From 4f88d937fd8984d76086556e4934c2a0af2c9698 Mon Sep 17 00:00:00 2001
From: weipohsu <pohsu.wei@gmail.com>
Date: Sat, 19 Apr 2014 13:16:53 -0400
Subject: [PATCH] Programming Assignment2

---
 cachematrix.R | 57 +++++++++++++++++++++++++++++++++++++++++++++------
 1 file changed, 51 insertions(+), 6 deletions(-)

diff --git a/cachematrix.R b/cachematrix.R
index a50be65aa44..43ad594ce10 100644
--- a/cachematrix.R
+++ b/cachematrix.R
@@ -1,15 +1,60 @@
-## Put comments here that give an overall description of what your
-## functions do
+## The followinfg two functions are used to demonstrate a programming 
+## method to cache results of some expensive calculations.
+## The <<- operator is also introducted to assign a value to an objecct that
+## is in different environment.
 
-## Write a short comment describing this function
+## Thw makeCascheMatrix function creates a customized matrix object that can cache the inverse of the matrix.
+## The function is a list of functions to
+## 1. Set the value of the matrix
+## 2. Get the value of the matrix
+## 3. Set the value of the inverse of the matrix
+## 4. set the value of the inverse of the matrix
 
 makeCacheMatrix <- function(x = matrix()) {
-
+    i <- NULL
+    set <- function(y) {
+        x <<- y     ## Set the new matrix value
+        i <<- NULL  ## Set the cache to be NULL since a new matrix value is set 
+    }
+    
+    ## return the value of the current matrix
+    get <- function() x 
+    
+    ## put the inverse of the matrix in cache
+    setinv <- function(inv) i <<- inv 
+    
+    ## get the inverse value from cache
+    getinv <- function() i    
+    
+    ## return the list of functions 
+    list(set = set, get = get,
+         setinv = setinv,
+         getinv = getinv)
 }
 
 
-## Write a short comment describing this function
+## The function computes the inverse of the special "matrix" returned by
+## makeCacheMatrix. In the case that the inverse of the matrix is already
+## calculated and cached, the function should retrive the inverse from the 
+## cache
 
 cacheSolve <- function(x, ...) {
-        ## Return a matrix that is the inverse of 'x'
+    ## Return a matrix that is the inverse of 'x'
+    i <- x$getinv()
+    
+    ## if the cache returns a value
+    if(!is.null(i)) {
+        message("getting cached data")
+        return(i)
+    }
+    
+    ## If there is no cache value for the current matrix
+    ## Retrieve the value of th ematrix
+    data <- x$get()
+    ## calculate the inverse of the matrix
+    i <- solve(data, ...)
+    ## Put the inverse value back to the special matrix object
+    x$setinv(i)
+    ## Return the newly calculated inverse value of the matrix
+    i
 }