Implement the cachematrix helper methods (and document them)

Author: jits

cachematrix.R

@@ -1,15 +1,68 @@
-## Put comments here that give an overall description of what your
-## functions do
+# ------------------------------------------------------------------------------
+# Helper functions to compute and cache the inverse of a matrix (i.e. "solve" a
+# matrix).
+#
+# Note: we assume a square matrix (i.e. one that is invertible) is always
+# provided when using these functions. Therefore there is no erro handling for
+# cases where this is not true.
+#-------------------------------------------------------------------------------
 
-## Write a short comment describing this function
 
+# Given an initial matrix, this method returns a special wrapper, in the form of
+# a list with functions that can be used to:
+# - set the underlying matrix again
+# - get the underlying matrix
+# - set the inverse of the underlying matrix
+# - get th inverse of the underlying matrix
 makeCacheMatrix <- function(x = matrix()) {
-
+  i <- NULL  # Cache value for the inverse matrix of `x`
+  
+  # Setter method, if we want to overwrite the underlying matrix `x`
+  set <- function(y) {
+    x <<- y
+    i <<- NULL
+  }
+  
+  # Getter method, to access the underlying matrix `x`
+  get <- function() {
+    x
+  }
+  
+  # Set the inverse of the underlying matrix `x`, into the cache
+  setinverse <- function(inverse) {
+    i <<- inverse
+  }
+  
+  # Get the inverse of the underlying matrix `x`, from the cache
+  # Note: this will return `NULL` if the cache hasn't been set yet
+  getinverse <- function() {
+    i
+  }
+  
+  list(set = set, get = get,
+       setinverse = setinverse,
+       getinverse = getinverse)
 }
 
 
-## Write a short comment describing this function
-
+# Use this method to "solve" (i.e. get the inverse) of a matrix, doing so in a
+# way that caches the very first computation of the inverse and uses the cache
+# subsequently.
+#
+# The argument `x` MUST be the special wrapper obtained from `makeCacheMatrix`.
 cacheSolve <- function(x, ...) {
-        ## Return a matrix that is the inverse of 'x'
+  ## Return a matrix that is the inverse of 'x'
+  
+  # Try to get the cached value
+  i <- x$getinverse()
+  if(!is.null(i)) {
+    message("getting cached data")
+    return(i)
+  }
+  
+  # … otherwise compute the inverse and cache
+  data <- x$get()
+  i <- solve(data, ...)
+  x$setinverse(i)
+  i
 }