cache matrix inversion

twoerth · twoerth · commit ae5eefe9ebbf · 2014-04-26T20:02:27.000+02:00
diff --git a/cachematrix.R b/cachematrix.R
@@ -1,15 +1,54 @@
-## Put comments here that give an overall description of what your
-## functions do
-
-## Write a short comment describing this function
+## Special wrapper of a matrix in cases where we need cheaper repeat access to the inverse.
 
+## Build the matrix wrapper
 makeCacheMatrix <- function(x = matrix()) {
+	## by default we do not have an inverse.
+	inv <- NULL
 
+	## Allow the caller to set a new matrix in place of the use used at initialisation.
+	set <- function( y ) {
+		x <<- y
+		## make sure an eventual cached inverse is cleared
+		inv <<- NULL
+	}
+	
+	## Return the matrix
+	get <- function() {
+		x
+	}
+	
+	## cache the inverse
+	setinverse <- function( inverse ) {
+		inv <<- inverse
+	}
+	
+	## return the inverse
+	getinverse <- function() {
+		inv
+	}
 }
 
 
-## Write a short comment describing this function
-
+## Given an object returned by makeCacheMatrix, calculate the inverse and cache it for further calls.
+## If a cached inverse has already been calculated on the wrapped matrix, the cached version is returned.
 cacheSolve <- function(x, ...) {
         ## Return a matrix that is the inverse of 'x'
-}
+		
+		inv <- x$getinverse
+		
+		if( ! is.null( inv ) ) {
+			## The wrapped matrix has a non NULL inverse
+			message( "cache hit" )
+			return inv
+		}
+		
+		## Calculate the inverse using solve
+		## TODO: handle special cases more carefully, not all matrices can be inverted.
+		inv <- solve( x$get() )
+
+		## Cache the result of solve()
+		x$setinverse( inv )
+
+		## return the inverse
+		inv
+}
