From ba3fa7577415b5abd352d8c802e49340a0f84b5e Mon Sep 17 00:00:00 2001
From: Manolis Stamatogiannakis <mstamat@gmail.com>
Date: Wed, 20 May 2015 17:40:54 -0700
Subject: [PATCH] Update cachematrix.R

---
 cachematrix.R | 71 ++++++++++++++++++++++++++++++++++++++++++++-------
 1 file changed, 62 insertions(+), 9 deletions(-)

diff --git a/cachematrix.R b/cachematrix.R
index a50be65aa44..3c152037d43 100644
--- a/cachematrix.R
+++ b/cachematrix.R
@@ -1,15 +1,68 @@
-## Put comments here that give an overall description of what your
-## functions do
-
-## Write a short comment describing this function
-
+##  Creates a special "matrix" object that can cache its inverse.
 makeCacheMatrix <- function(x = matrix()) {
+	inv <- NULL              # cached inverse
+	
+	# mutator for the encapsulated matrix - also resets cached result to NULL
+	set <- function(y) {
+		# only reset inverse if matrix are not equal
+		if ( !(all(dim(x) == dim(y)) && all(x==y)) ) { inv <<- NULL }
 
+		# set encapsulated matrix in all cases
+		x <<- y
+	}
+	
+	# accessor - return encapsulated matrix
+	get <- function() { x }
+	
+	# mutator for the inverse matrix
+	set_inv <- function(inv_new) { inv <<- inv_new }
+	
+	# accessor for the inverse matrix
+	get_inv <- function() { inv }
+	
+	# wrap everything into a list
+	list(set = set, get = get, set_inv = set_inv, get_inv = get_inv)
 }
 
-
-## Write a short comment describing this function
-
+## Computes the inverse of the special "matrix" returned by makeCacheMatrix().
+## The inverse is computed only once. Cache results are used in subsequent calls.
 cacheSolve <- function(x, ...) {
-        ## Return a matrix that is the inverse of 'x'
+    ## Return a matrix that is the inverse of 'x'
+	
+	# return any non-NULL cached result
+	inv <- x$get_inv()
+	if(!is.null(inv)) { return(inv) }
+	
+	# no cached result exists - calculate it and return
+	realmtx <- x$get()
+	inv <- solve(realmtx, ...)
+	x$set_inv(inv)
+	return(inv)
 }
+
+## This function implements a loop for demonstrating the speed gains
+## from caching the matrix inverse.
+timeloop <- function(x, usecache=FALSE, n=1) {
+	if (usecache) {
+		message("Loop using cached inverse...")
+		xc <- makeCacheMatrix(x)
+		for(i in 1:n) {
+			inv <- cacheSolve(xc)
+		}
+	}
+	else {
+		message("Loop without cached inverse...")
+		xc <- x
+		for(i in 1:n) {
+			inv <- solve(xc)
+		}
+	}
+	message("Done.")
+	print(inv)
+	message()
+}
+
+## Run the timing loop with caching disabled/enabled.
+mtx <- matrix(c(3,1,2,1,1,1,2,1,1), nrow=3, ncol=3)
+print(system.time(timeloop(mtx, usecache=FALSE, n=80000)))
+print(system.time(timeloop(mtx, usecache=TRUE,  n=80000)))