my version of the cachematrix.R

cachematrix.R

@@ -2,14 +2,40 @@
 ## functions do
 
 ## Write a short comment describing this function
-
+# makeCacheMatrix: create a matrix that can store itself and a cached copy of its inverse once its computed
 makeCacheMatrix <- function(x = matrix()) {
-
+  inv <- NULL                        # cache for inverse 
+
+  set <- function(y) {               # replace matrix and reset cache
+    x <<- y
+    inv <<- NULL
+  }
+
+  get <- function() x                # return current matrix
+
+  setinverse <- function(inverse)    # store computed inverse
+    inv <<- inverse
+
+  getinverse <- function() inv       # return cached inverse (or NULL)
+
+  list(set = set, get = get,
+       setinverse = setinverse,
+       getinverse = getinverse)
 }
 
 
-## Write a short comment describing this function
 
+## Write a short comment describing this function
+# cacheSolve: returns the cached inverse or computes & caches it
 cacheSolve <- function(x, ...) {
-        ## Return a matrix that is the inverse of 'x'
+  inv <- x$getinverse()              # try cache first
+  if (!is.null(inv)) {
+    message("getting the cached inverse")
+    return(inv)
+  }
+  mat <- x$get()                     # fetch matrix
+  inv <- solve(mat, ...)             # compute inverse (assumes invertible)
+  x$setinverse(inv)                  # cache result
+  inv                                 # return inverse
 }
+