working version of assignment functions

battez · battez · commit 4927e6501f58 · 2015-02-17T12:06:45.000Z
diff --git a/cachematrix.R b/cachematrix.R
@@ -1,15 +1,87 @@
-## Put comments here that give an overall description of what your
-## functions do
+# WEEK3 ~ ASSIGNMENT 
+# by John Barker
+# cachematrix.R
+# Pair of functions which cache the inverse of a matrix. 
+# example usage:
+# cacheSolve(makeCacheMatrix(x = someInvertibleMatrix))
+# 
+# The first function, makeCacheMatrix creates a special "matrix" object, makes a function to
+# set - the value of the matrix & reset cache
+# get - the value of the matrix object
+# 
+# set - the value of the matrix that is the inverse of matrix x
+# get - ditto
+# 
+makeCacheMatrix <- function(x = matrix()) {
 
-## Write a short comment describing this function
+	# variable to store the inverse
+	i <- NULL
 
-makeCacheMatrix <- function(x = matrix()) {
 
-}
+	# function to invalidate the cache. 
+	# It sets x to new value y. It also resets the stored inverse: i.
+    set <- function(y) {
+
+    	#both lines make use of "Superassignment" operator:
+        x <<- y #looks outside the current scope starting with the parents environment.
+        i <<- NULL #looks outside the current scope starting with the parents environment.
+    }
+
+    # simple getting of x.
+    get <- function() x
 
+    # function that sets i (again, looks in another environment) to the "solved" passed
+    setinverse <- function(solved) i <<- solved
+
+    # returns the inverse
+    getinverse <- function() i
+
+    # returns a list of functions: 
+    # it is a holder for the functions to get and set the values, 
+    # that are used by the partner function below, cacheSolve().
+    list(set = set, get = get, setinverse = setinverse, getinverse = getinverse)
+	
+
+
+}
 
-## Write a short comment describing this function
 
+# computes the inverse of the  "matrix" inside the obj. returned by makeCacheMatrix()
+#  (i.e. the list of functions). 
+#  
+# If the inverse has already been calculated (and the matrix has not changed), 
+# then  cachesolve should retrieve the inverse from the cache.
+#
+# 
+# NB assumes matrix supplied is invertible.
 cacheSolve <- function(x, ...) {
-        ## Return a matrix that is the inverse of 'x'
+
+	
+
+	# Return any cached matrix we may have that is the inverse of 'x', 
+	# by referencing the getinverse() function our partner function gave us.
+	# 
+	i <- x$getinverse()
+
+	# if i was obtained with a value, we can just exit here 
+	if(!is.null(i)) {
+	   message("getting cached data")
+	   return(i) #exit here
+	}
+
+	# if no cache exists,  we use the passed matrix to generate the inverse:
+	data <- x$get()
+
+	# R-function for obtaining the inverse of a matrix is solve(), assign this to i.
+	# 
+	i <- solve(data, ...)
+
+
+	# we need to cache this before exiting:
+	x$setinverse(i)
+
+	# return  the inverse value:
+	return(i)
+
+
 }
