="ruby" %}
+[import, lang="ruby"](code/ruby/thomas.rb)
 {% endmethod %}
 
 <script>