cachematrix.R

# This module contains functions for memoization (caching) of results
# on an associated matrix.


# makeCacheMatrix(src_matrix) wraps the src_matrix with an object that
# can cache the values of computations on the matrix.
#
# Sample usage, finding the transpose and inverse of a matrix:
#
# x <- makeCacheMatrix(matrix(c(3,0,2,1,5,3,9,2,1),nrow=3))
# x_transpose <- x$cacheGet("t", t)
# x_inverse <- cacheSolve(x)
#
makeCacheMatrix <- function(src_matrix = matrix()) {
  # Create an environment, which can store keys and the corresponding
  # cached values.
  cache_store <<- new.env()
  
  # Overwrite the existing matrix with new_value
  set <- function(new_value) {
    src_matrix <<- new_value
    cache_store <<- new.env()
  }
  
  # Retrieve the current matrix
  get <- function() src_matrix
  
  # Return the value associated with the given key.  If the value is not
  # available in the cache, it is generated by calling the passed
  # calc_function, and will be returned from the cache thereafter.
  #
  # If called with no calc_function, a value is only returned if it's 
  # already in the cache.
  cacheGet <- function(key, calc_function =NULL) {
    
    # Check for a cached value, and either use it or calculate and cache.
    if (exists(key, envir=cache_store, inherits=FALSE)) {
      # Because we define a get() function above, I have to call the
      # built-in get() using the base environment.
      cache_value <- baseenv()$get(key, envir=cache_store)
    } else {
      # If the value was not cached and we can calculate it, do so and 
      # cache it.
      # print("Calling function to store in cache")
      if (!is.null(calc_function)) {
        cache_value <- calc_function(get())
        assign(key, cache_value, envir=cache_store)
      } else {
        cache_value <- NULL
      }
    }
    return (cache_value)
  }
  
  list(set = set, get = get, cacheGet = cacheGet)
}

# The cacheSolve() takes a matrix created by makeCacheMatrix(),
# and cache the result to be used on future invocations.
cacheSolve <- function(cache_matrix) {
  cache_matrix$cacheGet("solve", solve)
}