Update cachematrix.R

lancetw · lancetw · commit f775ca795f4f · 2014-04-27T02:20:47.000+08:00
diff --git a/cachematrix.R b/cachematrix.R
@@ -1,15 +1,43 @@
-## Put comments here that give an overall description of what your
-## functions do
+## Assignment: Caching the Inverse of a Matrix
+## (Programming Assignment 2 for R Programming on Coursera)
+##
+## Matrix inversion is usually a costly computation and their may be some 
+## benefit to caching the inverse of a matrix rather than compute it repeatedly
+## (there are also alternatives to matrix inversion that we will not discuss 
+## here). Your assignment is to write a pair of functions that cache the 
+## inverse of a matrix.
 
-## Write a short comment describing this function
+## This function creates a special "matrix" object that can cache its inverse.
 
 makeCacheMatrix <- function(x = matrix()) {
-
+  inv_x <- NULL
+  set <- function(y) {
+    x <<- y
+    inv_x <<- NULL
+  }
+  get <- function() x
+  setInv <- function (inv_mat) inv_x <<- inv_mat
+  getInv <- function() inv_x
+  list(set = set, get = get, 
+       setInv = setInv,
+       getInv = getInv)
 }
 
 
-## Write a short comment describing this function
+## 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 the cachesolve should retrieve 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'
+  inv_x <- x$getInv()
+  if(!is.null(inv_x)) {
+    message('getting cached inverse matrix')
+    return(inv_x)
+  }
+  data <- x$get()
+  inv_x <- solve(data, ...)
+  x$setInv(inv_x)
+  inv_x
 }
