From d3dd2c0aa08e2798ec120ed3780c774feb910f50 Mon Sep 17 00:00:00 2001
From: Steven Tobin <steventtobin (at) gmail.com>
Date: Thu, 19 Jun 2014 22:46:35 +1000
Subject: [PATCH] completed, including usage axample

---
 cachematrix.R | 53 ++++++++++++++++++++++++++++++++++++++++++++-------
 1 file changed, 46 insertions(+), 7 deletions(-)

diff --git a/cachematrix.R b/cachematrix.R
index a50be65aa44..8c5612f22f5 100644
--- a/cachematrix.R
+++ b/cachematrix.R
@@ -1,15 +1,54 @@
-## Put comments here that give an overall description of what your
-## functions do
-
-## Write a short comment describing this function
+# makeCacheMatrix takes an invertible matrix x and creates getters and setters
+# to compute, cache, store, and retrieve its inverse x^{-1} via solve()
 
 makeCacheMatrix <- function(x = matrix()) {
-
+    m <- NULL
+    set <- function(y) {
+            x <<- y
+            m <<- NULL
+    }
+    get <- function() x
+    setinverse <- function(solve) m <<- solve
+    getinverse <- function() m
+    list(set = set, get = get,
+         setinverse = setinverse,
+         getinverse = getinverse)
 }
 
 
-## Write a short comment describing this function
+# this is a cache-aware variant of solve(), which takes an enhanced matrix
+# makeCacheMatrix and returns the inverse. It returns the cached value if
+# present, and if not computes it "the hard way" and stores the result.
 
 cacheSolve <- function(x, ...) {
-        ## Return a matrix that is the inverse of 'x'
+    ## Return a matrix that is the inverse of 'x'
+    m <- x$getinverse()
+    if(!is.null(m)) {
+        message("getting cached data")
+            return(m)
+    }
+    data <- x$get()
+    m <- solve(data, ...)
+    x$setinverse(m)
+    m
 }
+
+# _Usage example_
+# Create a 2000*2000 matrix, solve takes ~2 seconds
+# then add the caching functions
+# first run of cacheSolve again takes ~2 seconds, but second run simply
+# returns cached data and takes ~0 seconds
+
+# > A <- matrix(runif(2000*2000), ncol=2000)
+# > system.time(solve(A))
+#    user  system elapsed
+#   3.589   0.051   1.926
+# > z <- makeCacheMatrix(A)
+# > system.time(cacheSolve(z))
+#    user  system elapsed
+#   3.826   0.012   2.004
+# > system.time(cacheSolve(z))
+# getting cached data
+#    user  system elapsed
+#       0       0       0
+# >
\ No newline at end of file