cachematrix.R

## matrix functions to cache a matrix inverse
## solve for inverse matrix only once and 
## cache inverse to return on subsequent requests

makeCacheMatrix <- function(x = matrix()) {
  ## crate a matrix object 
  ## eg use >myMatrix = makeCacheMatrix(matrix(data=c(1:4),nrow=2,ncol=2))
  inv <- NULL  
  set <- function(y) {
    x <<- y
    inv <<- NULL
  }
  get <- function() x
  setinverse <- function(solve) inv <<- solve
  getinverse <- function() inv
  list(set = set, get = get,
       setinverse = setinverse,
       getinverse = getinverse)
}

cacheSolve <- function(x, ...) {
  ## Return a matrix that is the inverse of 'x'
  ## x must be a square invertible matrix created with makeCacheMatrix 
  ## eg. use >cachSolve(myMatrix)
  inv <- x$getinv()
  if(!is.null(inv)) {
    message("getting cached data")
    return(inv)
  }
  data <- x$get()
  inv <- solve(data, ...)
  x$setinv(inv)
  inv  
}


# example usage
# > v4 <- makeCacheMatrix() 
# > v4$set(matrix(c(99,13,5,26),nrow=2,ncol=2)) 
# > v4$get()
# [,1] [,2]
# [1,]   99    5
# [2,]   13   26
# > cacheSolve(v4)
# [,1]         [,2]
# [1,]  0.010362694 -0.001992826
# [2,] -0.005181347  0.039457951
# > cacheSolve(v4)
# getting cached data
# [,1]         [,2]
# [1,]  0.010362694 -0.001992826
# [2,] -0.005181347  0.039457951