First working version thomas algorithm in lisp.

Author: Trashtalk217

contents/thomas_algorithm/code/clisp/thomas.lisp

@@ -0,0 +1,30 @@
+;;;; Thomas algorithm implementation in Common Lisp
+
+(defun thomas (a b c d)
+  "Returns the solutions to a tri-diagonal matrix destructively"
+  (setf (svref c 0) (/ (svref c 0) (svref b 0)))
+  (setf (svref d 0) (/ (svref d 0) (svref b 0)))
+
+  (loop
+    for i from 1 upto (1- (length a)) do
+      (setf
+        (svref c i)
+        (/ (svref c i) (- (svref b i) (* (svref a i) (svref c (1- i))))))
+      (setf
+        (svref d i)
+        (/
+          (- (svref d i) (* (svref a i) (svref d (1- i))))
+          (- (svref b i) (* (svref a i) (svref c (1- i)))))))
+
+  (loop
+    for i from (- (length a) 2) downto 0 do
+      (setf (svref d i) (- (svref d i) (* (svref c i) (svref d (1+ i))))))
+
+  (return-from thomas d))
+
+(defvar diagonal-a #(0 2 3))
+(defvar diagonal-b #(1 3 6))
+(defvar diagonal-c #(4 5 0))
+(defvar last-column #(7 5 3))
+
+(print (thomas diagonal-a diagonal-b diagonal-c last-column))