cachematrix.R

### Functions to allow caching the inverse of a matrix.  Implementation 
### creates a closure over a given matrix; calls to the new object returns 
### the cached value if available, otherwise generates the inverse and stores
### it for future calls.

# Helper function that creates a closure over a given matrix that allows the 
# inverse of the matrix to be cached to potentially avoid expensive 
# computations. The resulting object is a list containing four functions:
#
#   set() to set the value of matrix; setting the contents clears the cache
#   get() to return the matrix
#   setinverse() to cache the inverse of when computed
#   getinverse() to retrieve the cache value (will be NULL if yet to be
#     computed)

makeCacheMatrix <- function(x = matrix()) {
        inv <- NULL
        
        set <- function(y) {
                x <<- y
                inv <<- NULL
        }  
        get <- function() x  
        setinverse <- function(inverse) inv <<- inverse  
        getinverse <- function() inv
        
        list(set = set, get = get, setinverse = setinverse, 
             getinverse = getinverse)
}


# Main function call to retrieve the inverse of a matrix. If the inverse was
# previously computed and cached, return the cached value.  Otherwise, 
# compute the inverse and cache the resulting value for future use.

cacheSolve <- function(x, ...) {
        inv <- x$getinverse()
        if(!is.null(inv)) {
                message("getting cached data")
                return(inv)
        }  
        
        data <- x$get()
        inv <- solve(data, ...)
        x$setinverse(inv)
        inv
}