cache matrix and its inverse

Author: Annie

cachematrix.R

@@ -1,15 +1,71 @@
-## Put comments here that give an overall description of what your
-## functions do
-
-## Write a short comment describing this function
+## This progrom computes a matrix inverse
+## For this assignment, assume that the matrix supplied is always invertible
+## 
+## The program consists of two functions:
+##
+## One function returns a matrix inverse either
+##   from cache or from computation with cache miss
+##   a computed inverse is cached by design
+##
+## The other function provides handles to the original matrix and its inverse
+##   In this function, we can set/get the original matrix
+##   we can also set/get the matrix inverse
 
+## This function creates a special "matrix" object
+##   that can cache its inverse.
 makeCacheMatrix <- function(x = matrix()) {
+  ## initialize the invertible matrix to null
+  b <- NULL
 
-}
+  ## populate the original matrix per argument
+  ## and initialize its inverse to null
+  set <- function(y){
+    x <<- y
+    b <<- NULL
+  }
+
+  ## fetch the original matrix 
+  get <- function() {
+    x
+  }
 
+  ## populate the invertible matrix per argument
+  setinverse <- function(inverse) {
+    b <<- inverse
+  }
 
-## Write a short comment describing this function
+  ## fetch the invertible matrix 
+  getinverse <- function() {
+    b
+  }
+
+  list(set = set, 
+    get = get,
+    setinverse = setinverse,
+    getinverse = getinverse)
+}
 
+
+## This function computes the inverse of the special
+##   "matrix" returned by `makeCacheMatrix` above. If the inverse has
+##   already been calculated (and the matrix has not changed), then
+##   `cacheSolve` should retrieve the inverse from the cache.
 cacheSolve <- function(x, ...) {
-        ## Return a matrix that is the inverse of 'x'
+  # fetch matrix inverse from cache
+  b <- x$getinverse()
+
+  # if in cache, return cached inverse
+  if (!is.null(b)){
+    message("getting cached data")
+    return(b)
+  }
+
+  # otherwise, compute the matrix inverse
+  data <- x$get()
+  b <- solve(data)
+  # cache the computed inverse matrix
+  x$setinverse(b)
+
+  ## return the computed inverse matrix
+  b 
 }