Implement cached matrix and inverse solver

Author: swalberg

cachematrix.R

@@ -1,15 +1,86 @@
-## Put comments here that give an overall description of what your
-## functions do
-
-## Write a short comment describing this function
+## Assignment 2
+## Implements a function that returns a matrix wrapper that caches the result of
+## the inverse calculations, and a function to return the inverse from such an object.
+## Also includes a test function.
 
+## A matrix wrapper that memoizes[0] the inverse calculation.
+## Returns a list containing getter/setter functions for the matrix
+## and its inverse.
+## 0 - https://en.wikipedia.org/wiki/Memoization
+##
+## Example
+## x <- matrix(rnorm(9), 3, 3)
+## x.cached <- makeCacheMatrix(x)
+## x.cached$get() # same as x
+## x.cached$getInverse() # Inverse of x, only calculated on the first call
 makeCacheMatrix <- function(x = matrix()) {
-
+  cachedInverse <- NULL
+  cachedMatrix <- x
+  
+  # Setter function
+  set <- function(y) {
+    cachedMatrix <<- y
+    cachedInverse <<- NULL # Inverse has changed, so invalidate cache
+  }
+  
+  # Getter function
+  get <- function() cachedMatrix
+  
+  # Setter function for the inverse
+  setInverse <- function(inverse) cachedInverse <<- inverse
+  
+  # Getter function for the inverse
+  # implements a read through cache rather than requiring
+  # the caller to know about internal details
+  getInverse <- function(...) {
+    if(is.null(cachedInverse)) {
+      message("Cache miss - recalculating inverse")
+      cachedInverse <<- solve(cachedMatrix, ...)
+    }
+    cachedInverse
+  }
+  
+  # Return a structure containing the getters and setters
+  # Because of lexical scoping rules, they have a private copy of the cache
+  list(set = set,
+       get = get,
+       setInverse = setInverse,
+       getInverse = getInverse)
 }
 
 
-## Write a short comment describing this function
-
+## Returns the inverse of a matrix previously wrapped by makeCacheMatrix
+## Note the wrapper is responsible for cache, so this is effectively a pass through
 cacheSolve <- function(x, ...) {
-        ## Return a matrix that is the inverse of 'x'
+  ## Return a matrix that is the inverse of 'x'
+  x$getInverse(...)
 }
+
+test <- function() {
+  a.normal <- matrix(rnorm(9), 3, 3) 
+  b.normal <- matrix(rnorm(9), 3, 3) 
+  
+  a.cached <- makeCacheMatrix(a.normal)
+  b.cached <- makeCacheMatrix(b.normal)
+  
+  if (identical(a.cached$get(), a.normal)) {
+    print("Good, a is the same")
+  }
+  
+  if (identical(b.cached$get(), b.normal)) {
+    print("Good, b is the same")
+  }
+  
+  if (identical(solve(a.normal), cacheSolve(a.cached))) {
+    print("Good, a's inverse is correct")
+  }
+  
+  if (identical(solve(b.normal), cacheSolve(b.cached))) {
+    print("Good, b's inverse is correct")
+  }
+  
+  if (identical(solve(a.normal), cacheSolve(a.cached))) {
+    print("Good, a's inverse is still correct and you should not see a cache miss")
+  }
+  
+}