From: Michael Orlitzky <michael@orlitzky.com>
Date: Sat, 8 Feb 2014 00:24:14 +0000 (-0500)
Subject: Make safe indexing slower but not dependent on the number of rows/columns.
X-Git-Url: https://gitweb.michael.orlitzky.com/?a=commitdiff_plain;h=d889338fd30522f43643c77e33b941f076f597b5;p=numerical-analysis.git

Make safe indexing slower but not dependent on the number of rows/columns.
---

diff --git a/src/Linear/Matrix.hs b/src/Linear/Matrix.hs
index a5fbb84..611545f 100644
--- a/src/Linear/Matrix.hs
+++ b/src/Linear/Matrix.hs
@@ -38,7 +38,6 @@ import qualified Data.Vector.Fixed as V (
   fromList,
   head,
   ifoldl,
-  length,
   map,
   maximum,
   replicate,
@@ -145,23 +144,54 @@ instance (Show a) => Show (Mat m n a) where
 toList :: Mat m n a -> [[a]]
 toList (Mat rows) = map V.toList (V.toList rows)
 
+
 -- | Create a matrix from a nested list.
 fromList :: (Arity m, Arity n) => [[a]] -> Mat m n a
 fromList vs = Mat (V.fromList $ map V.fromList vs)
 
 
--- | Unsafe indexing.
+-- | Unsafe indexing. Much faster than the safe indexing.
 (!!!) :: (Arity m, Arity n) => Mat m n a -> (Int, Int) -> a
 (!!!) (Mat rows) (i, j) = (rows ! i) ! j
 
 
 -- | Safe indexing.
+--
+--   Examples:
+--
+--   >>> let m = fromList [[1,2],[3,4]] :: Mat2 Int
+--   >>> m !!? (-1,-1)
+--   Nothing
+--   >>>  m !!? (-1,0)
+--   Nothing
+--   >>>  m !!? (-1,1)
+--   Nothing
+--   >>>  m !!? (0,-1)
+--   Nothing
+--   >>>  m !!? (0,0)
+--   Just 1
+--   >>>  m !!? (0,1)
+--   Just 2
+--   >>>  m !!? (1,-1)
+--   Nothing
+--   >>>  m !!? (1,0)
+--   Just 3
+--   >>>  m !!? (1,1)
+--   Just 4
+--   >>>  m !!? (2,-1)
+--   Nothing
+--   >>>  m !!? (2,0)
+--   Nothing
+--   >>>  m !!? (2,1)
+--   Nothing
+--   >>>  m !!? (2,2)
+--   Nothing
+--
 (!!?) :: (Arity m, Arity n) => Mat m n a -> (Int, Int) -> Maybe a
-(!!?) matrix (i, j)
-  | i < 0 || j < 0 = Nothing
-  | i > (nrows matrix) - 1 = Nothing
-  | j > (ncols matrix) - 1 = Nothing
-  | otherwise = Just $ matrix !!! (i,j)
+(!!?) matrix idx =
+  ifoldl2 f Nothing matrix
+  where
+    f k l found cur = if (k,l) == idx then (Just cur) else found
 
 
 -- | The number of rows in the matrix.
@@ -841,5 +871,3 @@ ifoldl2 f initial (Mat rows) =
 
     row_function :: b -> Int -> Vec n a -> b
     row_function rowinit idx r = V.ifoldl (g idx) rowinit r
-
-