cachematrix.R

##  makeCacheMatrix
#
#   Creates an "object" for a (square!) matrix to prepare for caching its inverse
#   containing a getter and setter (returning the value set) 
#   for the matrix and its inverse

makeCacheMatrix <- function(x = matrix()) {
    i <- matrix()            # clear cache for inverse initially
    set <- function(y) {
        i <<- matrix()       # setter changes matrinx and clears cache
        x <<- y
    }
    get <- function() x
    setinv <- function(inv) i <<- inv
    getinv <- function() i
    list(set = set, get = get, setinv = setinv, getinv = getinv)
}

##  cacheSolve
#
#   returns the inverse of a (square!) matrix's caching object
#   if no inverse exists yet will store it will determine it an via its setter
#   or return the cached value otherwise
#

cacheSolve <- function(x, ...) {
    emptyMatrix <- function (i) {
        # an "empty" matrix has dimension 1 by 1 and its only value is NA
        (identical(dim(i), as.integer(c(1, 1))) && is.na(i[1,1])) 
    }

    i <- x$getinv()                    # try to get cached value
    if(!emptyMatrix(i)) {              # found one, return it
        message("getting cached data")
        return(i)
    }
    data <- x$get()                    # not found, get matrix contents
    i <- solve(data, ...)              # invert it
    x$setinv(i)                        # store / return the inverse
}