From: Michael Orlitzky <michael@orlitzky.com>
Date: Sun, 21 Jul 2013 14:13:50 +0000 (-0400)
Subject: Add 'diagonal' function.
X-Git-Url: http://gitweb.michael.orlitzky.com/?p=numerical-analysis.git;a=commitdiff_plain;h=d05e7a5cec0e76a6ebb4cbc49651211c5a7cf18e

Add 'diagonal' function.
---

diff --git a/src/Linear/Matrix.hs b/src/Linear/Matrix.hs
index 8490b22..20769b4 100644
--- a/src/Linear/Matrix.hs
+++ b/src/Linear/Matrix.hs
@@ -520,3 +520,23 @@ angle v1 v2 =
   where
    theta = (recip norms) NP.* (v1 `dot` v2)
    norms = (norm v1) NP.* (norm v2)
+
+
+
+-- | Given a square @matrix@, return a new matrix of the same size
+--   containing only the on-diagonal entries of @matrix@. The
+--   off-diagonal entries are set to zero.
+--
+--   Examples:
+--
+--   >>> let m = fromList [[1,2,3],[4,5,6],[7,8,9]] :: Mat3 Int
+--   >>> diagonal m
+--   ((1,0,0),(0,5,0),(0,0,9))
+--
+diagonal :: (Arity m, Ring.C a)
+         => Mat m m a
+         -> Mat m m a
+diagonal matrix =
+  construct lambda
+  where
+    lambda i j = if i == j then matrix !!! (i,j) else 0